On Motions, Wetness, and Such: The USS Midway Blister Story

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1 SNAME Transactions, Vol. 97, 1989, pp On Motions, Wetness, and Such: The USS Midway Blister Story Myron V. Ricketts, 1 Member, and Peter A. Gale, 2 Member USS Midway (CV 41 ) was commissioned in September During her 4O-plus years of superior service to our Navy and country, she has twice had blisters added to her hull to offset the effects of topside weight growth on draft, hull strength and damage stability. The first blister was added in 1957, the second in When Midway put to sea in late 1986 with her second blister, she experienced undesirable rolling and flight deck wetness in some sea states which adversely affected handling aircraft on deck. An accelerated effort was undertaken to analyze these problems and define solutions. The paper describes this effort. Topics addressed include: (a) Full-scale tests to establish the ship's natural roll period. (b) Alternatives studied to improve the ship's motions, including blister width reduction in way of the waterline, free-flood tanks, internal cross-connected tanks, and larger bilge keels. (c) Alternatives studied to improve the ship's wetness, including hull forebody shape modifications, bulbous bows, shoulder bulbs, spray rails, and removal of scupper extensions. (d) Extensive and unique model tests performed to validate the proposed solution. (e) Other issues related to the proposed solution; for example, anchor handling, accommodation ladder support, trim and list correction, speed and directional stability. (f) Limiting ship motion criteria, ship motion correlations (analytic predictions versus model tests) and seakeeping operability assessments. DEDCATON Thispaper is dedicated to the professional men andwomen, civilian and military, who have flown from, operated and maintained Midway during her illustrious career. Midway's many commendations, her high state of readiness and her excellent material condition during a career of 44-plus years are a tribute to the skill and diligence of all those who have served her. ntroduction USS Midway (CV 41) was commissioned in September 1945, just after the end of World War. She has since had a long and illustrious career in the service of the United States Navy. Blisters were added to Midway in 1957 in response to topside weight growth, primarily resulting from the changes made to accommodate the Navy's shift from propeller-driven to jet aircraft. The 1957 blisters were nominally 4 ft-0 in. wide per side and increased the ship's molded beam amidships at the 34 ft-6 in. design waterline (DWL) from 113 ft-0 in. to 121 ft-0 in. The blister length was about 672 ft or 74 percent of the ship's waterline length. At the time the 1957 blister was added to the Rear Admiral, USN (Ret.); formerly Director, Fleet Support Atlantic, Naval Station, Norfolk, Virginia. Professor of Naval Architecture, Webb nstitute of Naval Architecture, Crescent Beach Road, Glen Cove, New York. Presented at the Annual Meeting, New York, N.Y., November 15-18, 1989, of THE SOCETY OF NAVAL ARCHTECTS AND MA- RNE ENGNEERS. 429 hull, the skegs enclosing the two inboard propeller shafts were shortened about 40 ft in an effort to solve a propeller cavitation/erosion problem. An effect of these two changes was to make the ship directionally unstable, a fact apparently not recognized at the time. Originally Midway had eighteen 5-in. / 54 single mounts topside but gradually lost them all as weight compensation; by the late 1960's this weight compensation resource had been depleted. Also, in the late 1960's, a major modernization of the ship was accomplished. Weights continued to grow and by the mid-1980's the ship's hull strength and damage stability characteristics had been degraded beyond acceptable limits. Blisters were suggested by per- sonnel in the field. Naval Sea Systems Command (NAVSEA) studies validated the basic concept; however, a greatly modified blister configuration was required to satisfy damage stability considerations. The blister below the waterline was required to add buoyancy and the blister above the waterline was required to provide reserve buoyancy well outboard and hence righting moment to counteract off-center flooding in the event of damage. The blister and associated structure was also required to restore necessary hull strength. Midway's flight deck does not act in strength and thus her hull girder is shallow. This is one reason why she needed strengthening as displacement continued to increase. The decision was made in May 1985 to proceed with a greatly accelerated program to design and install the blisblisters during a scheduled overhaul period in The primary blister design objective has been stated. A secondary objective was to reduce ship draft and hence the wetness of lowered aircraft elevators. Midway (as well as the other ships of her class) had long had the reputation

2 DECK 8"r-6" ABL ND DECK m,.~. Xi't DECK ~'-0" ABL 4TH DECK ~D'-0" ABL 'o: o,o O O to - e lo olo ;o i i OO~O i i / ORGNAL 8HELL Metric Conversion Factors l ft = m 1 ft 2 = m 2 1 in. = 2.54 cm ~.~ / 1M7 BLSTER ll i ~ J ~ / lm OUSTER f R~ 212 i FR 41 Fig blister configuration and extent of being a wet ship, probably due to her low freeboard in comparison with the later Forrestal and Nirnitz class carriers. The blister contract design was completed on 15 August 1985, less than four months after design start. The nominal blister width was 10 ft-0 in. per side; blister length was 683 ft-0 in. Molded beam at the DWL (now the 36 ft-0 in. WL) had increased from 121 ft-0 in. to 141 ft-0 in. Figure 1 depicts the blister extent and configuration and Table 1 presents some principal ship characteristics preand post-blister. The blister fully satisfied its primary objective to restore hull strength and damage stability and also reduced draft about 8 in., increasing elevator freeboard by the same amount. Draft could have been further reduced if the ship's shell plating behind the new blister had been removed. This was not done as it would have greatly increased design and installation time and cost. ncreased deck motions were recognized and advertised as a risk area with the new blister; however, motion predictions placed the resulting deck motions within the cri- teria developed more than a decade earlier and still in use in The flawed nature of those motions criteria is discussed later in this paper. Model tests to validate the seakeeping behavior of Midway with her new blisters were performed several months after the contract design was completed. To save time and cost, the model used was an over 30-year-old, 30-ft-long, wooden resistance and propulsion model, built up only to the hangar deck level. The 1986 blisters were modeled in glass-reinforced plastic (GRP). The model lacked sponsons, elevator cutouts and other topside detail although it was fitted with a lowered forward elevator. Comparative seakeeping model tests of the baseline (pre-1986 blister) configuration were not performed making it more difficult to interpret the test results. The model tests were performed in sea states 5 and 6 and did indicate a wetness problem in sea state 6 with the elevator down. The conclusion at the time was "Do not run in sea state 6 with the elevators down." No one directly involved with the model tests Table 1 Principal ship characteristics CV 41 BLSTER CV 41 CV 41 MODFCATON PRE-1986 BLSTER CURRENT 1 CONTRACT DESGN 2 CV 63 LENGTH, WL, FT BEAM, WL, FT DRAFT, FL, FT DSPL, FL, LT GM T FL, FT NATURAL ROLL PEROD, FL, SECS TRM BY STERN, FT FULL LOAD LST, DEG FULL LOAD 1.5 PORT 1.0 PORT STBD 1 - CURRENT SHP DATA BASED ON NOVEMBER 1986 NCLNNG RESULTS 2 - NCLUDES ALL FX COMPONENTS, NCLUDNG 575 TONS NEW LST/TRM CORRECTON LEAD 3 - CORRECTED FOR FREE SURFACE (0.25 FT FOR CV 41, 0.40 FT FOR CV 63) 430 The USS Midway Blister Story

3 voiced concern that the ship had elevator cutouts and a sponson forward of the elevator that were below the hangar deck and could not be raised like the elevator. The new blisters were installed at the U.S. Navy's Ship Repair Facility (SRF) in Yokosuka, Japan by Sumitomo Heavy ndustries (SH) and Midway put to sea in November Midway had also long been known for her "Dutch roll," in naval aviator's parlance. This term refers to a "lazy eight" motion of her stern ramp at the aft end of the flight deck ( ~ ) which many aviators who have landed on Midway have remarked on. n the course of the blister design effort in 1985, which included maneuverability assessments, the existing directional instability of Midway was discovered and later corrected by increasing movable rudder area by 50 ft 2 each and adding a 200 ft ~ fixed fin on the ship's centerline aft. The Dutch roll was related to Midway's prior directional instability and, after the foregoing fixes were installed along with the new blisters, the Dutch roll has not been observed. The current ship is also reported to be much more responsive to her helm when alongside during underway replenishment (UNREP) operations. The blister and directional stability modifications achieved their design goals. However, it quickly became obvious that the deck motions were often unsatisfactory, as reported almost immediately by the ship [1 ],3 and a totally unexpected phenomenon, excessive flight deck wetness, was experienced. While deck motions met the existing criteria, they sometimes precluded safe handling of aircraft on the flight deck. On the plus side, in relatively calm seas the ship could use large amounts of rudder without inducing unsatisfactory heel angles. A mid-december trip to the ship by three NAVSEA and David Taylor Research Center (DTRC) naval architects generally confirmed the ship's report and led to an intensive effort to analyze and solve the problems. The ensuing seven-month effort, begun in late December 1986, is the subject of this paper. During the December ship visit, which included two days at sea, the often unsatisfactory motion behavior of the ship was observed. n addition, unusual flight deck wetness in the form of "geysers" rising vertically above the deck edge near the forward quarter point was observed. Notable was the ship's short roll period: on the order of 10 to 11 sec based on crude measurements with a stop watch. This was a surprise, since during the blister design the ship's roll period post-blister had been predicted to be 13 to 14 see. Aided by a portable ship motion recorder, a simple experiment was performed in order to establish the ship's roll period. This consisted of steaming the ship for 20 min on five courses, each differing from the previous by 45 deg, a]l the while recording roll am: plitude with the motion recorder. The experiment was designed to ensure that any swell components hidden by the local wind-generated seaway would be encountered close to the beam on one of the legs. Spectral analysis of the roll records later revealed the ship's natural roll period at the peak of the response spectrum to be 11.6 sec. Thus, at the outset of the motion improvement study we were confronted by a dilemma: how could we explain the nearly two-second difference between the ship's actual roll period and our prediction? Compounding the confusion was the fact that a 13.1-sec roll period had been measured during 3 Numbers in brackets designate References at end of paper. a sally 4 performed in the course of an inclining experiment on the ship shortly after installation of the 1986 blister. This agreed well with the design prediction. The roll period mystery took some effort to unravel but much was learned in doing it. At the outset of the Midway fix effort the focus was on the reported ship motions problems. nitial efforts were severely constrained by the requirement that the "fixed ship" be deployed in June Allocating the absolute minimum time for detail design and installation, this meant that any modifications had to be selected and developed to the contract design level of detail by mid- February 1987, less than two months from the start. The team worked to this schedule through January. By that time, further reports of flight deck wetness and aggravated speed loss, also attributed to the new blisters, were being received [2-5]. The ship stated that more boilers had to be placed on the line in order to make the higher ship speeds required under low ambient wind conditions. With the Midway engineering configuration, each boiler in a separate space, demands on the crew were increased as each additional boiler was placed on the line. Still photos taken from a helicopter showing the ship's forebody in a seaway were received in NAVSEA in late January, followed shortly by an even more dramatic video tape. These photos and the tape showed the flight deck wetness and also the presence of a steep shoulder wave, often breaking, which diverged from the ship's hull near the forward end of the new blister. t was judged likely that there was a relationship between the flight deck wetness and the breaking shoulder wave. Recognizing that hull form modifications would be required to reduce the shoulder wave and that such modifications would have to be validated by model tests prior to adoption, plans were changed in early February and the fix schedule was extended. The revised schedule, still highly accelerated, called for the development and assessment of hull form changes, including extensive model tests, leading to an early June decision on the modifications to be adopted. This decision would be followed by a brief period of design refinement. Completion and signature of a contract design package defining the modifications was scheduled for late July Subsequent detail design and installation of the modifications would be done in Yokosuka, Japan by SH with completion by spring This schedule was adhered to until September 1987, at which time the Chief of Naval Operations directed that further work on the project cease as a result of severe Navy funding constraints. n the course of this intense study and development effort, much was learned about numerous topics of interest to members of this Society. Notable were findings related to the determination of natural roll period, reduced waterline beam (notch) behavior, bilge keels, free-flood tanks (with and without tuning), spray rails and the assessment of topside wetness by model testing. Also, an extensive NAVSEA/NAVAR (Naval Air Systems Command) effort resulted in the updating of aircraft carrier (CV) limiting 4 A sally is a simple experiment, normally performed with the ship at rest in calm water, in which a ship is made to roll and the natural roll period is measured, usually in order to check the transverse metacentric height. The ship is generally excited in roll by moving weights back and forth across the deck in a periodic fashion synchronized with the ship's period of roll. Historically, ships were sallied by having the crew rush from side to side across the weather deck. Hence the term. The USS Midway Blister Story 431

4 deck motion criteria which are now much more stringent than the criteria existing in Ship motion improvement n setting out to analyze and correct the Midway's motion problem, the first order of business was to establish a firm baseline, that is, to establish the pertinent characteristics of the ship as she existed. The roll period mystery and other allegations heard during our December 1986 visit to the ship--for example, "the ship now has excessive trim by stern," and "the ship when undocked lifted off the keelblocks forward at 2.5 ft less water depth than had been predicted "--had created uncertainty regarding the ship's true hull shape, displacement, and vertical center of gravity position (KG). (ncidentally, both of the above allegations later proved to be unfounded.) Concerning hull form, offsets were carefully measured on the model used for all tests of the 1986 blister and also measured at several locations on the ship itself. We compared these measurements with the corresponding values from the contract design lines for the ship with blister as well as from the shipbuilder's mold loft offsets. The ship measurements agreed very well with both the mold loft offsets and the contract design offsets and were well within the GENSPEC requirements for allowable deviations. The model exhibited greater differences in some areas, for example, at the design waterline (34.5 ft full scale) halfbreadth deviations between +0.2 in. (6 in. full scale) and in. (10.5 in. full scale), but calculations showed that these differences would have a negligible effect on hydrodynamic performance: resistance, motions, etc. Midway had been inclined in the dry dock at SRF, Yokosuka shortly after the blister installation. The inclining ultimately provided a firm estimate of the ship's transverse metacentric height (GMT) and KG, but several months were required to do the necessary detailed analysis after the inclining itself. Preliminary "quick look" data were provided in late December. n accordance with standard practice, the ship had been sallied during the experiment to get a quick check on GAT and hence KG based on the observed roll period. The roll period measured on that occasion had been 13.1 sec, which agreed well with the estimate made during the blister design phase. Confusion reigned after roll periods' on the order of 10 to 11 sec were observed at sea in mid-december. These observations were confirmed by spectral analysis of roll motion records taken during the mid-december ship visit. This analysis indicated a natural roll period of about 11.6 see. t was essential to get a firm handle on the ship's roll period and the factors which influence it, including the transverse radius of gyration of the ship's mass. f A = ship displacement 8A = added displacement for roll due to entrained water A' = virtual displacement of ship for roll =A+~A Kxx = radius of gyration of ship mass for roll about a longitudinal axis through the center of gravity (CG) xx = mass moment of inertia of ship mass for roll about same axis = (A/g). Kxx 2 81xx = added-mass moment of inertia for roll due to entrained water = (SA/g) Kxx 2 xx' = virtual mass moment of inertia for roll = xx + ~xx then a ship's natural roll period can be expressed as ]',(seconds) = 2rr since lxx' can be expressed as then or A xx' : G-MT ~ / Kxx 2 or -- Kxx 2 g f; 2,n'Kxx T,(seconds) = a[~ X/g : G--MT V~ :r,(seconds) /--- _ 2rrKxx 41 8A t was postulated that the restrictions imposed by the dry dock had affected the ship's roll period and we decided to resally the ship in deep open water. The only guidance we could find on the necessary clearances for an accurate assessment of natural roll period was reference [ 6 ], which implied that at least one ship length lateral clearance and a water depth of two to three times the ship draft should be provided. The ship was resallied in Tokyo Bay in early January. The results yielded a roll period of 11.6 sec based on the ship motion recorder and 11.9 sec based on the ship's inertial navigation system gyro. The latter figure was adopted and, when corrections to the full load condition were made, resulted in a full load natural roll period of 12.2 sec2 Detective work ultimately revealed two reasons for the original error in the estimated roll period for the postblister ship. First, the effects of entrained water were accounted for twice. When predicting the ship motions using the Ship Motions Program (SMP), the inertial characteristics of the ship in air are input and SMP predicts the entrained water effects and adds them to the input value for xx in air. For the SMP predictions of Midway motions made during the 1986 blister design, the input lxx value was based on a previous sally of CV 41 and thus already included entrained water effects. The SMP program then dutifully included them again. Second, the effect of the blister on the inertial characteristics of CV 41 in air was greatly exaggerated. n fact, the blister internals are mostly 5 A model sally was performed in the DTRC MASK facility to gain knowledge on the effect of restricted water on natural roll period. The model was ballasted to have a roll period of 11.9 sec (ship scale) in the center of the facility, essentially unrestricted water. The model was then resallied in the fitting-out dock with and without the dock gate in place. The dock's length was 1.90 times the model WL length, its width was 1.15 times the model WL beam, and its water depth was 2.15 times the model draft. The roll period in dock was 12.3 sec, an increase of 0.4 sec. The roll period was not affected by the clock gate being closed or open. Reference [ 7 ] documents this experiment. 432 The USS Midway Blister Story

5 air and the blisters have a small effect on xx. However, the estimate of their effect made during design was arrived at by multiplying the Kxx in air of the pre-blister ship (from an earlier sally) by the ratio of the ship's beam, that is, by maintaining Kxx at the same percentage of the ship's beam. This assumed that the transverse distribution of mass of the pre- and post-blister ships was similar which was not true. The Midway motions problem resulted from the increased stiffness of the ship due to the blister addition. Transverse metacentric height (GM-r) was increased from 9.5 to 25.3 ft and the ship's natural roll period decreased from 18.6 sec to about 12.2 sec fully loaded. The decreased roll period increased lateral accelerations topside and, even worse, brought the ship into synchronism with wave periods frequently encountered in the Western Pacific Ocean. Roll angles in synchronous conditions are greatly increased, thus increasing the lateral component (in the plane of the deck) of the acceleration due to gravity, which is the dominant factor in the lateral accelerations perceived by objects on deck. t is important to note that, although the predictions of roll period were in error by 1 to 2 sec, the ship motions would have been frequently unsatisfactory even if the 13.5-see roll period had been achieved. This point is fully discussed later in the paper. The Midway Motions mprovement Project was strongly influenced by two considerations. First, any solution had to be effective at low ship speeds since aircraft carriers often launch and recover aircraft at 10w ship speeds due to the limitation on maximum wind over deck: about 35 knots. Thus, in the presence of a 30-knot true wind, for example, ship speed will be limited to about 5 knots during aircraft launch and recovery. Second, any solution had to be low risk; that is, it had to work when put to sea on Midway in 1987; developmental approaches were out. These considerations effectively eliminated active stabilization options which need substantial water velocity over control surfaces, such as rudder roll stabilization or active fin stabilization. Six options which met the above criteria were identified for study. They are depicted in Fig. 2. Five of the options represented attempts to increase the natural roll period by reducing GMT, recognizing that significant increases in Kxx as a way to increase roll period are impractical. The sixth option was to increase bilge keel span or add additional bilge keels to the ship. This would primarily affect roll damping and have only a small effect on NOTCH HULL CROSS-CONNECT TANKS WAB BUt/ LARGER BLGE KEELS (ADD DAMPNG) ADD HGH WEGHT Fig. 2 FLOOD BLSTER VODS "FM SLO-ROL CONCEPT Motion improvement options ~1~ RPE roll period through the increase in entrained water. Based on brief analyses, three of the six options were dropped from the study. Additional bilge keels or a bilge keel span increase, to the maximum feasible based on constraints imposed by construction in the SRF dry dock, were simply not effective enough for the primary motion solution. ntroducing large amounts of free surface by cross connecting outboard voids within the new blister was determined to be infeasible because of the very large cross-connect ducts which would be required and the intolerable impacts they would have on the already crowded internal spaces of the ship. Adding weight topside to reduce GMv by raising KG was also ineffective. For example, 1500 LT of topside weight addition would reduce GMr by 1.0 ft and increase the natural roll period by only 0.3 sec, whereas a GMT reduction of about 10 ft would be required to increase the natural roll period to 15 sec. Fifteen seconds as a minimum acceptable roll period had been stipulated by SEA 05 as the result of preliminary ship motions study. This left three options for further study: 1. "Notching" the hull; that is, reducing ship beam at the waterline by cutting away part of the new blister. 2. Free-flood tanks, that is, opening a portion of the new blister to the sea with large enough openings to the air and water so that the tank contents would act as true free surface. 3. SLO-ROL TM tanks, a patented concept for lowering the free surface of a free-flood tank below the external waterline by cross-connecting opposite tanks by an air pipe and pressurizing the tanks. Attention was focused on the first two of these options since it was recognized that SLO-ROL TM tanks were really a variant of free flood tanks and were considered to have somewhat more development risk. Three notch alternatives 4, 7 and 10 ft deep (measured horizontally) were sketched and briefly assessed. Note that 10 ft is the full width of the 1986 blister. n section the notch consisted of three straight-line segments, the upper and lower portions sloping and the middle portion vertical. The height of the vertical portion was initially set at 13.5 ft based on accommodating a single-amplitude roll angle of 5 deg within the full height of the wall-sided portion of the 10-ft-deep notch for a likely range of operating drafts and then decreasing the height slightly so that the upper, inner corner of the notch was located at the third deck level for structural reasons. Later the lower, inner corner of the notch was raised to the fourth deck level, again for structural reasons, so that the notch was no longer symmetrical in section with respect to the 34 ft-3 in. WL. Figure 3 depicts the notch configuration and Fig. 4 shows the effect of notch depth on GMr and roll period. The principal conclusions of the brief notch assessment were: 1. The deeper the notch, the better from the standpoint of GM-r reduction and hence motion improvement. Draft increase and drag effects would be acceptable for any feasible notch depth. 2. t could not be determined if a notch 10 ft deep was feasible in the time available. Detailed structural analyses using finite element techniques would be required to establish structural feasibility and equally lengthy studies would be needed to establish that damage stability would be satisfactory. n addition, cutting back to the previous shell in the mid-portion of the notch would require some very difficult and costly structural details to ensure continuity. (NOTE: Later the 10-ft-deep notch option was revisited as a result of the project schedule extension and The USS Midway Blister Story 433

6 MAXMUM FEASBLE NOTCH MAN DECK UPPER NOTCH SLOPE 2NO DECK 49'-9" ADL 3RD DF.CK -41'..6" Am. 4TH DECK / ~ W.L 32'-8" AOL! - 22'-8" ABL PRE-BUSTER ~ BUST[K SHELL REDUCE TWST N SECTONS AFT 48'-r' KHC(LE 48'-9" KNUCKLE UPPER NNER NOTCH R.~DUS 41'"6" (UUCKL[ 6UOYACY V~J 41'4". liuckl,[ 3 Dg OK., ~ " / J "'~" ~ W ~ / / 6'(ALTERS ~ N 41' ABL / ~ l S ~ ) ) / ~ 32'-6" KNUCgl " 32'-6" KC~ 36'WL EXSTNG REDUCED TWST Fig. 3 Notch configuration Rg. 4 GM- (FEET] 15j TROLL ~" x (SEES} ~_ ' f 0 T ~L UST QT$ 0t 11[ tll~ tell UT tl TRill.,C t t t f i i O 10 NOTCH DEPTH (FEET( autl e Effect of notch depth on GMT and roll period detailed studies showed that damage stability was unsatisfactory. ) 3. Below 10 ft, the maximum feasible notch depth was 7 ft-6 in. This resulted from the minimum acceptable 30- in. lateral clearance between the previous shell and the vertical portion of the notch to permit welder access. 4. A 7 ft-6 in.-deep notch would increase the ship's natural roll period to about 15.5 sec and significantly reduce the ship's roll amplitude in commonly occurring sea states, for example, 5 or 6. An example of the dramatic effect of the notch on predicted ship roll motion is shown in Fig A 7 ft-6 in.-deep notch was judged to be feasible from the structural and damage stability standpoints although additional capacity to counterflood would be required. Resistance increase was estimated to be small (up to 4 percent across the speed range). Draft increase was about 8 in. The principal concern was the potential for slamming of the upper notch surface and possibly aggravated wetness resulting from slam-generated spray. Other recognized difficulties introduced by a notch related to tug handling, the design of the camels required to hold the ship the necessary distance off a pier when alongside, and the method used to secure the lower ends of accommodation ladders to the hull near the waterline. 434 The USS Midway Blister Story

7 1986 BLSTER 5 ROLL (DEGREES) (SGNFCANT SNGLE AMPLTUDE) FOLLOWNG SEAS SEAM SEAS HEAD SEAS SHP AT FULL LOAD SHP SPEED = 10 KNOTS SEA STATE 6 SHORT CRESTED WAVE HEGHT = 16.4 FEET WAVE MODAL PEROD = 13 SECONDS HEADNG (DEGREES) NATURAL ROLL PEROD PRE-BLSTER 18 6 SEC CURRENT 12.2 MAXMUM NOTCH 15.5 Fig. 5 Roll motion comparisons Free-flooding pairs of voids in the newly created Midway blister were also studied. The U.S. Navy had installed free-flood tanks in several ton cruisers in the early 1930's as described in references [8] and [9]. The tanks had been effective in improving the ships' roll behavior although problems were experienced related to noise, foul odors and maintenance. Earlier, free-flood tanks had also been fitted to several German ships, as described in references [10] and [11]. The watertight transverse subdivisions in the Midway blister are spaced 16 ft apart longitudinally. Flooding 8, 16 and 20 pairs of voids was assessed. The possibility of tuning the tanks to increase their effectiveness was recognized Attention focused on the 20-pair option for quantitative assessments. Rough estimates were made of the minimum air and water hole sizes needed to achieve true free flood effectiveness. Estimates were also made of the drag of the free-flood tanks in calm water and with the ship rolling -+ 5 deg in calm water These estimates were made using a simplified technique for estimating momentum drag. The principal results of the brief free-flood tank assessment were: 1. The technique is an effective way to improve ship roll behavior. Flooding 20 pairs of tanks would increase the ship's apparent roll period to about 16.5 sec and significantly reduce the ship's roll amplitude in sea states 5 and The powering penalty is unacceptable. At 27 knots, the estimated shaft horsepower (SHP) increases were 22 percent in calm water and 63 percent with a - 5 deg roll. At full power, predicted speed loss was 1.8 knots in calm water and 4.6 knots with ---5-deg roll. Tuned tanks would have somewhat smaller penalties. 3. The tanks are feasible from the structural and damage stability standpoints although additional capacity to counterflood would be required. 4. Other concerns with the tanks related to noise (deemed to be treatable), increased maintenance, odor, technical risk, and the observation that operators generally tend to lose their affections for passive antiroll tanks over time. The performance benefits and powering penalties of SLO-ROL TM tanks would be similar to those of a like number of free-flood tanks. Based on these results, primarily the very large powering penalty associated with large numbers of free flood tanks, the 7 ft-6 in.-deep notch was selected as the primary solution to the Midway's motion problem. Due to the lead times required to build and test ship models, analytical techniques were used in the initial assessment of motion improvement options. Ship motions were predicted using the Navy's SMP computer program with care taken to correctly input KG and in-air mass moment of inertia values derived from the early January deepwater sally experiment Seakeeping operability was assessed for four typical Midway operating areas at the worst season of the year. This methodology essentially compares predicted motions with assumed limiting motion criteria over a range of speeds, headings and sea states and then weights the results by expected probabilities of occurrence. The ultimate result, a seakeeping operability percentage, can be thought of as a "percent time that ops can be safely conducted" although it is not a percent time strictly speaking. Prior to making the seakeeping operability assessments, the established limiting motion criteria were hastily updated as described in Appendix 1. Events had already shown that the existing specification which required a limiting roll angle of 5 deg significant single amplitude was too liberal and that the roll angle limit should have been sensitive to natural roll period. Lateral acceleration is the true limiting factor for aircraft handling on deck, not roll angle, even though the dominant lateral acceleration term is the gravity component in the plane of the deck resulting from roll angle. Seakeeping operability was assessed for two scenarios: handling aircraft on deck and aircraft launch/recovery. Each scenario had unique limiting motion criteria and speed/heading restrictions. The assessments were made for Midway pre- and post-1986 blister, Midway with the proposed 7 ft-6 in. notch and CV 63, typical of the "big deck" carriers. Sample results from the late January 1987 The USS Midway Blister Story 435

8 ARCRAFT HANDLNG LAUNCH & RECOVERY 100" 9O t:go ~o PRE-BLSTER CURRENT NOTCH CV 63 CV 41 CV 41 CV 41 a-k~ 10 0 i J ):1 i PRE-BUSTER CURRENT NOTCH CV 63 CV 41 CV 41 CV 41 Fig. 6 Seakeeping operability assessment (preliminary) (northern Arabian Sea--June-August) time frame are shown in Fig. 6 for one operating area. These assessments were updated later in the study when the limiting motion criteria were updated; see Appendix 1. The results show that roll has a major effect in the aircraft handling scenario, where all headings are considered, and little or no effect in the launch/recovery scenario, where headings are restricted to near-head sea cases and vertical plane motions dominate. t should be noted, however, that these assessments assumed a theoretical short-crested seaway with a cosine-squared energy spread and did not account for frequent real-world situations where a low-frequency ground swell is present in combination with a local wind-generated sea, the two coming from very different directions. n such situations, roll could also have a significant effect in the launch / recovery scenario although few reports of problems from the field have been received which involve landings or takeoffs. A special study was performed to evaluate the effects of real world seaways on Midway motions, both the preand post-1986 blister configurations [12]. Motion predictions were made using a representative sample of hindcast directional wave spectra for the period at two grid points in the North Western Pacific Ocean. The significant wave heights studied ranged from 3.3 to 16.4 ft. Predictions were made in head and both port and starboard beam seas relative to the primary wave direction in each spectrum. n brief, the results showed that the pre-1986 blister Midway was less sensitive to the multidirectional, asymmetric spectra contained in the sample. Typically, the roll angles predicted for the two hull configurations differed by a factor of 4 in beam seas and by a factor of 3 in head seas. Notably, the roll angles predicted for the post-1986 blister ship in head seas were substantial. For example, significant single-amplitude roll angles in the range of 1.4 to 3.3 deg were predicted in head seas for spectra in the sample with significant wave heights of 12 to 15 ft. These figures indicate the importance of roll motion in the launch / recovery scenario, when the ship is headed into the wind and has virtually no flexibility in heading selection, as well as in the aircraft handling scenario. Unfortunately, the computer program used to assess seakeeping operability could not handle multidirectional, asymmetric wave spectra at the time the Midway assessments were made. Subsequent to basic notch definition and selection, potential refinements of the notch shape were evaluated during the second, wetness improvement, phase of the project. These refinements concerned upper notch slope or deadrise, radii of the notch knuckles, and the notch "twist" aft. These variables are depicted in Fig. 3, which also shows the basic notch configuration. Extensive and, in some cases, unique model tests were used to evaluate several of the proposed notch refinements. Ship impacts were also assessed. Results and conclusions are discussed later in the paper. Once the 7 ft-6 in.-deep notch had been selected, the motion improvement effort turned toward the study of possible supplements to the notch which would further improve roll behavior and enable the ship to more closely approach her pre-blister seakeeping operability. Attention focused on: (1) larger bilge keels and (2) a small number of free-flood tanks. An additional parallel set of bilge keels with the same span (5 ft-0 in.) as the baseline ship was briefly considered but quickly rejected when the roll decay experiments briefly mentioned in Appendix 2 showed them to be ineffective. For the larger bilge keels, analytical performance predictions and ship impact studies were made. For the supplementary tanks, model tests were performed at two facilities and ship impacts were assessed. The results of these studies and the decisions based upon them are discussed later in the paper. Before leaving the subject of motions, it should be mentioned that during the motion improvement phase of the overall project, considerable effort was also expended to validate the SMP program, that is, to demonstrate that SMP could be relied upon to produce realistic predictions of Midway motions in waves. These efforts, which are summarized in Appendix 2, were not completely successful. Ship wetness improvement The second phase of the project focused on the flight deck wetness problem which manifested itself in the form of" geysers" which rose above the flight deck in the region of the forward quarter point of the hull and were then carried across the deck by the relative wind. These geysers often occurred in mild seas and rose on either side of the ship, depending on conditions at the time. The video tape received from the ship in late January showed these geysers but the source wasn't clear. Nor was it clear whether the geysers rose to the flight deck on the hull surface or away from it. Continuing efforts were made to get clearer videos which would reveal the source of the geysers and their route to the flight deck. This did not occur until several months had passed and even then the results were not definitive. Meanwhile, the problem had to be attacked and with alacrity! Based on the initial videos from the ship, two things were clear. First, the scupper extensions and their 436 The USS Midway Blister Story

9 boat guards mounted on the hull near the waterline were generating a great deal of spray. These extensions, shown in Fig. 7, had been installed to direct often unpleasant overboard dischar.ges down to the waterline to prevent their being caught by the wind and carried back aboard or onto the crews of small boats alongside. Second, there was a large secondary wave generated by the hull shoulder at ship speeds over about 16 knots. This wave originated near the forward end of the blister and was breaking almost constantly, even in calm water. Also, on the starboard hull side, the shoulder wave sat right under the low sponson forward of the aircraft elevator, decreasing the freeboard of this long-known wetness source. The assumption was made that both the scupper extensions and the breaking shoulder wave contributed to the wetness problem. A three-pronged approach was taken in seeking a solution. First, the decision was made to remove the scupper extensions and their boat guards from the ship's forebody at the earliest opportunity. This was done in early April. Forward of midship, there were seven scupper extensions on the port side and 10 on the starboard side. Those forward of frame 85 (just aft of the No. 1 aircraft elevator) were removed along with all boat guards fore and aft. Second, studies were made of hull form modifications and bulb options intended to reduce the breaking shoulder wave. Third, spray rafts and other appendages designed to catch and deflect any remaining spray that was generated were studied. The 1986 blister had introduced a discontinuity, or unfairness, as well as a hard shoulder into the ship's sectional area curve, as can be seen in Fig. 8. Note that Hull O reflects the ship pre-1986 blister, Hull X reflects the ship post-1986 blister, and Hull A is Hull X with the notch selected as the primary motions solution. These hull designations are used henceforth in the paper. Two modified hull forms were developed to ease this discontinuity and reduce the hard shoulder. t was hoped that by doing this the breaking shoulder wave would be reduced or eliminated. Hull B represented the "clean sheet of paper" approach. The entire ship forebody was fair game. The decision was made to fair the Hull B lines without a bow bulb and then to design and test several modern bulbs on Hull B. Hull C was restricted to forebody hull form changes only in way of the 1986 blister. Further, modifications which cut into the pre-1986 blister hull form (Hull O) were not permitted. Thus Hull C had the same Taylor bulb that the current ship, Hull X, has. Figures 9 and 10 show these two forebodies in comparison with Hull A. Note SHELL -~, F " _L / i,,.~'" ~,~ scumn F.XTE.=Oa o u"-~ SHELL ~** 12"--'~ Fig. 7 6UARO 36,.O,,,WL NOTE: DMENSONS ARE FOR LARGEST SCUPPER EXTENSON/BOAT GUARD NSTALLED. Scupper extensions and boat guards E mo zooo. ~ oo i Fig HULL C vs HULL A /H!LL X,HUK A "" ~ /i "utt \ \ \ HULL B vs HULL A Hua X TATON \ ~, HU,LL B Sectional area curves (to 36-ft WL) that all the hull forms reflect the same notch 6 and have the same afterbody. All six hull forms addressed in this paper are listed in Table 2 along with their designations. Twenty-foot-long models of each of the three hulls were constructed and tested extensively in calm water and in waves at DTRC, Carderock. The Hull A model was built with a removable plug in way of the notch so that it could represent Hull X as well. Each model also had a removable section at the forefoot so that it could be tested with alternative bow bulbs. Note that a model of Hull O was not constructed; time and resources did not permit it. From a research point of view, this was unfortunate. 5 After Hulls B and C had been designed, a panel of retired senior naval architects with extensive surface warship design experience was convened to review the" entire Midway motions/ wetness correction effort. The panel recommended another hull form, designated Hull D. This form reflected a smoothly curved version of the notch in section and had no knuckles. Due to its section shape, it was dubbed the "Grecian Urn" by the design team. Hull D was studied on paper but there was neither time nor resources to build a model and test the form. nitial damage stability analyses showed that Hull D was not satisfactory in that it exceeded the criterion for initial heel after damage. Further work indicated that Hull D could have been made to meet the damage stability criterion if model tests showed the notch to be a poor performer; this didn't happen. \ \ o The USS Midway Blister Story 437

10 MAiN OECK HULL B - - HULL A (W/O BULB) HULL O Table 2 Hull designations MDWAY WTH 1957 BLSTER, i.e. PROR TO 1986 BLSTER HULL X HULL O WTH 1986 BLSTER ADDED HULL A HULL X WTH PROPOSED NOTCH MODFCATON HULL B HULL A WTH ENTRE FOREBODY BELOW DWL REFARED, NO BOW BULB STATDN O EXTENDED HULL C HULL A WTH FOREBODY REFARNG RESTRCTED TO 1986 BLSTER REGON EXTERNAL TO HULL O FORM l //1~/~ BASEUN[ Fig. 9 Hull B versus Hull A forebody HULL D VARANT OF HULL A WTH SMOOTHLY CURVED SECTONS N WAY OF NOTCH; THE "GRECAN URN" i MAN DECK,,,,,'. D/ -! ly Fig. 10 Hull C versus Hull A forebody HULL C - - HULL A ij /i / 3S' OWt BASEUNE As soon as each hull form was developed, it was analyzed using a 3-dimensional, potential-flow, free-surface computer program developed at DTRC termed "SWFT." This was done to ensure that none of the hulls displayed undesirable flow characteristics. Particular attention was paid to three aspects: free surface profile along the hull, free surface profile perpendicular to the hull in way of the shoulder wave, and flow along the hull surface in way of the lower, outer notch knuckle. These assessments were reassuring and revealed the power of this analytical approach. Figure 11 is an example of the SWFT output: predicted wave profile along the hull for Hulls O, X, A, B and C at 24 knots. The analytical results compared well with the later model test results, especially on the hull surface aft of the immediate bow, that is, aft of about station 2. Figure 12 is a comparison of the model test and SWFT wave pro des for Hulls A and C at 24 knots. While models were being built and tested, ship impact assessments and considerable design development was car u u. 44 i z ug z 42,..,.g 40 w > 38 0 m < 36 ~- 34 w -r 32 FEET FROM FORWARD PERPENDCULAR Computation By D]NSROC Code /3/87 ~.~.~. B HAS NO BULB; ALL OTHERS HAVE TAYLOR BULB 1 ~- ~.., % Ks Model 0 Model X... Model A... Model 8... Model C 45~ SHP STATON Fig. 11 Wave profiles on hull predicted by SWFT; ship speed = 24 knots 438 The USS Midway Blister Story

11 u_ 0.c: 46 -~ 44 Feet from Forward Perpendicular Model A (Expenment) [] Model C (Experiment) - - Model A (SWFT) Model C (SWFT) 2 42 / ~ 40 o ~ 3a o Fig Ship Stations Comparison of wave profiles on hull predicted by SWFT versus model experiment; ship speed = 24 knots ried out on each of the alternative hull forms. n order to meet the 31 July 1987 date established for completion of a contract design level modification package, initial contract design (CD) development for each hull was necessary in advance of the hull form selection date. Most of this work was in the structural area; other areas developed included weights, general arrangements and stability assessments. Contemporary bulbous bows were designed for each of the three hull form alternatives. For Hull A, two bulbs were designed. One was a hydrodynamically optimum bulb with a nabla section shape designated Option. This bulb, developed using the design approach of Kracht [ 13 ], had a cross-sectional area of 9 percent of the immersed midship section area; see Fig. 13. The width of this bulb would cause severe anchor handling problems A second, narrower bulb was designed with an elliptical section which had a cross-sectional area of 6 percent of the midship area. This bulb, designated Option and also shown in Fig. 13, would reduce but not eliminate anchor handling problems. Without hawse bolster extensions, the anchor flukes could strike the bulb when raising and lowering but EXSTNG HULL WTH TAYLOR BULB / OL i DlL --"-"=B OPTON BOW BULB PROFLE DWL PROFLE 0.0' ll -?.T - - D&~U[ i J ~u,t CETEllU SECTON AT STATON 1/2 SECTON,~ -.4-~- 6.5' PROFLE ~DWL OPTON V BOW BULB OPTON BOW BULB f f SECTON i Ox~Nt L~W.UN Fig. 13 Bow bulb options PROFLE SECTON AT STA 1/4 The USS Midway Blister Story 439

12 not the crown. The Option and bulbs were also tested on Hull C. The forward portions of Hull C being nearly identical to Hull A, the Kracht design approach would not yield significantly different bulbs for these two Hulls. For Hull B, a hydrodynamically optimum nabla-shaped bulb with a cross-sectional area of 6 percent of the midship area, designated Option V, was designed. n addition, the decision was made to test the hull with no bulb and with the Option bulb. The after portions of the Option bulb had to be carried further aft to fair into the Hull B lines. The Hull B Option V bulb is also shown in Fig. 13. Several other promising bulbs were sketched for Hull B and evaluated using the SWFT potential flow program but not in time to be considered for model construction and testing. At the outset of Phase, it was hoped that longitudinal wave cut experiments could be used to optimize the bulb volume and bulb longitudinal position for the several hull forms. t was soon learned that the project schedule would not permit this. However, such experiments were performed on Hull A with the Taylor and Option bow bulbs over a range of speeds. The results showed that a 20 percent bulb volume decrease and a longitudinal position change of 2 percent of the ship length forward would yield the most favorable resistance reductions, especially at the higher speeds examined. These results became available too late to influence the ongoing Hull A model test program. Also, the indicated forward shift of bulb position would certainly have undesirable impacts on ship structure and anchor handling. Shoulder bulbs were also defined and tested on the several hulls. These are faired shapes appended to the hull sides in the shoulder region. t was felt that even large bow bulbs might not affect the ship's shoulder wave to a sufficient degree due to the extreme length of the hull. n that case a surface wave generator closer to the shoulder wave might be needed to provide the desired cancelling effect. Spherical shoulder bulbs with circular arc maximum sections of 6, 8, 10 and 12-ft radii were defined as well as two equal volume variants of the smallest (6 ft radius) shoulder bulb, one with a nabla section and the other with a rather strange "reentrant" shape. The spherical and nabla bulbs are shown in Fig. 14. Models of the various shoulder bulbs were built and tested, first on Hull A with the Taylor bow bulb. The test approach and the manner in which the test results were analyzed are described later. Suffice it to say here that the bulb tests for each hull were conducted in calm water. Resistance and surface wave patterns were measured and evaluated. Based on these results a decision was made as to what bulb(s) to use on each model for the subsequent seakeeping tests. Time did not permit seakeeping tests with more than one bulb suite per hull. Ship impact assessments were made for a typical bow and shoulder bulb as were rough order-of-magnitude (ROM) cost estimates. This was done in parallel with the model testing. Extensive studies of the anchor handling problem were also performed. After considerable discussion, the Office of the Chief of Naval Operations (OPNAV) agreed that, if necessary, the Midway anchors could be raised and lowered by dragging them over a large bow bulb. nterestingly, the major concern of the operators was the risk of anchor failure rather than bulb failure in the case of an emergency drop onto the large bulb. Studies of the hawse bolster extension needed to clear the elliptical bow bulb were performed and an alternate bolster location 440 The USS Midway Blister Story PROFLE SPHERCAL SHOULDER BULB SECTON ~ Fig, 14 4 $112! 112 M NABLA SHOULDER BULB./_ 1 /l 211! ~ ll. SECTON Shoulder bulb options PROFLE on the ship's stem to clear the front of the bulb was also studied. t was recognized that the efforts to solve the ship's wetness problem by preventing spray generation might not be fully successful. Therefore, ways of trapping or deflecting the spray to prevent its reaching the flight deck were also studied. Three approaches were considered: extending the flight deck outboard in way of the wetness (just forward of the existing flight deck sponsons port and starboard), raising and increasing the size of the existing missile sponson at the hangar deck level just forward of the starboard elevator and adding a similar sponson on the port side, and spray rails. Of these, spray rails proved to be by far the most practical and cost-effective alternative. Flight deck extensions were severely limited in size: on the port side by a 15-deg down angle clearance requirement related to aircraft which miss the arresting wires on recovery and "bolter"--accelerate and take off from the angle deck, and on the starboard side by the arc of fire requirements of the missile launcher located on the sponson at the hangar deck level below. Adding large sponsons below the flight deck level would be very expensive, have numerous undesirable ship impacts and could very well aggravate the ship's wetness problem (when rough seas impacted the sponsons) rather than alleviate it. Having settled upon spray rails as the preferred wetness deflector, the spray rail trace on the forebody was established by an initial guess, followed by confirmatory model tests--piggybacking on the extensive ongoing seakeeping model experiments to be described later. Visual observations during the tests showed the spray rails to be effective in deflecting sheets of water and spray riding up the hull surface in a seaway. Advice from experts was sought regarding the most desirable width and section shape for the spray rails. n just a few weeks, references [ 14] and [ 15] were created. Both contain valuable insights and arrive at remarkably similar

13 1 )- SPRAY RAL LOCATONS STARBOARD SPRAY RAL CROSS SECTON 1!,.,...,...,...,...,...,..--,--..,-..-,.-.-,----,---.,-.--,----, PORT Fig. 15 Spray rail configuration conclusions from very different points of view. Reference [14] is a theoretical analysis based on a potential flow model. Reference [ 15] draws on extensive model test experience with high-speed planing and semi-displacement hulls. The recommendations of these references, merged and then tempered by structural and producibflity considerations, resulted in the spray rail configuration shown in Fig. 15. The spray rail was not designed to withstand the theoretical maximum impact force it might experience if impact occurred over a substantial length simultaneously. Rather, the structure was designed so that failure, ff it did occur, would not damage the adjacent ship shell and backing structure. Model test programs Twenty-foot-long seakeeping models were tested extensively at DTRC in the spring of 1987 to validate the notch motions fix and to assess the alternative bulb and hull form wetness fixes. The models were designed with realistic topsides to gain maximum value from the seakeeping experiments. They were built up to the flight deck and all sponsons, deck edge elevator recesses, catwalks etc. were simulated. Due to physical constraints in the DTRC MASK facility, data cannot be collected in seas' on the starboard bow from dead ahead to 45 deg Off the bow and on the port bow from 45 deg off the bow aft to the beam. Thus the models were built so that.all asymmetrical features forward of midship, for example, the sponsons and No. 1 starboard deck edge elevator, could be installed on either side of the model. This enabled data to be collected at "any" heading but complicated testing and data analysis. These sophisticated models were well built in an extremely short time, two by DTRC and one by Chicago Bridge and ron Co. nitially, calm-water tests were performed with each model to select a single bulb for each hull's seakeeping test series. This was done by testing the various bulb suites on each hull and recording and analyzing the forebody surface waves generated, especially the shoulder wave. The effectiveness of each bulb suite in reducing the shoulder wave and also ship resistance was evaluated. The forebody shoulder wave was recorded by two means. The wave trace along the hull was recorded visually with the aid of a grid painted on the hull side which defined each station as well as waterlines spaced vertically at 5-ft intervals (full scale). Tick marks spaced 1 ft apart (full scale) were also painted on the station lines. On each test run the wave profile was read by carriage riders and also recorded on videotape. The wave profile could later be consistently read from the videotape by different observers. The shoulder wave off the hull was measured by five vertical capacitance wire wave probes fixed in the tank and penetrating the free surface. These probes were equally spaced horizontally and were located in a vertical plane perpendicular to the model's path of travel. As the model was towed past the probes, each probe recorded the wave profile in a plane parallel to the ship's longitudinal centerplane. Five points on the wave profile in a transverse plane corresponding to each ship station could be determined by reading the data for each wave probe at the proper instant of time. The drawback with this approach was that there were not enough probes and the innermost probe was not close enough to the hull to fully define the transverse wave profile. The inner probe had to be offset far enough to clear the model as it passed by. For each hull / bulb combination tested, the wave trace on the hull was recorded at three ship speeds: 18, 24, and 30 knots. Eighteen knots was about the lowest speed at which the shoulder wave became significant and thirty knots was a representative high speed. The wave probe data could be collected and analyzed at only one speed due to time and resource limitations; 24 knots was chosen. Model resistance was measured at all three speeds. A simple numerical scoring scheme was developed to facilitate and standardize the bulb selection process. The methodology is outlined in Table 3. Wave heights along the hull at station 4, 5 and 6 and also the highest and lowest elevations between stations 2 and 8, all at three speeds, and the highest and lowest wave elevations measured at the five off-hull wave probes at 24 knots at station 5 were utilized. These elevations and two maximum slopes of the shoulder wave front--( 1 ) in profile between stations 2 and 8 on the hull, and (2) transversely between wave The USS Midway Blister Story 441

14 Table 3 Bulb selection methodology MEASURED REFERENCE ATTRBUTE UNTS VALUE SCORNG WAVE PATTERN ALONG HULL (AT SPEF~DS OF 18.24, 30 KNOTS) WAVE ELEVATON AT STATONS 4, 5, 6 HGHEST WAVE ELEVATON BETWEEN STATONS 2 AND 8 (AT STA ) LOWEST WAVE ELEVATON BETWEEN STATONS 2 AND 8 (AT STA ) FEET FEET FEET MAXMUM SLOPE OF WAVE PROFLE BETWEEN NVERSE STATONS 2 AND 8 MAXMUM WAVE SLOPE (l/h) TRANSVERSE (AT 24 KNOTS ONLY; STATONS 5 AND 6) HGHEST WAVE ELEVATON AT PROBES FEET LOWEST WAVE ELEVATON AT PROBES FEET MAXMUM SLOPE OF WAVE PROFLE BETWEEN NVERSE WAVE PROBES MAXMUM WAVE SLOPE (l/h),: BELOW 50' WL BELOW,50' WL BELOW 50' WL 7/1 NVERSE SLOPE BELOW 50' WL BELOW 50' WL 7/1 NVERSE SLOPE ELEVATONS: >20FT. = 10 <OFT. = 0 intermediate elevations scored proportionally SLOPES: >35/1 = 10 <7/1 = 0 intermediate slopes scored proport=onally AS ABOVE probes off the hull--as indicators of shoulder wave steepness, were used to indicate a tendency toward wetness. Higher, steeper waves were deemed worse as they were more likely to break at full scale. Attention was directed to the vicinity of the forward quarter point (station 5) for two reasons: first, it was the region where flight deck wetness was most prevalent, and second, it was the region just aft of where the shoulder wave diverged from the hull at the higher ship speeds. Thus a transverse wave cut at this station would intersect the shoulder wave. Ship resistance was factored into the scoring scheme but wave heights and slopes were given much more weight. Using this approach, a numerical score was developed for each bulb suite tested on each hull. This score, the videotapes of the wave patterns and other considerations such as anchor handling difficulty and technical risk were used to select a single bulb suite for each hull to be used in the seakeeping model tests. Results are presented in the next section of the paper. The calm-water bulb tests for each model were performed with the existing ship's 5 ft-0 in. bilge keels. Once the tests for each hull were completed and the bulb to be used in the seakeeping tests selected, the bilge keels were removed and the optimum bilge keel trace established using oil dots. For Hull A, the first notched hull form tested, oil dots were also used to study the flow in way of the lower outboard shoulder of the notch. This test showed that the notch shoulder was aligned well with the flow at 27 knots and there was little or no crossflow. The 5 ft-0 in. bilge keels were also used for the seakeeping tests of each hull form. Prior to testing in waves, roll decay tests were conducted in calm water over a large speed range to determine the nonlinear roll damping coefficient. The prediction of roll damping was known to be a weak point in SMP. t was planned to use damping values determined from the roll decay experiments in the SMP predictions to improve the accuracy of the final seakeeping operability assessments. Ultimately, this could not be done in the time available due to the complexity of SMP, that is, the large number of places and ways roll damping appears within its various modules. For Hull A, roll decay tests were performed with both 5 ft-0 in. and the maximum feasible 8 ft-3 in. bilge keels. This had previously been done for Hull X using the large, old resistance and propulsion model. The seakeeping tests on each hull were performed at 10 and 20 knots in sea states 5 and 6 at six headings: head seas, seas 30 deg and 60 deg off the port and starboard bows and starboard beam seas. At 10 knots in sea state 6, tests were also performed in starboard quartering and following seas. Limited tests were conducted in a simulated survival condition: sea state 9 at zero knots in head, beam and following seas. n an attempt to quantitatively evaluate wetness, the test results at 20 knots in seas 30 deg and 60 deg off both port and starboard bows were used. Both high-quality color video recordings and quantitative measurements were made. The latter were measurements of wave elevation relative to the hull at stations 3, 4 and 5 measured 77.4 ft off the ship centerline using vertical capacitance wire wave probes suspended from the flight deck. The measured wave elevations were converted into the percent probability that the water surface would exceed the 49-ft waterline, assuming that the wave height peak values on each wire followed a Rayleigh distribution. For reference, the lowest point on the missile sponson at the hangar deck level, starboard side, forward of the No. 1 aircraft elevator, is at the 49 ft-3 in. WL, 13 ft-3 in. above the 36 ft-0 in. DWL. The video tapes were carefully reviewed to observe and count seven specific wetness events. The selected events were: 1. waves breaking on or off the ship's bow in the vicinity of stations 0 to 3, 2. waves breaking on or off the ship's forward quarter point in the vicinity of stations 3 to 6, 3. spray or green water reaching the flight deck at the bow (stations 0 to 3), 4. spray or green water reaching the flight deck near the forward quarter point (stations 3 to 6), 5. green water entering the No. 1 starboard elevator recess (elevator at the flight deck level), 6. waves striking the underside of the missile sponson at the hangar deck level forward of No. 1 aircraft elevator, and 7. waves breaking over the same sponson. 442 The USS Midway Blister Story

15 These events and their letter designations are indicated in Fig. 16 along with the three wave height exceedance probabilities. Several test runs were made for each test condition. Run times were noted. The resulting video tapes were spliced together and the total number of events occurring then scaled up to one hour full scale ship time for each test condition. Since judgment was involved in deciding what constituted "an event," all the official event counts used in the subsequent analysis were made by the same dedicated person at DTRC. He reviewed each test series several times before deciding on the correct count. For the four wave breaking and flight deck wetness event counts, the results for test runs with seas on the port bow were generally used to avoid the visual interferences with seas on the starboard side caused by the wave probes, which sometimes threw spray, and the missile sponson, which slammed often. An example of the seven event counts and the three 49- ft WL exceedance probabilities for the four hulls in sea state 6 at 20 knots is shown in Table 4. These results were combined to create a wetness merit factor for each hull as shown in Table 5. The weights were assigned to give greater influence to events near the forward quarter point and to those which would most affect flight deck wetness. Clearly, the entire wetness evaluation procedure was both arbitrary and subject to the vagaries of human interpretation of sometimes unclear or misleading video tapes. The events selected, the weights assigned to them and each individual event's scoring system, zero baseline and range, were all arbitrary. Also, it was not always clear from the tapes whether a wave had broken or not. On the other hand, due to the exaggerated effect of surface tension and the absence of wind in model seakeeping experiments, when breaking waves are seen in a model test, they are felt to be certain indicators of spray sources full scale. Also lending some credence to the wetness scoring system was the fact that the overall ranking of the hulls remained unchanged when a subset of the data was evaluated in a particular instance. This will be discussed in the next section of the paper, where all test results are summarized. ATtRbUTES: STA 6 STA 3 FWO STBO SPONSON ('P C 'CP ', f E O ~ jwl $TA: 4 3 A FREOUENCY OF HAVE BREAKNG STATON 0-3 AFD - FREQUENCY OF FLGHT DECK NETNEG$ STATON 0-3 a FREQUENCY OF HAVE BREAKNG STATON 3-~ BFO - FREQUENCY OF FLGHT DECK HETNE$$ STATON 3-~ C - FREQUENCY OF NETNESS ELEVATOR NO. CUTOUT US FREQUENCY OF NETNESS UNDERSDE FND STARBOARD SPONSON S FREQUENCY OF NETNESS OVER FNO STARBOARD SPONSON 0 PROBABLTY OF EXCEEDNG q9 FOOT HL AT STATON 3 E PROBABLTY OF EXCEEDNG q9 FOOT VL AT STATON q F PROBABLTY OF EXCEEDNG q9 FOOT WL AT STATON 5 Fig. 16 Wetness event designations Motions were recorded during all test runs, specifically the following components: pitch, heave, roll, sway, vertical displacement at ramp (at aft end of flight deck), vertical velocity at the aircraft touchdown point (derived from acceleration), vertical and lateral accelerations at the No. aircraft elevator and vertical acceleration at the ship's center of gravity. Notch behavior in a seaway was a major concern, specifically slam or wave slap loads on the upper portion of SEA STATE Table 4 Wetness event counts and WL exceedance probabilities, sea state 6, 20 knots HEADNG US S A AM B B~ C O E F (DEG) (PER HR) (PER HR) (PER HR) (PER HR) (PER HR) (PER HR) (PER HR) (PERCENT) (PERCENT) (PERCENT) HULL 6 X , HULL A HULL B , HULL C The usa Midway Blister Story 443

16 Table 5 Wetness merit factor--combination and weighting A'-rRBUTE RANGE EQUVALENT RATNG* WEGHT RANGE 1-10 NUMBER OF WAVE BREAKS: STATON 0-3 [A] NO./HR SHOULDER WAVE [B] NO./HR UNDERSDE OF SPONSON [US] NO./HR NUMBER OF WETNESSES AT FLGHT DECK: STATON 0-3 [AFO] 50-0 NO./HR STATON 3-6 [BFD] 50-0 NO./HR NUMBER OF SPONSON SUBMERGENCES: S] NO./HR NUMBER OF ELEVATOR WETNESSES: [C] NO./HR PROBABLTY OF EXCEEDANCE OF 49' WL (13' FREEBOARD) AT STATON 3 [D] 20-0 % AT STATON 4 [E] 15-0 % AT STATON 5 [F] 10-0 % SHP SPEED RELATVE WAVE SEA STATE (KNOTS) HEADNG (DEGREES} WEGHT (330) (300) (330) (300) 2 * NTEGER VALUES, UNEAR NTERPOLATON the notch and the potential of the notch to generate spray which might aggravate flight deck wetness. To assess loads, the notch in the Hull A model was instrumented with seven panel gages (load cells) scaled from the ship's structural configuration. Six gages were located on the upper, vertical and lower surfaces of the notch at stations 5 and 8; the seventh gage was located at station 12 on the upper notch surface. Wetness tendencies of the notch were observed visually. t was desired to perform tests on the largest possible model of the notch at forward speed in waves to examine the effects of: (1) upper notch deadrise angle, (2) upper inner notch radius, and (3) spray rails at the upper, outer notch knuckle (at the top of the notch). Load panels would be fitted to the model in the upper and vertical portions of the notch at two longitudinal positions. The tests could not be performed using the 20-ft-long 1/45 scale seakeeping models as they were fully committed to the seakeeping experiments previously described. Also, those models had not been built to permit variations in notch geometry; to have done so would have substantially increased model construction time and cost. n addition, there was the possibility that scale effects would distort the behavior of the 1 / 45 scale notch in the DTRC seakeeping tests. t was decided to build a 1 / 16 scale "partial beam" model of the Hull A forebody and perform tests at Arctec Offshore Corp. The partial beam model is a model of the port and starboard sides of the ship forebody outboard of the 28-ft buttock (ship scale), mated together as depicted in Fig. 17. This approach greatly reduced model displacement and permitted a substantial scale increase. n order to assess the validity of the partial beam model for examining notch behavior in a seaway, two 1 / 45 scale models of the ship's forebody were first built: one full beam and one partial beam. These were tested in equivalent conditions to study the effect of the partial beam model's distorted hull form on the wave conditions in the notch. FULL BEAM M O D E L ~ RM. WDTH ~.~ /~/~f- -,x~,.,/ ",um. A w,, / TAL FARN6 PARTAL BEAM MODEL Fig. 17 Partial beam model (Hull A forebody) 444 The USS Midway Blister Story

17 A 1 / 75 scale partial beam model was also built to assess scale effects on the wetness / spray behavior of the notch. None of these three smaller models were fitted with load panels; nor could their upper notch deadrise be varied. The initial tests of the three small models were performed in calm water and regular head waves with the models forced in pitch and heave. Runs were also made in irregular head waves with the model fixed (the model test apparatus was not capable of realistically simulating forced pitch and heave motions in irregular seas). All tests were recorded on video tape. Observations from the initial test series included: 1. Differences in the wetness / spray patterns of the full and partial beam 1/45 scale models. The partial beam model was wetter near its bow and the full beam model was wetter aft in way of the notch. 2. Differences in the calm-water waves generated by these same two models. The waves generated by the partial beam model were somewhat more like the actual shipgenerated waves (probably because its proportions, such as L/B, were closer to those of the ship). 3. Absence of any obvious scale effects in the observed wetness / spray patterns between the 1 / 75 and 1 / 45 scale models. The tests also revealed certain problems; these were: 1. Poor video quality related to vibrations caused by the forced model motion and the cantilevered camera positions. 2. Difficulty in obtaining the desired extreme values of forced motion amplitudes and wave heights. 3. Difficulty in obtaining the desired phasing between the forced motions and the encountered waves. Based on these initial results, there was concern about the ability of the carriage, drive and hydraulic oscillation systems to perform adequately during the extensive tests planned for the 1/16 scale model. These systems were strained by the 1 / 45 scale model. Reluctantly, the 1 / 16 scale model tests were cancelled and construction of the model stopped. Contributing to this decision were the initial results of notch tests in waves at Tracor Hydronautics (to be described) which showed excellent notch behavior. A second series of tests was performed on the 1 / 45 scale partial beam model using a modified test plan. Tests were conducted at zero speed in irregular waves in head, bow and beam seas with the model fixed and in both calm water and regular waves with the model forced in roll and heave up to very large amplitudes. The principal conclusions from the second phase of tests were: 1. The notch behaved extremely well. To generate significant spray/wetness, the model had to be tested in unrealistic conditions: extreme amplitudes or unrealistic phasing between waves and motions. 2. There was no evidence that a spray rail fitted at the top of the notch was needed. Another series of model experiments was performed in the Spring of 1987 at Tracor Hydronautics, nc. (TH). This series utilized a large (1/14 scale) 2-D free floating model of a transverse section of CV 41 at frame 95, approximately station 8.4 of 20 stations. The model, depicted in Fig. 18, could be forced to roll in calm water by moving a topside weight to and fro athwartship. The weight was driven by a rotatable threaded rod. This model was originally conceived for investigating the effect of notch section shape on roll damping and to improve the SMP roll damping predictions with data taken from the model. Ultimately, however, the model was not used for these purposes, which proved to be impractical, but rather for tests of supplementary SLO-ROL TM tanks and studies of the effects of notch geometry on wetness, spray and impact loads. Due to the configuration of the model and the test basin, all these tests were restricted to beam seas and zero speed. For the notch wetness/spray tests, the model was constructed so that the upper notch deadrise angle could be set at 36 deg (baseline) as well as 45 and 55 deg. Two spray rail configurations were tested in each of two locations: one within the notch and one 3 in. above the upper, outer notch knuckle. Both configurations were 27 in. wide (full scale). One had a horizontal lower surface, the other a 15-deg declivity on the lower surface. Load panels were fitted to the upper and middle portions of the notch in the same locations as on the DTRC seakeeping ship models. The model was also built so that a very large radius on the upper inner notch knuckle could be tested in lieu of the baseline 5 in. radius. The TH tests consisted of forced oscillations in calm water at two amplitudes and two roll periods followed by tests with the model free to roll in regular beam waves over a range of wave heights and periods. The principal conclusions from these tests were: 1. Wetness or spray occurred only in the most extreme test conditions when the top of the notch or spray rail was immersed; even then the wetness was moderate. S "J J ~ l,, ~ " ~ = / ~ ' ~ BEAM WAVES.TANK SECTON FOR SLO-ROL TANK Fig. 18 "2-D" free-floating model The USS Midway Blister Story 445

18 2. ncreasing the upper-notch deadrise decreased observed wetness / spray. 3. The large 6-ft-radius fillet at the upper, inner notch knuckle had no observed effect on wetness/spray generation. 4. Spray rails above the notch had little effect on wetness/spray; in the notch they increased wetness/spray. 5. Spray rail declivity increased wetness / spray in comparison with a horizontal lower surface. 6. Notch loads were almost entirely due to hydrostatic submergence; there was no evidence of slamming. 7. Loads were not significantly affected by varying the upper, inner notch radius or by adding spray rails. 8. There was a slight load decrease with increasing notch angle. The 2-D model was also used for tests of the SLO-ROL TM tank. These tests are fully described in reference [ 16] and will only be briefly summarized here. Free and forced oscillations in calm water were performed, as well as free roiling tests in regular waves. Tank damping ratios and natural periods were ascertained; these were required to predict ship roll with tanks. Tank fluid motions and hydrodynamic pressure at the tank inlet were measured. The effects of tank inlet size, louver angle, air crossover duct area and adjacent bilge keels were assessed. Principal findings included: 1. The SLO-ROL TM tanks operated as expected; there were no surprises. 2. The 70-deg louvers performed better than 40-deg louvers. 3. The tanks began to vent at roll angles greater than about 6.7 deg. 4. Air crossover duct backpressure was modeled successfully. 5. Bilge keels have a negligible effect on tank performance. Free-flood tank models were also built and tested by Ship Research, nc. (SR). These tests are also described in detail in reference [ 16]. The tests were conducted in three phases. n the first phase, bench tests were performed on free flood, SLO-ROL TM and tuned tank candidates for the primary motions solution. Thus these models did not reflect the hull notch later adopted. n the second phase, attention focused on an improved tuned tank configuration and the SLO-ROL TM tank, both in way of (behind) the notch since, by this time, tanks were being considered as supplements to the notch. Bench-type Plexiglas models, 1:16 scale, were tested at zero speed by SR in the University of California at Berkeley towing tank. mpulsive changes in the tank fluid level were used to determine tank damping ratios and natural periods. Tests were also performed with the tank fixed in incident waves to determine the tank fluid response to incident wave excitation. The objective of the third-phase tests was to determine SLO-ROL TM tank performance, fluid motions, and momentum drag at Froude-scaled forward speeds, the tuned tank having been dropped from consideration by this point. A 1:24 scale Plexiglas model of the outer portion of the hull, including notch, and containing one SLO-ROL TM tank section was built as depicted in Fig. 19. The model had fore and aft fairings to minimize wave diffraction. The tank model was towed in close proximity to the tank side wall at speeds of 0, 10 and knots ship scale. Tests were performed in irregular seas with significant wave heights of 10 to 16 ft ship scale. The SLO-ROL TM tank air side was modeled and two cases were studied: pure rolling VARABLE LOUVERS Fig The USS Midway Blister Story PLAN VEW ' TANK MODULE J AT VM'flN6 SPEEO AR PRESSURE SMULATON ELEVATON ED6E 6F TOW TANK SLO-ROL TM tank model for forward speed tests (constant air volume) and pure heaving (constant air pressure, variable volume). Tests were performed with open water inlets and with 40 and 70-deg inlet louvers. Primary conclusions from these tests were: 1. Measured momentum drag agreed well with predictions. 2. The tanks worked well at speeds up to 18 knots; no problems anticipated at higher speeds. 3. Seventy-degree louvers were more effective than 40- deg louvers in admitting water at forward ship speed. 4. The tank phase angle relative to an incident wave was 20 deg -10 deg at ship resonance; this agreed well with TH results. Phase study results As mentioned previously, the bulb to be used on each hull for the seakeeping experiments was selected based on calm water test results. These calm water test results and the bulb decisions based upon them will be discussed first. The most extensive bulb testing was done on Hull A, the first model to be completed. nitial exploratory tests were done with Options and bow bulbs and the largest and smallest circular arc shoulder bulbs; each bulb was tested separately. The shoulder bulbs were tested at several longitudinal and vertical positions in conjunction with the existing ship's Taylor bulb. These initial tests showed that both the new bow bulbs dramatically reduced the amplitude and steepness of the ship's shoulder wave. They also significantly reduced ship resistance at speeds above about 17 knots. The shoulder bulbs increased ship resistance, were effective in reducing wave heights locally (a station or so aft) at higher speeds but generated a breaking wave immediately behind the bulbs at lower speeds due to the sharp change in pressure near their trailing edges. The breaking wave generated by the 12-ft-radius shoulder bulb showed that the bulb was clearly too large and it was dropped from consideration. The remaining shoulder bulbs were then all tested in conjunction with the Option bow bulb; a few were tested with the Option bow bulb. The conclusions from these tests were that the 6-ft nabla was the most effective shoulder bulb, that the bulb should be located with its maximum section at station 3.5 and its top at the ft WL, about 10 ft below

19 Table 6 Calm-water wave reduction merit factor and resistancemhull A OPTON + TAYLOR OPTON 6 ' NABLA OPTON BOW BULB BOW BULB SHOULDER BULBS BOW BULB WAVE REDUCTON MERT FACTOR* WAVE ALONG HULL : SPEED = 18 KNOTS SPEED = 24 KNOTS L~ 393 SPEED = 30 KNOTS WAVE OFF HULL : SPEED = 24 KNOTS ~ 425 RESSTANCE (EHP)** SPEED = 18 KNOTS 16,707 15,927 16,569 16,438 SPEED = 24 KNOTS 42,286 40,328 41,522 40,228 SPEED = 30 KNOTS 97,342 94,402 96,579 93,696 * HGHER SCORE BETTER ** CA = , NO MARGN, NO STLL AR DRAG the ft test LWL and that, of all the bulb combinations tested, the choice came down to either the Option bow bulb alone or a combination of the Option bow bulb with the 6-ft nabla shoulder bulbs. Table 6 gives the calmwater wave reduction merit factors based on wave slopes and elevations for these and two other cases, along with bare hull ehp values at ship scale. The video assessment showed that the "Option plus shoulder bulbs" combination produced smoother, flatter waves on the hull and outboard near station 5 and moved the breaking shoulder wave aft and closer to the hull. There was a very small hydraulic jump near station 5, but heeling the model 2.5 deg to either side at forward speed showed no deleterious effects (this brought the shoulder bulb closer to the free surface). Based on these results and other considerations, including anchor handling, the project team recommended the "Option plus 6 foot nabla shoulder bulbs" combination. This recommendation was not endorsed by higher authority based on technical risk concerns. To our knowledge shoulder bulbs have never been put to sea on ships and proven sueeessfifl. Thus the Option bow bulb was chosen for the Hull A seakeeping tests. Hull B was tested in four configurations: plumb bow (no bulb), Option bow bulb, Option V bow bulb alone, and "' Option V bow bulb plus 6 foot nabla shoulder bulbs" located as on Hull A--maximum section at station 3.5 and top at ft WL, 10 ft below the test LWL. The merit factor analysis of the test results as well as video review indicated that the two best bulb candidates were the Option and V bow bulbs without shoulder bulbs. The Option bulb was chosen for the seakeeping tests because of its higher merit factors and improved wave profile along the hull at most speeds. However, both bow bulbs exhibited off-body breaking waves near the bow in the 18- to-24-knot speed range and the Option bulb also showed a strange and disturbing wave pattern at 15 knots. These Hull B test results were surprising and disappointing to the project team. We had expected Hull B to have the best wave patterns. Hull C was also tested in four configurations: Taylor (existing) bulb, Option bow bulb, Option bow bulb alone, and "Option bow bulb plus 6 foot nabla shoulder bulbs" located as on the two previous hulls--maximum section at station 3.5 and top of bulb at ft WL, 10 ft below the test LWL. All three bulb configurations showed large wave pattern improvements over the exist- ing Taylor bulb. The Option bulb had the highest merit score at 24 and 30 knots but was by far the poorest performer of the three configurations at 18 knots. The addition of nabla shoulder bulbs slightly improved the merit factor of the Option bulb alone at all three speeds. This small improvement was not enough to offset the drag increase and added technical risk of shoulder bulbs. The Option bulb alone was chosen for the Hull C seakeeping experiments. Table 7 gives the calm water wave reduction merit factors and ehp values for Hull X and for Hulls A, B and C with their selected bulbs. Based solely on the Table 7 data, Hull B ranks highest, followed closely by Hull C, and Hull X ranks lowest. The seakeeping model test program and wetness assessment methodology were described previously. The results of the wetness assessment are shown in Fig. 20. The relatively poor performance of Hull B was unexpected but consistent with the surprising wave breaking observed during her calm-water tests. During the seakeeping tests with Hull A, a limited series of runs were inadvertently made with the Taylor bulb rather than the Option bulb. For these runs, the model was also fitted with spray rails. The bulb error was discovered and the tests rerun. However, the data set from these inadvertent tests was analyzed using the same wetness assessment methodology and compared with the corresponding limited data set for the oo 100 x "~ 80-- l-,,~ 70-- O 60-- h : 100 i::~:~ i A B HULL Fig. 20 Wetness assessment The USS Midway Blister Story 447

20 Table 7 Calm-water wave reduction merit factor and resistance comparison WAVE REDUCTON MERT FACTOR* HULL X HULL A HULL B HULL C TAYLOR OPTON OPTON ll OPTON BULB BULB BULB BULB WAVE ALONG HULL : SPEED = 18 KNOTS SPEED = 24 KNOTS SPEED = 30 KNOTS WAVE OFF HULL : SPEED = 24 KNOTS? REDUCTON (EHP)** SPEED = 18 KNOTS 17,077 16,438 16,878 16,219 SPEED = 24 KNOTS 44,218 40,226 38,800 39,136 SPEED = 30 KNOTS 99,473 93,696 90,320 90,453 * HGHER SCORE BETTER ** CA = , NO MARGN, NO STLL AR DRAG other hull/bulb combinations. The results are shown in Fig. 21 and are interesting on two counts. They show that the Option bulb contributes greatly to the success of Hull A and also that the overall wetness merit factors of ~" tu 46 the other hulls are essentially unchanged in shifting to the limited data set. This result was gratifying and increased our confidence in the assessment approach. ~- While the various models were being built and tested, ~ 4o initial versions of contract designs were prepared for each,,', of the hull form alternatives. The designs for Hulls A and z B were developed before C since they represented the ~ least and greatest modifications in terms of required labor, materials, time and cost. Cost estimates were prepared based on these initial contract design (CD) packages; the Hull C CD was not complete by the time cost estimates 3o 0 were required, but it was far enough along to permit a credible estimate to be made. As expected, the Hull A modification was estimated to cost the least, about 17 per- Fig. 22 cent less than Hull C, and Hull B the most, about 23 percent more than Hull C (total program cost deltas; estimates of program costs themselves are omitted from this paper). The same trend held for schedule. The study results showed clearly that the choice was between Hulls A and C; Hull B was the most expensive and provided the least wetness improvement over Hull X. Hull A with the Option bulb was the least expensive and had a somewhat higher wetness merit factor than Hull C with the Option bulb. However, Hull A's wetness ! 86 X Fig HULL 93 A A B OPT 61 TAYLOR BULB BULB Limited wetness assessment l STBD SPONSON HULL "A" OPTON 3 BOW ~- - HULL "C" OPTON 2 BO~ NO. 1 ELEV AT REST WL "" STATON NUMBER Model test wave profiles, Hulls A and C; ship speed = 24 knots merit was to a large degree dependent on the large Option bow bulb. Both groups which independently reviewed our fix efforts, Newport News Shipbuilding and the retired Navy designers, referred to as the "oldtimers," strongly recommended against trying to cancel, by means of large bow bulbs, wave patterns created by a poor hull form. Both groups recommended hull form modifications instead, getting at the root of the problem directly. Also, the large Option bow bulb would introduce anchor handling problems and inerease technical risk. Hull C has significantly less resistance than Hull A at the higher speeds. The wave profiles at 24 knots on Hulls A and C, with Option and bow bulbs, respectively, are shown in Fig. 22. Note the difference in wave elevation between stations 4 and 5 in way of the starboard side missile sponson, about 1.5 ft. Other factors considered in the decision were: (1) The breaking shoulder waves on Hull C are farther aft and inboard than on Hull A and thus less likely to generate wetness (not reflected in the wetness merit factor); (2) Hull C rolls slightly more than Hull AT; (3) Figure 23 shows the final seakeeping operability assessment for the four hulls based on SMP ship motion predictions and the final limiting motion criteria described in Appendix 2. Figure 24 shows a sample of the roll and pitch motions measured for Hulls X, A and C during the seakeeping model tests. Figure 25 is a sample of the off-hull wave elevation excursions measured during the same seakeeping tests, expressed in terms of probability of exceedance of any WL. 448 The USS Midway Blister Story

21 AJRCRAFT HANDLNG LAUNCH & RECOVERY " 7o ////~ r//// 100, , ~ 60' ~.,o- ~"" 10- "''" ~fff: HULLO HULLX HULLA ~-//// ~-//// f////, :511" //// ////~ ~///½ //// rll// CV 63 ~- 30' 20' 10' 0 i!! HULLO HULLX HULLA CV63 HULL H = 61 HULL B = 53 HULL C=UO HULL C =53 HULL 0=56 HULL D=52 Fig. 23 Seakeeping operability assessment (northern Arabian Sea--June-August; all hulls with existing 5 ft-0 in. bilge keels) LL D..J O CC < U) i O. (/) Fig. 24 ROLL 0 O Hull &-HXP 6 A Hull X-EXP / 0 O Bull C-EXP 9 9 Hull B-EXP / / / / / i t i HEADNG o D Hull A-$ ft Btge Kee-EXP 6 A Hull X-EXP o o HUll C-EXP V 9 HUll B-EXP PTCH ~ - ""~a "~ A z 0 % o i i i i i ?5 90 HEAD HEADNG BEAM Model test motion comparison, sea state 5, 20 knots tx Hull C has less stability reserves than Hull A, about 0.8-ft KG margin versus 1.1 ft for Hull A; and (4) Hull C has about 1 ft less stern trim than Hull A. Based on all of the above, the decision was made to adopt Hull C with the Option bow bulb as the primary wetness solution. As mentioned earlier, notch shape refinements were studied after selection of the notch as the primary motions solution. Upper notch slope, notch knuckle radii and the issue of eliminating the twist in the vertical portion of the notch aft were addressed. The most difficult of these issues to resolve was the upper notch slope. The baseline configuration was essentially a fallout of the notch development process whereby the vertical portion was carried up to the third deck (actually 6 in. above the 3rd deck for structural reasons) and then drawn out to the existing hull at the second deck (actually 6 in. below the 2nd deck, again for structural reasons). This yielded an upper notch slope, or deadrise (measured above the horizontal) which varied somewhat along the hull but reached a minimum of 36.1 deg at station 8 (of 20 stations). Slamming of the upper notch surface was a major concern and the consensus of the hydrodynamicists was that this angle should be increased to a minimum of 45 deg. We did not wish to increase the angle by lowering the intersection of the vertical and the upper, sloped portion of the notch as this would reduce the limiting roll angle within which the hull was wall-sided at the waterline. nitially there was concern that the sloped upper and lower notch surfaces might introduce a "corkscrew" type of roll motion when ship roll angles exceeded this limit. The later seakeeping model tests revealed no signs of any such unusual roll motions. Thus, an upper notch slope increase could be handled in two ways (refer to Fig. 3 ): ( 1 ) Raise the top of the notch, thus cutting into the 2nd deck, or (2) retain the existing 2nd deck, cut inboard below it and add a lightened longitudinal bulkhead between the main (hangar) and 2nd decks to support the upper end of the notch. Both of these approaches were studied for the port side of the ship, but only the lightened bulkhead option was considered feasible for the starboard side due to the presence of an external sponson between the 2nd and main decks which The USS Midway Blister Story 449

22 J 03 m o n- o. ~, 0.2 ",.,"V",,,"-, 0.1 u X (FEET ABOVE CALM W.L.) Fig. 25 Off-hull wave elevation exceedance probabilities: sea state 5, speed = 20 knots, heading 30 deg, station 5, 77.4 ft off centerline (*probability that sea surface elevation is greater than X) presented structural, arrangement, and construction difficulties. Ship impacts and costs were evaluated for "full notch length" and "forebody only" increased slope options. Meanwhile, the hydrodynamic aspects were studied by a three-pronged model test program described previously. The results showed conclusively that the baseline notch configuration was entirely acceptable and that slamming loads were not measurably reduced by increasing the deadrise angle. The seakeeping model tests at DTRC showed that the notch behaved well in all the sea conditions examined. n fact, even in the sea state 9 survival condition, wave action in the notch was generally mild and there were no discernable motion anomalies, that is, corkscrew motions. Based on these results, the baseline upper notch slope was retained. The radii of the four notch knuckles were established as follows. nitially, each of the four radii was set at 4 in. based on structural considerations. Subsequently, the lowest knuckle radius was increased to 2 ft based on hydrodynamic considerations, that is, concern that crossflow over the knuckle would add drag and create vortex shedding leading to noise and maintenance problems. Later the radii of the three upper knuckles were increased to 5 in. based on the inability of SH to roll steel plate to smaller radii. The upper, inner notch knuckle radius was a special concern; see Fig. 3. t was thought that a small radius at this location might lead to spray generation as water rose across the knuckle in a seaway. This effect was studied in unique model tests described earlier. Radii up to 6 ft were evaluated and, based on the benign test results, the baseline 5 in. radius was retained. The baseline notch had a twist in its after sections, also shown in Fig. 3. Reducing this twist was studied as a producibility enhancement. The study showed that reducing the twist would indeed improve notch module producibility but that the notch length would be increased, adding three modules per side. The cost increase would be substantial. Less important was a modest degradation of damage stability, somewhat offset by a small increase in natural roll period. The "reduced twist" option was dropped from consideration. As mentioned earlier in the paper, larger bilge keels were studied as a possible motion improvement supplementary to the notch. Midway is fitted with 5 ft-0 in. span bilge keels, the largest known to the authors. These bilge keels were installed in 1980 in an earlier effort to improve the ship's roll behavior; the previous bilge keels had a span of 3 ft-0 in. s Studies showed that the maximum feasible bilge keel span was 8 ft-3 in. n brief, this dimension was set by the rectangle bounding the midship section of the notched hull and was driven by the notch module installation requirements, that is, the need to transport modules longitudinally along the floor of the dry dock at SRF, Yokosuka from the bow or stern of the ship toward midship, past the turn of the ship's bilge. The maximum span bilge keel would be located higher on the ship's hull than the current bilge keel and this relocation would have two costly impacts: (1) Twenty-four large damage control valves located near the turn of the bilge would have to be relocated (they had previously been relocated outboard to the new blister shell in 1986); and (2) the existing shell at the turn of the bilge in way of the relocated bilge keel would have to be removed and replaced with new, crackresistant HY-80 to serve as a proper backing plate. The effects of the larger bilge keel on ship motions and seakeeping operability were predicted analytically with the aid of SMP. Higher-priority test requirements did not permit comparative seakeeping model tests with the alternative bilge keels before the required early June decision date. However, roll decay tests were performed in s The ship's roll problem circa 1980 was, in fact, a result of her low metacentric height due to years of weight growth, mostly topside, coupled with her directional instability. The problem manifested itself as the "' Dutch roll" at the ramp and a tendency to "'hang" in turns and at the extremes of slow rolls in stern quartering seas. 450 The USS Midway Blister Story

23 ~ ~ HEAO 5'-0" BLGE KEEL 3--~ r-3" BLGE KEEL LMT,,-o,, r BEAM 10 KNOTS FOLLOWlN6.= tl---o '-0" llle KEEL t,---o O'-s" KLGt Kilt LMT 20 KNOTS,,-o,, i' ~...""" \ 8' 3" -.L,....B'" S T HEAD BEAM FOLLOWN6 Fig. 26 Effect of bilge keel span on roll motion: Hull A/Option bulb, SMP predictions, short-crested, sea state 5 (H~,~ = t0.7ft, To = 11 sec) calm water. We had planned to modify the roll damping terms in SMP based on these roll decay test results but this proved to be impractical due to the complexity of SMP. Thus the final decision on bilge keel span was based on SMP predictions without model test inputs. Typical roll motion predictions are shown in Fig. 26. The results show that roll amplitudes are reduced 10 to 20 percent by the larger bilge keels. Seakeeping operability is increased 11 percent in one typical operating area in the aircraft handling scenario (where all headings are considered). n the aircraft launch and recovery scenario, dominated by pitch motion, there is no predicted improvement in operability. As mentioned earlier, there would have been some improvement in this scenario also, if the assessment methodology took into account the likelihood of cross seas. The effects of the larger bilge keels on ship weight, -buoyancy and calm water speed / power were assessed and found to be small. Added cost would be several million dollars. nstalling the larger bilge keels was estimated to add one month to the installation time required for the notch. There was concern about the technical risk associated with such large bilge keels. n the end, this consideration was the dominant one and the decision was made to model test the larger bilge keels but not install them. That could always be done later if at-sea experience with the modified ship showed that further motion improvements were necessary. We said earlier that two variants of free-flood tanks in the blister voids were also studied extensively as possible supplements to the notch. These were tuned tanks and the SLO-ROL TM tank concept. The tanks were assumed to be located in way of the notch, generally forward of midship to assist in reducing stern trim. The tank configurations studied are shown in Fig. 27. Due to the notch, the blister width at the waterline was reduced to 30 in. Thus the free-surface effect, or GMT loss, associated with a specific length of free-flood tank ( tuned or not) is greatly reduced by the presence of the notch. A significant advantage of the SLO-ROL TM concept for Midway is that, by pressurizing the tank and thus lowering its free surface below the notch region, its free-surface width and hence tank effectiveness can be substantially increased. Four pairs of SLO-ROL TM tanks and six pairs of tuned free-flood tanks were studied for Hull A. n addition, five pairs of SLO-ROL TM tanks and eight pairs of tuned free-flood tanks were studied for Hull C. These tank sets had approximately equivalent waterplane areas; the numbers selected were quite arbitrary. n fact, analytical performance predictions were ultimately made for a wide range of numbers of tank pairs. Each tank is 16 ft long, the distance between watertight bulkheads in the outer blister void. The tuned tank has an internal tapered vertical duct, rectangular in plan view. This tapered duct is used to tune the tank, that is, to adjust the period and phase of the water motion in the tank relative to the ship motion so as to improve tank effectiveness. n essence, to suit the relatively long roll period of an aircraft carrier, the time required for the water in the tank to travel from the inlet opening up toward the free surface needs to be increased by increasing the "effective length" of the tank. This could also be done by introducing a system of horizontal baffles, but this concept was dropped when it was discovered that there are nontight transverse web frames in the ship's outer blister voids spaced every four feet. The tuned tank design was not carried as far as that of the SLO-ROL TM tank. As far as it went though, the design process was an iterative one, relying heavily on model testing. nitially rules of thumb based on previous work and simplified theories were used to size the water inlets and air vents, as well as the tuning duct. Then bench tests '~AR N AND OUT j J AR FLOW TO.,,.. P,RE.m.Js'rB PRt-OUST~ / '~."',\ SH~BJ~~ (H ~-~ TUNiN6 OUCT (HULL O) SHELL ~ ~ H / BL6[ KEEL (BOSTNB) / BLB[ KEEL (EXS'gNG) TM TYPCAL TUNED TANK TYPCAL SLO-ROL TANK Fig. 27 Supplementary free-flood tank alternatives The USS Midway Blister Story 451

24 were performed at Ship Research ncorporated (SR) using a simple model of the tank and the critical tank parameters were modified to maximize performance. The final series of tuned tank model experiments evaluated a refined tank design. Tests with a bench-type model were performed by SR at zero speed in the model tank at the University of California at Berkeley. These tests were described previously. Data obtained from the tests were used to validate a d fine tune an analytical tank performance prediction program developed for the Midway Project. This program was coupled with SMP to enable predictions of the ship seakeeping behavior to be made with and without freeflood tanks of all types: untuned, tuned and SLO-ROL TM. The development of this prediction program by John DalzeU of DTRC in less than four months was one of the most extraordinary achievements during the project. The SLO-ROL TM tank design development followed much the same process as did the tuned tank. The biggest issue with the SLO-ROL TM tank was whether the required crossover air ducts could be installed in the crowded internals of Midway without unacceptable penalties. Shipchecks were performed to identify the least-impact crossover routes. Then the ship impacts were evaluated and cost estimates made. The model test program for the SLO- ROL TM tanks, described earlier, involved bench tests and tests in two model tanks. By April enough ship impact, cost and performance data had been developed for both tank options to conclude that the tuned tank should be dropped from consideration. On Hull A, the six tuned tank pairs' performance was not significantly better than that of the four SLO-ROL TM tank pairs and the technical risks were deemed to be significantly greater for the tuned tanks. Tuning duct installation inside the Midway blister voids would be difficult and costly. The ducts would degrade the effectiveness of the ship's torpedo defense system. The tuned tanks could not be readily adjusted once installed, whereas the SLO- ROL TM tank air crossover pipe would permit adjustments. similarly, the tuned tanks couldn't readily be deactivated at ship speeds and headings where the tanks were destabilizing, whereas the SLO-ROL TM tanks could. Damage control considerations also favored SLO-ROL TM. n a damaged ship condition, the SLO-ROL TM tank air crossovers could be shut and, at least potentially, the tanks on the low side of the ship emptied by pressurized air to add righting moment. Other concerns with the tuned tanks were the possibility of noise and odor emanating from the air vents. The SLO-ROL TM tank design was carried to a logical stopping point: about the preliminary design level. The tank design was not completed since it was decided to hold the supplementary SLO-ROL TM tank in reserve until at-sea experience with the notched ship had been gained. f that experience showed that further motion improvements were needed, the SLO-ROL TM tanks and/or larger bilge keels could always be installed on the ship. The hydrodynamic design of the tank was complete and validated by the SR and TH model tests. The tank and crossover pipe configurations had been defined, as well as louvers on the water inlet opening to smooth the flow and prevent resonance phenomena from causing noise and erosion problems. The tank pressurization and control system concepts had been defined. Ship performance with and without the tanks had been predicted analytically based on tank behavioral characteristics which had been confirmed by model tests of the tank alone. Figure 28 is an example of such predictions. n bow seas, 8.25-ft bilge keels without tanks are about equivalent to 5-ft bilge keels plus four pairs of SLO-ROL TM tanks. n quartering seas, however, the larger bilge keels alone are better. Note that "' ship + tank" performance was not predicted by using ship models containing tank models. There were two reasons for this: 1. the tank models would be so small that the anticipated scale effects would destroy the credibility of the results, and 2. there was no time to dedicate a seakeeping model to such tests; the ship models as they became available were dedicated to wetness evaluations. For five tank pairs on Hull C (each tank 16 ft long; total flooded length 80 ft), estimated installation cost would be about twice that of the maximum span bilge keels. Four air crossover pipes would be utilized; three 16-in.-diameter pipes for three tank pairs and one 22-in.-diameter pipe for two tank pairs. These pipes would displace 27 berths; space to relocate the berths could not be found in the time available. Speed loss in calm water due to four tank pairs on Hull A would be about 0.2 knot; in sea state 5 about 0.7 knot (the latter figure does not include the effects of Q d HEAD SEAS 8 FT. BLGE, NO TANKS <> 8.25 FT. BLGE KEELS, NO TANKS 5 FT. BLGE KEELS + 4 TANK PARS 8.25 FT. BLGE KEELS + 4 TANK PARS, i,,, BOW BEAM QUARTERNG FOLLOWNG SEAS SEAS SEAS SEAS Fig. 26 Predicted roll motion --SLO-ROL TM tanks: Hull A/Option bulb, ship speed = 10 knots, sea state 6 (short-crested, H~ = 16.4 ft, To = 13 sec) 452 The USS Midway Blister Story

25 sea state 5 on the ship itself). Similar deltas would be expected for five tank pairs on Hull C. The tanks would increase the notched ship's natural roll period about 0.8 sec, from 15.7 to 16.5 sec in the full load condition. Considering a range of ship speeds, headings and sea states, the supplementary SLO-ROL TM tanks would be less effective overall than the maximum bilge keels. This is because the SLO-ROL TM tanks lose effectiveness in stern quart- ering seas, more so at higher ship speeds. n fact, in certain conditions, ship roll with tanks would be greater than ship roll without tanks. n such destabilizing conditions, the tanks could be "shut off" by closing valves in the air crossover pipes and ship roll reduced, if the ship's officers could be confident the conditions were destabilizing. That's a big "if." The larger bow bulbs being considered for Midway created potential anchor handling problems. Both the Option and Option bow bulbs would be contacted by Midwag's anchor as it was raised and lowered. Bolster extensions and a new stem bolster to eliminate this contact were studied. The Navy design criterion states that the anchor should clear the bulb by a minimum of 12 in. with 1-deg adverse list on the ship for the full range of possible anchor rotation as it is suspended from the bolster. The studies showed that bolster extensions to clear the Option bulb were impractical; they would be huge and extend too close to the water. A stem bolster could be designed to clear the forward end of both bulbs but it would also be large and likely to generate additional flight deck wetness. A bolster extension to clear the Option bulb was feasible but it would weigh and cost about twice as much as a stem bolster. t would also increase the risk of wetness/spray generation but not to the degree a stem bolster would. Either bolster would require a minimum of nine months to design, model test, construct and install. While bolsters were being studied, discussions were taking place with the OPNAV representatives of the carrier operating forces. The result of these discussions was that OPNAV agreed to accept anchor contact with the bulb during raising and lowering ff they could be assured that the anchor would not be shattered in the event of an emergency drop from the stowed position onto the bulb. Studies showed that an anchor drop onto the Option bulb would result at worst in a glancing, fluke-only impact. The anchor would survive; the impact would be no worse than that suffered in a standard Navy anchor proof test: a drop from 12 ft onto a hard surface. The potential impact area on the bulb could be designed to absorb the impact; ~-in. ductile plate would provide the needed capability through.plastic deformation. The bulb would be permanently dented by such a drop, about 2 in. maximum deflection. With the current ship anchor and bolster, the potential contact area on the Option bulb shell was estimated to be 80 ft 2 per side with an adverse 1-deg list on the ship and 40 ft ~ with a beneficial 1.5-deg list. The lower portion of the bulb would be built of 2-in. plate to resist side loads and abrasion from the anchor and chain (it was estimated that Midway's anchors would be raised and lowered 90 to 200 times during her remaining service life). The final decision was that no change would be made to the current Midway anchor handling system. The Option bulb would be designed to absorb potential anchor impacts and chain abrasion. Accommodation ladders have been a perennial problem in the U.S. Navy. They are typically difficult to rig and unrig; the process is slow and manpower-intensive. They are also difficult and expensive to maintain. Midway has three accommodation ladders: two aft port and starboard and one forward on the starboard side, aft of the No. 1 aircraft elevator. Midway was the first U.S. Navy carrier to receive accommodation ladders of an improved design, developed after several years of effort, to reduce maintenance and rigging time. These ladders were installed at the time of the 1986 blister installation and have been highly rated by the ship's company. The notch necessitated the development of a new standoff or support for the lower platform-of these ladders. The problem was more difficult for the forward ladder because the notch is deeper forward and the risk of wave impact is greater. Through a collaborative effort involving NAVSEA, Puget Sound NSY and private contractors, a number of concepts were studied. Ultimately, a folding support was developed which shows great promise; see Fig. 29. When the ladder is raised and stowed in the sponson above, the support is folded up and stowed against the upper surf~ace of the notch. The support stows between two fairings designed to smooth the flow of waves past the support to reduce impact forces and, as important, the risk of spray generation. The folding support would further reduce the rigging time of the current Midway ladders and could be safely and quickly retrieved in building seas. Maintenance would be moderate. A much less expensive fixed support alternative was developed for the aft ladders but this was ultimately rejected. The fixed support would reduce operating efficiency in that it would require the manhandling of a large shell bumper between the support and the ladder base near the WL. This bumper, raised and lowered from above, could not be quickly and safely retrieved in building seas. Also the fixed support, whose lowest part is 3 ft-0 in. above the 36 ft-0 in. WL, would be subject to wave and floating object damage and would generate spray at higher speeds, even in moderate seas. The issue of trim and list correction, while largely unrelated to the motion and wetness problems, nevertheless remained on the "front burner" throughout most of the fix effort and, to some degree, acted to distract the project team from the primary issues. Also, in retrospect although not at the time, the tale of our trim-and-list correction efforts is somewhat amusing. Prior to the 1986 blister installation, it was known that Midway normally operated with a port list and stern trim. The blister addition reduced the ship's port list (due to the associated GMT increase) and increased stern trim 1.0 ft. The plan all along had been to firmly establish the ship's post-blister trim and list through an inclining experiment and then take corrective action as indicated. However, the outcry from the ship after Midway went to sea in December '86 included protests over the effect of the blister on stern trim. Because stern trim increases aircraft landing gear loads, this was a sensitive issue and the design team was directed to address it as a high priority. Table 8 gives list and trim for Hulls X and A over a range of liquid loads assuming no ballasting. Studies were performed of three ways to correct only list and both list and trim. These were: directed tank sequencing, lead ballast and fuel tank relocation. t was concluded that directed tank sequencing was not an effective list-and-trim solution because the ship's diesel fuel marine (DFM) and JP-5 ser- -vice systems are split fore and aft and port and starboard and a significant amount of fuel rotation is required to maintain fuel "freshness." Fuel relocation was a feasible solution but it would cost significantly more than correction by means of lead ballast and would require careful tank sequencing, reducing the ship's operational flexibility. The USS Midway Blister Story 453

26 MAN 9ECK TO WNCH STOWAGE. ~ 2NO DECK! ;" ';"1 SUPPORT A ' ~ ~-" FOLDNG eract W/t.6CK SECT LKG FWD i ', FWO STOWED SUPPORT %$TmEO BRACE (RETRACTED POSTON) Fig. 29 Accommodation ladder folding support (forward ladder shown) Lead ballast would be a sure correction and it would retain the ship's operational flexibility. On the other hand, it would reduce the ship's service life weight and KG reserves but not to an unacceptable degree. While the above studies were being performed using Hull A as a baseline, additional information was being collected. The current ship has a gravity bilge drain system which depends upon stern trim to function. Trim elimination would necessitate replacing this system with a stripping system at considerable expense. The ship trims by the bow as speed is increased. At 30 knots, Hull X trims 3.2 ft more by the bow than at zero speed; Hull C with the Option bulb trims 2.4 ft. Thus, if the current ship were trimmed 3.2 ft by the stern at zero speed, for example, at 30 knots it would have zero trim. Aircraft landing gear loads are greatest on calm days when the ship is operated at high speeds to generate wind over deck. Listand-trim correction would also reduce freeboard on the starboard side forward: a 2-deg list correction would reduce freeboard at the No. 1 aircraft elevator and at the missile sponson just forward of it by 2 to 3 ft; a 3.2-ft trim correction would reduce the same freeboard less than 1 ft. Any trim correction would degrade directional stability and might necessitate a fix such as increased skeg area aft. Lastly, Hull C, due to the blister shaving forward, would inherently have about 1 ft less stern trim than Hulls X or A, other things being equal. Based on all of the above, the decision was made to use lead ballast to eliminate list and to locate it fore and aft so as to trim the ship 1 ft by the stern in the full load condition. As fuel was consumed, this stern trim would slowly increase, permitting retention of the gravity bilge drain system. The amount of lead ballast required to eliminate the list on Hull C was 575 tons located 66 ft to starboard of the ship's centerline; to trim the ship one ft by the stern FL the lead had to be located 186 ft aft of midshipl One of the January 1987 messages from Midway [3] complained about the effeet of the blister on the ship's speed. The message alleged that the blisters had caused a speed loss of 1.0 to 1.5 knots and that the ship could now only make 27 knots on eight of her twelve boilers whereas previously she could make 29. This message resulted in speed being added to the list of issues to be addressed by the project team. A careful review of the self-propelled Table 8 List and trim for Hulls X and A t/ FULL LOAD 80% DFM/ 60% DFM/ 50% JP-5 0% JP-5 lj HULL X LST, DEG TRM, FT 1.0 PORT 1.9 STERN 1.1 PORT 4.7 STERN 1.9 PORT 6.1 STERN HULL A LST, DEG TRM, FT 2.0 PORT 1.8 STERN 2.3 PORT 4.8 STERN 3.9 PORT 6.2 STERN /TANKS ASSUMED NOT BALLASTEC TANK SEQUENCNG BASED ON CV 41 LQUD LOADNG NSTRUCTONS, CH. 31 OF SHPS NFORMATON BOOK, CA The USS Midway Blister Story

27 model test data for the existing ship, Hull X, and the previous pre-blister ship, Hull O, was undertaken. This review confirmed our earlier predictions that the 1986 blister would degrade speed, but not nearly to the extent alleged by the ship. Also, our data showed that the pre-1986 blister ship could not come close to 29 knots on eight boilers in the full load condition. We predicted about 0.7-knot speed loss at full power of which part was due to the blister drag increase and part was due to the drop in full power output resulting from reaching the torque limit on the inboard shafts. On eight boilers, the predicted speed loss due to the blister addition was considerably less. However, there was somewhat more than the usual uncertainty regarding these data. The Hull X data had been derived from experiments using 30+year-old ship and propeller models, the ship model having been modified by the addition of GRP blisters. The white metal propeller models were in poor condition. Also, the existing Hull O data (circa 1954) were suspect because of inconsistencies in the derived hullpropeller interaction factors. Standardization trials for Hull X had been planned and partially executed immediately after the 1986 blister installation. These trials were incomplete and the results suspect due to strong and variable currents on the trials range. Another complete set of standardization trials was therefore planned and executed in April The ship was ballasted to approximately zero trim and full load displacement. The trials went smoothly and the results were judged to be of high quality. They showed that the ship required significantly less power for a given ship speed than previously predicted; the ship made 27.6 knots on eight boilers and over 31 knots on twelve boilers. These results satisfied the ship but did not alter our earlier predictions, based on model tests, of the speed loss due to the blisters. Meanwhile, studies were performed of ways to increase ship speed. The options considered were: modifying the pitch of the existing inboard propellers (to increase rpm at the torque limit and hence SHP output), new design inboard propellers and the installation of inflow modifying vanes ahead of the inboard screws. The study results showed great promise for the propeller options but ultimately the decision was made not to pursue propeller modifications due to time / cost considerations and the encouraging April '87 trial results. Also, other study results were showing that the proposed notch and Option bow bulb were reducing ship resistance rather than increasing it. Figure 30 shows the final ship speed predictions resuiting from the project's work. The predictions are a synthesis of the new 20-ft model resistance data, the previous Hull X self-propelled model propulsion data, and an updated correlation allowance derived from the April 1987 Hull X standardization trials. Notable is the fact that the notch reduces resistance as do the Hull C blister shaving and the Option bow bulb. Design refinement On 5 June 1987 the major decisions summarized in the preceding section were made. n early February, NAVSEA had committed to a target date of 31 July 1987 for signature of the completed Midway fix contract design package. Thus the project team's efforts in June and July focused on completion of the contract design to meet that date. As mentioned earlier, CD packages had been completed for Hulls A and B prior to 5 June; the Hull C CD package was in-process and only partially complete on that date. As fate would have it, the 5 June decision was to HULL H.O HULL 0 1o HULL A U 11 O HULL O HULL C TAYLOR TAYLOR,0t'1 ii,op BULB ULB BULB BULB * 1'-1" OK E,l J HULL X 27.6 HULL X.,4o0 HULL O st,tin HULL l HULL C HULL C ROLL C TAYLOR TAT.011 oopt t *OPT BULB BULB OULB BU lit ',0'-3" OK 16,~0 34,990 36,700 MAX SPEED (12 BOLERS) MAX SPEED (8 BOLERS) SHP AT 20 KNOTS Fig. 30 Speed-power estimates for CV 41 modifications. (Hull X baseline data from April 1987 standardization trials; estimates of deltas from baseline based on model tests) adopt Hull C. Thus the first order of business was to complete the Hull C CD package. Much of the effort in this regard concerned the structural design. Hull C presented very difficult structural problems in way of the No. 1 aircraft elevator. Maintaining structural continuity around the shell cutouts in way of deck edge aircraft elevators is a difficult problem in all aircraft carriers. The two blisters added to Midway over the years had added structural complexity in these areas. The blister shaving reflected in Hull C meant that, immediately below the No. 1 elevator, the new shell was too close to the 1957 blister shell behind it for welders to gain access and work. Thus, in this area, the 1957 shell would have to be removed and transitions provided fore and aft. Other difficulties related to providing adequate paths for stresses to flow up and aft from the bottom of the No. 1 elevator cutout into the hangar deck sponson structure aft of the elevator. n this region the structure became so complex that even the experts designing it had trouble describing the resulting configuration. As part of the design refinement process, a critical look was taken at the extent of the Hull C blister shaving. f the area affected by this shaving could be reduced, substantial savings would accrue. Not only would less ripout be required but also less new steel and the labor to install same would be required. This was especially true at the lower edge of the shaved area as this was in a region where thick, high-strength steel shell insert plates were located for longitudinal strength and bilge keel crack arrest purposes. Through a trial-and-error process, aided immeasurably by the computer, the lower blend line (defining the line at which Hulls X and C diverge) was raised several feet as depicted in Fig. 31. The net effect was to reduce the shaved area of the blister about 2900 ft 2 on each side The USS Midway Blister Story 455

28 2o.o' n~nsto / 12.o' o,,. u,e ~.r ]., /. ",::,i, _./,,,., i SHELL EXPANSON ld-x (STARBOARD SDE SHOWN) //" t ~//~//.~." MOD o/v;,,'::...,//"... ADDTONAL HULL SURFACE c - ~ ~ - ' - - " / l~'l l";t/~lili RETANED iper SDEi = 2900 sn. FT. st;lllit~:~ //'~--X STATON 6.. 4' t (NOT TO SCALE) ~ Fig. 31 Hull C lower blend line of the hull with very substantial savings. There was no criterion for deciding how much the blend line could be raised without unduly distorting Hull C, introducing flow problems and perhaps nullifying the model test results. The decision was based on the collective judgment of the naval architects on the project team. The consensus was that a 3-in. deviation perpendicular to the Hull C shell at the blend line could be accepted. t was interesting to note that a few of those present were in favor of going to as much as a 6-in. deviation. Additional seakeeping experiments were performed on Hull C during this period. These tests had three objectives: to validate the final spray rail shape and location, to assess the effect of retaining the Taylor bulb on Hull C's wetness characteristics, and to evaluate the effectiveness of the maximum, 8 ft-3 in. span, bilge keels. The tests were performed and evaluated using the same approach as before but the number of test conditions was restricted so that they were the same set as those run for the inadvertent April tests on Hull A fitted with the Taylor bulb and spray rails. The test conditions were: 20 knots ship speed, sea states 5 and 6, headings 30 deg off the port bow and 60 deg off the starboard bow. Three Hull C configurations were tested, all with spray rails: Option bulb with 5 ft- 0 in. bilge keels, Taylor bulb with 5 ft-0in, bilge keels, and Option bulb with 8 ft-3 in. bilge keels. A wetness assessment was made based upon the resulting limited data set. The results are shown in Fig. 32 along with equivalent data from the previous tests on Hulls X, A, B and C, all but one test without spray rails. The results show that spray rails significantly improve Hull C's wetness behavior; this was confirmed by visual observations. The Taylor bulb significantly degrades Hull C's wetness behavior relative to the Option bulb, confirming the trend previously observed on Hull A. Surprisingly, the large bilge keels also aggravated wetness even though they significantly re- LMTED MERT FACTOR SPEED 2O KNOTS LMTED HEADNGS OATA NORMALZED TO 10.7 FT. SG. HEGHT FOR S.S FT. SlG. HEGHT FOR S.S X T A A T Bll Cii C C T C + SPRAY + SPRAY + SPRAY + SPRAY RALS RALS RALS RALS (+ ir- 3" BK) NOTE: T = TAYLOR BULB; = OPTON BULB; = OPTON BULB Fig. 32 Limited wetness assessment 456 The USS Midway Blister Story

29 SPEED, O KNOTS SPEED, 20 KNOTS 6 S' D~ ~ 1 ~ 8'-3" BK Si-'-d'iT HEAD BEAM FOLLOWNG HEADNG w 3 3 d o2! 5' O K ~,,~'r/~;:; ~, ~ \ 8'-3" Dl -'~-ffi~- HEAD BEAM FOLLOWNG HEADNG Fig. 33 Effect of bilge keel span on roll motion: model test results and SMP predictions; sea state 6, significant wave height = 16.4 ft, long-crested seas duced roll motion, as can be seen in Fig. 33. The reasons for this result are not known. Possibly the larger bilge keels adversely affect the phasing between the incident waves and the ship's pitch and heave motions. Figure 33 also shows that the SMP predictions of ship roll motion are generally less than measured on the model and that the model tests demonstrate the maximum bilge keel to be more effective compared to the 5-ft bilge keels than SMP does. Based on these results, the final spray rail shape and location was confirmed, the earlier decision to defer installation of the maximum span bilge keels was reaffirmed, and any thought of saving time and money by retaining the existing ship's Taylor bulb on Hull C was laid to rest. Observation of'videos of Midway wetness at sea showed three additional wetness sources which were addressed during the design refinement phase. The first of these was the lower end of the forwardmost guiderafl for the No. 1 aircraft elevator and its fairing. The guiderail is a deep transverse structural member mounted external to the hull. The 1986 blister significantly reduced the projection of the lower end of this guiderail beyond the molded hull form. Even so, the videos showed that the remaining projection and its fairing, an angled plate, were a frequent source of wetness at higher speeds in calm water, as well as in waves. t was recognized that the hull notch would increase the guiderail projection and thus the likelihood of wetness generation from this source. The second wetness source observed in the ship videos was the aft ends of the box-like recesses in the ship side in way of the aircraft elevators. With the elevators raised and the ship moving forward, waves would often sweep up into these recesses and impact against their aft ends. The resulting spray would sometimes enter the hangar, if the hangar side door was open, or, more often, would be thrown outboard and up against the side of the ship further aft. Figures 34(a) and 34(b) depict the results of our efforts to address these two wetness sources. The elevator guiderail fairing [Fig. 34(a)] is composed entirely of simple developable shapes and is designed to ease the flow of water past the guiderail projection as well as minimize the entry of water into the elevator recess immediately aft by the use of a wedge to throw the water outboard. The wave deflecting bulwarks [Fig. 34(b)], constructed of flat plates, sit on the bottom shelf of each elevator recess and direct entering waves outboard and down. The length and height of these bulwarks are restricted by the requirement that they not interfere with the aircraft elevator platform structure when the elevator is fully lowered. The third source of wetness and spray observed in the ship videos was the ship's stem. Due to the ship's weight growth over the years, coupled with the tendency of the ship to trim by the bow with increasing speed, the ship's waterline forward at high speeds is considerably above the original DWL. The resulting blunt WL endings near the free surface generate a high spray sheet above the bow wave which is often caught by the wind. To alleviate this problem, Don MacCallum, a NAVSEA member of the project team, conceived of a simple stem extension, termed the "bow knife." This concept, depicted in Fig. 35, was hastily modeled of clay and evaluated on Hull C in calm water in the DTRC MASK facility. The results were impressive, as shown in the figure. The height of the bow EL VATGU uo. t.-~--,jfr~ ~ Fig. 34(a) = OEFLECTGU GUDE RAL WEDGE FARli6 n (NEW) ~ (am) Elevator guiderail fairing (No. 1 elevator) 4" PPE Fig. 34(b) Wave deflecting bulwark (for elevators 1,2, and 3-- typical) The USS Midway Blister Story 457

30 5`5 J BOW / - KNFE EDGE STEM KNFE 3,5 DWL 32' W 2`5 ~ J J J J J ~ 12" RAOlUS L. KNFE ADDTON / ) / OPTON EX BULB EXSTNG STEM CASTNG \ TYPCAL ENDNG )WTH SUGGESTED STRUCTURE) EFFECT OF BOW "KNFE" ON CV 41 BOW WAVE BASED ON DTNSRDC VDEO Fig. 35 Bow knife wave was reduced about 5 ft, ship scale. This result would need to be confirmed with a more accurate model tested under the controlled conditions and wave height measurement instrumentation available with DTRC Carriage. The bow knife was not developed structurally nor was it incorporated in the contract design package. There was not sufficient time to do either by the 31 July signature deadline. Directional stability anc1 the ship's m~aneuvering characteristics were a continuing concern. During the study phase of the project, it was understood that we would be eliminating the ship's stern trim and possibly even trimming the ship by the bow in the full load condition, since the ship trimmed increasingly by the stern as fuel was consumed. The hull notch, which increased draft, the larger bow bulb and reduced stern trim would all tend to reduce the ship's directional stability, which had been sorely deficient prior to the 1986 blister addition and had been corrected when the blister was added by the addition of 50 ft ~ of movable area per rudder and a 200 ft ~ fixed fin on the ship's centerline aft. Extensive analytical studies were performed to assess the extent to which directional stability would be degraded by these changes and to evaluate various corrective schemes. These included larger or additional fixed fins aft and lengthened rudder horns with end plates at the horn tips. The latter were evaluated and rejected as being ineffective and risky. Rudder stock strength limitations precluded consideration of additional rudder area and rudder end plates. During this period, model tests were not possible as the available models were tied up with seakeeping experiments. Nor would model tests have been sensible, since we didn't know which hull form and bulb would be chosen; nor did we know what design trim condition would be selected. The conclusion of the analytical studies, which also included ship impact assessments and cost estimates, was that the most cost-effective solution, if directional stability improvements were found to be needed, would be to increase the area of the 1986 fixed fin aft. During the design refinement phase, after the supplementary seakeeping experiments were completed, the Hull C model was rigged for maneuvering experiments and transported to the MacMillan Reservoir in the District of Columbia. The great expanse of this reservoir makes it nearly ideal for conducting the spiral maneuvers needed to establish directional stability characteristics. The tradeoff is that the test team must often wait days for the necessary calm weather conditions to arrive. This was certainly true for the Hull C Midway tests in the summer of 1987! After a delay of about two weeks, the tests were finally run (after the 30 July contract design completion date). The results showed that the ship would be acceptable at full load displacement when trimmed 1 ft by the stern. There was a small hysteresis loop in the "turn rate versus rudder angle" plot, but the amount was similar to that previously established for Hull X and was within the accepted bounds for Navy ships: deg rudder angle at zero turn rate and a -+ turn rate at zero rudder angle equivalent to a tactical diameter over length (TD/L) ratio of 10. Space limitations permit only a brief mention of some other aspects of the design refinement phase. A careful weight estimate for the completed blister modification contract design showed the net effect of all the modifications to be: Light ship weight LT, loads LT, full load LT These figures include 575 LT of new lead ballast. The final damage stability analysis showed adequate displacement and KG reserves, more than had been predicted in the earlier study phase. Specifications and work requirements to define the desired modifications were developed. These were carefully reviewed with the planning yard, Puget Sound NSY, and with SRF, Yokosuka as well as the prospective shipbuilder, SH, to incorporate their comments prior to signature. The completed contract design specifications and drawings were signed on 30 July 1987, a date set nearly six 458 The USS Midway Blister Story

31 V'- ELEVATOR GUDERAL WAVE-DEFLECTNG BULWARKS ~ ~ ~" FARNG,N ELEVATOR OPEN,NGS \ ~ 1 7 t= ~r- SPRAY RAL U N BLSTER ~ BULBOUS BOW --~ ~/,~PRAY RALS ELEVATOR '/if J ~ \ / ~ /-- NEW HULL BOUNDARY GUDERAL ~ \ / = J / FA, R,NG ~ "~(,,~ (NOTCH) BLSTER HULL / - ' - " - - ~ t ~ 1 ~ ~ ~ WATERLNE BOUNDARY = ~ ~ i~... PRE-BLSTER ~ "~,. ~,,~ ~ NEW HULL BOUNDARY HULL BOUNDARY ~"~ (BLSTER "SHAVNG" -FORWARD ONLY) BULBOUS BOW Fig. 36 Proposed fix to improve roll motions and wetness months earlier. The elements of the proposed roll motions and wetness fix are collectively depicted in Fig. 36. Principal characteristics of the modified ship are noted in Table 1. Conclusions and recommendations The following are the principal conclusions we have drawn from the 1986 Midway blister experience described in the paper: Natural roll periods derived from sallies performed in restricted water can be significantly in error. Width and depth restrictions are both significant. f blisters are added to a high-speed hull in such a manner that hard shoulders are introduced into the sectional area curve, the result is likely to be a large, breaking shoulder wave which adds drag and increases the likelihood of undesirable deck wetness. Large transverse metacentric heights lead to large lateral accelerations topside with undesirable effects on equipment and payload handling evolutions, especially aircraft handling. These effects are magnified when the ship's natural roll period, operating area, and speed/heading profiles are such that roll resonance occurs often. A notch in way of the waterline along a ship's hull is an effective way to reduce transverse metacentric height. Such a notch reduces calm-water resistance a small amount and, surprisingly, shows no tendency to slam or generate wetness. n fact, the notch seems to channel waves along the hull in such a way that the normal wetness and slamming tendencies are reduced. Free-flood tanks inboard of a ship's side shell are an effective way to reduce transverse metacentric height. However, such tanks increase ship resistance a substantial amount and hence are impractical for high-speed ships if a large amount of free surface, that is, GMT reduction, is required. Also, free-flood tanks will increase ship roll motions at some speed/heading combinations, for example, in stern quartering seas at moderate forward speeds. This must either be accepted or means devised to inactivate the tanks in such situations. Very wide bilge keels can significantly reduce ship roll amplitudes. However, there is significant technical risk associated with such bilge keels since they have not been evaluated full scale and present methods of predicting bilge keel performance by analysis or model tests are uncertain at best. Unforseen side effects, such as increased wetness, may result. Relatively small protuberances on the shell of a highspeed ship near the waterline can readily generate extraordinary amounts of topside wetness. Hull form modifications to ease the shoulder on a ship's sectional area curve are effective in reducing the associated shoulder wave generated by the moving ship hull. Bow and shoulder bulbs can be used effectively to modify the bow and shoulder waves generated by a moving ship hull in order to achieve amplitude reductions in specific zones along the hull. The technical risks associated with shoulder bulbs are significant since they have not been evaluated full scale. Modern 3-dimensional potential-flow free-surface computer programs predict flow along the hull surface aft of the immediate bow which agrees well with model tests. Thus they are valuable, cost-effective tools for performing comparative assessments of candidate hull form modifications in the design process. Further improvements in these programs are needed to better predict the maximum bow wave height. Properly configured spray rails can be effective in alleviating the deck wetness experienced by large ships as well as small ones. However, they are not a substitute for a proper hull form and adequate freeboard. Video tapes are a valuable and cost-effective way to observe and record full-scale ship and model behavior in waves. They should be used more extensively at sea and in the model tank. The in-house engineering and scientific talent resident in NAVSEA 05 and at DTRC is second to none with respect to knowledge and ability. [ Capacity clearly limited by small number of people. ] The model test facilities at DTRC are the best in the free world. There is also top- The USS Midway Blister Story 459

32 notch talent resident in the private sector. Unfortunately, it seems to take an urgent project with top priority to assemble a dedicated "critical mass" of this talent. When that occurs and a team is created composed of dedicated in-house and private sector talent working shoulder to shoulder, the resulting productivity and quality of output is extraordinary. The following principal recommendations are made: An analytical method of predicting the effects of water width and depth restrictions on ship roll periods determined by sally experiments is needed. Criteria need to be established for the minimum water width and depth needed for the sally results to closely approximate openocean values. Model sally experiments for several principal ship types varying water width and depth could be used to establish preliminary criteria in the short term and also used to validate the ultimate analytical approach. Procedures are needed to calculate a ship's radii of gyration (mass moments of inertia) about the three principal axes during the design process and to routinely update these estimates as the ship weight estimate is developed and refined. Reliable data in this regard are needed in order to correctly assess ship dynamic behavior both analytically and in the model tank. This is certainly achievable given modern computer technology. The NAV- SEA weight community has recognized this need and is vigorously pursuing ways to accomplish it. High priority should be given to the refinement of the limiting motion criteria applied in assessing ship operability in rough water. Current criteria are deficient in many respects; for example, they are based largely on opinion rather than fact; they are too simplistic; limits are expressed in terms of limiting roll amplitudes, for example, rather than lateral accelerations; and they do not rationally address the full spectrum of ship size for a particular mission scenario, such as the motion limits for helo ops from a frigate versus a cruiser versus an amphibious assault ship versus an aircraft carrier. Full scale at-sea data collection, model tests and analytical work are required. Full scale, land-based tests of mission-critical equipment undergoing motions are also indicated, such as tests of a forklift truck on various deck surfaces being subjected to lateral and vertical accelerations. Since 1959, full-scale trials at sea have been conducted by the Naval Air Development Center to obtain statistics on a number of aircraft landing parameters, including distributions of hook-to-ramp clearances and relative vertical velocity between the aircraft and deck. By reanalysis of these data, backing out measured ship motions, it may be possible to apply statistical techniques to arrive at refined limits on ramp displacement, touchdown point vertical velocity, and pitch motion for safe landing of various aircraft types. Also, the use of flight simulators to verify roll, pitch and vertical ramp displacement limits beyond which aircraft landings are degraded should be explored. n Phase of the Midway Project, a flight simulator at the Naval Training Systems Center (NTSC), Orlando, Florida was used to evaluate the difficulties encountered by pilots landing on CV 41 Hulls O, X and A. This work was restricted to a T-2C trainer cockpit and the CV 59 deck image (input sinusoidal heave, roll, pitch and yaw motions were those of CV 41 ). Similar work could be done at NASA AMES in F/A-18 and A-6 aircraft cockpits with simulated irregular flight deck motions. We must be able to predict the lateral accelerations perceived by an object on the deck of a moving ship. Such an object is subject to accelerations which tend to t!p or slide the object laterally and also lift it off the deck. The accelerations perceived by the object result from (1) the rigid-body accelerations of the ship, expressed in a frame of reference fixed with respect to the earth and resolved into components in the plane of the deck and normal to the deck, coupled with (2) the acceleration due to gravity, also resolved into the same two components. The accelerations predicted by SMP and measured in a seakeeping model test are referenced to the earth's axes. The conversion to the accelerations perceived by an object on deck in axes relative to the deck is apparently very difficult. This problem must be addressed and, at the very least, approximation techniques developed for a range of ship types and sizes, as well as varous locations on the decks of each. The Navy's Ship Motion Program (SMP), a magnificent technical achievement, is an essential and invaluable tool for evaluating ship seakeeping behavior during design. However, a higher priority needs to be given to the "care and feeding" of SMP to make it even more useful and reliable and to enhance its credibility in the eyes of senior ship design decision-makers. First, the program should be made more "user friendly" so that it can be used more effectively in an interactive mode during design. Work should be done to adapt the program to modern mini- and microcomputers and make it easier for the practicing naval architect to use on a part-time basis in the course of design. Second, some parts of the program's internals need to be improved. For certain motion components and certain hull configurations, the correlation between predicted motions and those measured on a model, for example, is not satisfactory. The program's treatment of roll damping is an example of an aspect needing improvement. n the course of the Midway work described in the paper, significant differences were observed between SMP predictions and model test-derived values of roll damping for two bilge keel widths. Similarly, significant differences in predicted versus measured roll amplitudes were observed. Third, more work needs to be done to correlate the program's results with model test data and full-scale trial data to establish confidence in the computer predictions. More correlation work would also provide insights into aspects of the program which need further improvement. n recent years, a great deal of model seakeeping test data have been collected that, due to lack of funds, have never been correlated with SMP predictions. Also, there have been too few USN-sponsored full-scale seakeeping trials. Fourth, additional capabilities need to be added to SMP to enhance its usefulness. Examples include upgrades of the antiroll fin module, addition of a passive antiroll tank module, incorporation of the ability to routinely make predictions in swell-corrupted seaways, including the input of actual wave spectra instead of theoretical spectra, and to readily incorporate roll damping information derived from model tests into the SMP predictions for a particular ship. Deck wetness is an important contributor to the degradation of surface ship operational capability in higher sea states. The Midway wetness merit factor was an ad hoc attempt to compare the deck wetness behavior of several hull form alternatives. Research should be done to evaluate this and alternative approaches with the goal being to settle on an accepted evaluation methodology along with associated limiting criteria which could be routinely applied to future ship designs. The Midway effort indicated the importance of being able to measure wave elevations on and off the hull in calm water. The stationary wave probe technique suffers 460 The USS Midway Blister Story

33 from an inability to record data closer to the ship's centerline than one-half the maximum ship beam and also from reliability and maintenance problems, nonlinear response characteristics and signal drift. More flexible and reliable techniques are needed. The use of lasers to measure wave height and slope shows great promise. This method should be developed further so that it is routinely available for "production" work. Steps also need to be taken to streamline the analysis of wave cut data to improve productivity and reduce test time. Criteria of acceptability for the breaking waves generated by a ship hull in calm water need to be established. Model to full-scale correlation of breaking waves needs to be def'med in order to aid the interpretation of model test data. Epilogue The effort described in the paper was completed in the summer of n September 1987, due to severe Navy funding constraints, the Chief of Naval Operations directed that all work on the project cease. During the fall of 1987, Midway reported via ARPAC that the ship's wetness characteristics had been acceptable since the removal of the forebody scupper extensions and their boat guards the previous April. With this news, and in the face of a severe budget crunch, attention focused on the possibility of a less expensive "motion only" fix. t was agreed that the most cost-effective motion fix would be the Hull A notch alone. Since then a detail design package for the latter fix has been prepared and is available when and ff a decision to proceed is made. Meanwhile, the sailors of Midway have accommodated to the new ship motions to a remarkable degree. The ship remains a strong and viable asset to the U.S. Navy but could be improved by the notch modification described in this paper. Acknowledgments n the short space available here, it is not possible to adequately acknowledge the extraordinary efforts of the many people who contributed to the Midway fix effort. Even a complete list of the organizations and firms these contributors represented would be too long. We are grateful to all those who did contribute and we praise them for the excellence of their efforts and their sustained ability to meet extremely short deadlines. We trust that they will each always take pride inknowing that they successfully met all the challenges thrown at them in the course of the Midway Motions mprovement Project. Realizing that we can't mention everyone, and recognizing that some may be offended by our neglect, we feel bound to indicate a few of the people and organizations who made especially noteworthy contributions. Within NAVSEA 05, Mr. Richard Steward managed the contractor support effort, interfaced with the program manager, PMS 312, and generally expedited things. Mr. David Byers coordinated all technical effort. Mr. Edward Comstock directed the hydrodynamic studies which comprised the essential core of the effort. At DTRC, Carderock, Mr. Lewis Motter coordinated all technical effort. These four men were the "executive officers" of the project whose personal heroic efforts made it happen. Numerous NAVSEA and DTRC engineers were dedicated to the Midway project from start to finish. This precious in-house Navy talent performed a large portion of the project's engineering studies, model tests and design work. At NAVSEA, Mr. Tom Packard directed the extensive structural design effort which translated the reshapings of the hydrodynamicists into sound and producible ship structure. Mr. John Rosborough led the numerous complex stability analyses, more thorough and accurate than any ever done before. At DTRC, Mr. David Walden was instrumental in the development of the Phase Plan and was responsible for most of the seakeeping model test data presentations, SMP predictions, and seakeeping operability assessments. Mr. Gabor Karafiath developed the initial draft of the complex Phase model test plan and supervised the construction of the three 20-ft seakeeping models as well as all calm-water model testing. Mr. Harry Jones directed the extensive seakeeping model tests and Mr. Eric Baitis led the full-scale motion measurements. Other Navy organizations which played major roles were Puget Sound Naval Shipyard, the Ship Repair Facility at Yokosuka, Japan, and the Naval Air Systems Command. n the private sector, J. J. McMullen, Advanced Marine Enterprises, Designers and Planners, Ship Research nc., Tracor Hydronautics, nc., Westinghouse MTD, Chicago Bridge and ron, and NKF (in the person of Mr. Rod Barr) made notable contributions. Newport News Shipbuilding independently reviewed the work of the Project as did a group of retired senior Navy ship designers. The latter group comprised Mr. Owen Oakley, Mr. John Nachtsheim, Mr. Jim Mills, Mr. Herb Meier, and Mr. Phil Mandel. George G. Sharp, nc. prepared all project briefing materials as well as the figures for this paper. Ms. Marion Quinn patiently typed and edited numerous drafts of the paper. We gratefully acknowledge the important contributions of all of these individuals and organizations as well as the many not mentioned. References 1 COMNAVARPAC Z, Department o the Navy, Dee USS Midway Z, Department o the Navy, Jan (NOTAL). 3 USS Midway Z, Department of the Navy, Jan USS Midway Z (addressed wetness and videotape "Midway Roll Action from Helo, Water Flow and Scupper mpact and Wave Action from Flight Deck" forwarded separately), Department o the Navy, Jan Letter from Commanding Officer, USS Midway, to P.A. Gale, (forwarded still photos and additional copy of ref. [ 4 ] video tape), dated 23 Jan Gawn, R. W. L, "Rolling Experiments with Ships and Models in Still Water," Trans. Royal nstitution of Naval Architects, Vol. 82, Motter, L., "Model Sally in Restricted Water," Midway Motions mprovement Project Report, Vol., Phase Subtask Deliverables--Subtask 4.9, 23 Feb Rock, G. H., "The Behavior of the 10,000-ton Cruisers in a Seaway," Navy Department, Bureau of Construction and Repair, C and R Bulletin No. 4, Jan Bureau o Construction and Repair Newsletter No. 13, 22 Jan Principles of Naval Architecture, SNAME, 1967, pp Vasta, J., Giddings, A. J., Taplin, A., and StilweU, J., "Roll Stabilization by Means of Passive Tanks," TRANS. SNAME, Vol. 69, 1961, pp Bales, S. L. and Lee, W. T., "Stratified Sample Ship Motion Predictions," Midway Motions mprovement Project Report, Vol., Phase Subtask Deliverables--Subtask 8.4, 1 April Kracht, A. M., "Design,of Bulbous Bows," TRANS. SNAME, Vol. 86, The USS Midway Blister Story 461

34 14 Kalumuck, K. M., Chahine, G. L., and Johnson, V. E., Jr., "'An Analysis of Spray Deflector Design for the USS Midway, '" Technical Report , Tracor Hydronautics, nc., Laurel, Md., June Savitsky, D. and Roper, J., "Analysis of Spray and Deck Wetness, USS Midway," Technical Report ST-DL , Davidson Laboratory, Stevens nstitute of Technology, Hoboken, N.J., May Webster, W. C., Dalzell, J. F., and Barr, R. A., "Prediction and Measurement of the Performance of Free-Flooding Ship Antirolling Tanks," TRANS. SNAME, Vol. 96, Appendix 1 Limiting motion criteria n the course of most new ship designs, or modernization or conversion designs involving hull form changes, seakeeping operability assessments are made. The purpose of these assessments is to trade off seakeeping behavior against cost and other critical performance parameters, for example, transverse stability, in the hull form selection process and also to ensure that the hull form selected satisfies the customer's seakeeping requirements. n order to assess seakeeping operability, the effects of increasing sea state and the resulting ship motions on the operability of the ship's mission-critical systems must be known or assumed. n general, the operability of a given system will degrade with increasing ship motion, slowly at first, then more rapidly. For most ship systems, too little is known about how the various motion components contribute to this degradation and, for each component, how system effectiveness degrades with increasing motion amplitude (in this discussion the phrase "' motion component" refers to velocities and accelerations as well as to displacements). n order to permit seakeeping operability assessments to be made in spite of this lack of pertinent information, for a given mission scenario judgments are made concerning which mission systems are critical, which motion components are relevant to each system and, for each component, what is the limiting acceptable value for that motion component, beyond which the mission system is assumed not to function. This simplification converts the gradual degradation of system capability with increasing sea state and ship motion into a step function, assuming 100 percent capability up to a given motion level and zero capability beyond that level. For aircraft carrier design, the critical mission system is assumed to be the ship's aircraft and two mission scenarios are examined. One is the launch and recovery scenario, restricted to ship speeds which provide adequate, but not excessive, relative winds-overthe-deck and to a narrow band of headings to the wind and sea. The other is the so-called aircraft handling scenario, which concerns the problems associated with moving aircraft about the ship and performing aircraft maintenance. Both the handling and maintenance functions are performed at all headings to the wind and sea. For both scenarios, relative wind speed is a constraint and pitch and roll are assumed to be limiting motion components. For the launch and recovery scenario, vertical ramp displacement and vertical velocity at the touchdown point are also considered. Both of the latter components are functions of pitch and heave primarily, but also roll ( to a lesser extent ) since the recovery area centerline is not coincident with the ship's centerline. n the course of the Midway Motions mprovement Project, all of these assumed limiting motion components were reviewed and updated. n Phase, only the roll criterion was briefly addressed; in Phase, all criteria were studied in greater depth. Due to space limitations, the remainder of this brief discussion will address solely the roll motion component. Prior to the Midway experience discussed in this paper, the assumed limiting roll motion applied in aircraft carrier seakeeping operability assessments was 5-deg significant single amplitude (SSA). Neither roll period nor lateral acceleration limits were applied. When Midway put to sea with her 1986 blisters, this criterion was quickly revealed to be too lenient. Although Midway met the 5-deg criterion, she reported fast, heavy rolls that severely degraded air operations. Aircraft skidding and tipping incidents occurred often in sea states lower than those necessary to produce roll amplitudes of 5-deg SSA. t should be noted at this point that considerable confusion was created early in the project because Midway's bridge inclinometer, used by her officers to measure and report roll amplitudes, was giving readings on the order of twice the true values due to the high lateral accelerations on the inclinometer's fluid, caused by the ship's short roll period and the height of the inclinometer above the ship's roll center. This error was discovered by comparing roll amplitudes measured by inclinometer with those measured by the ship's gyro and by a portable ship motion recorder. A pertinent question is, Why wasn't the invalidity of the 5-deg SSA roll criterion discovered before the Midway incident? The answer is probably the lack of recent Navy experience operating conventional takeoff and landing (CTOL) aircraft from carriers with short roll periods. All of the Navy's current operational carriers except Midway and Coral Sea have roll periods greater than 20 sec and the latter two carriers, prior to Midway's 1986 blister, both had roll periods over 18 sec. Early in Phase an intense effort was made to quickly upgrade the 5-deg SSA limiting roll motion criterion to permit valid seakeeping operability assessments to be made for proposed motion fixes. The effects of Midway's short roll period on aircraft on the flight deck made it clear that the criterion should be sensitive to roll period as well as to amplitude; in fact, the ultimate criterion should probably be based on the lateral accelerations perceived by aircraft on the flight deck, that is the lateral accelerations acting to cause aircraft skidding or tipping. Lacking the time and data needed to develop a lateral acceleration criterion, an empirical limiting roll amplitude criterion sensitive to roll period was developed by John Pattison of SEA 05. The criterion is shown in Fig. 37 and was developed using the equations for lateral and vertical acceleration amplitudes at a given point on the ship shown in Fig 38. For a given aircraft or other object on deck, the friction coefficient acting against lateral movement, or sliding, is proportional to the ratio of the lateral to vertical forces acting on the aircraft, which is the same as the ratio of the corresponding accelerations, A r/az. Using SMP, motions were predicted for the current Midway at a representative speed, heading (bow seas) and sea state. Then Ayand Azwere computed at an extreme location, the forward outboard corner of the angled deck. The resulting Ar/Az ratio was held constant for a range of longer roll periods, in each case iterating roll amplitude until the value needed for constant Ay/Azwas determined (pitch, heave, and yaw motions were assumed to be unaffected by a change in roll period). The resulting curve of roll amplitude versus roll period was then multiplied by a constant so as to pass through 3.0-deg SSA at a natural roll period of 21.1 see. An experienced aircraft handling officer indicated that this was the limiting acceptable value for CVN 70 with a worn nonskid surface on the flight deck (CVN 70 natural roll period is 21.1 sec). This data point was confirmed to some extent by written aircraft handling policy from CV 66 which limited athwartship aircraft movements to the same maximum roll amplitude. The CV 66 natural roll period is 24.3 sec. The resulting curve, shown in Fig. 37, indicates a limiting roll amplitude of 2.2-deg SSA at a natural roll period of 11.7 sec, representative of CV 41 at somewhat less than full load displacement. This point was confirmed to some degree by observations on board Midway on 12 January On that date, rising seas caused air ops to be curtailed. Ship motion measurements showed the limiting acceptable roll amplitude on that occasion to be about 2.0- deg SSA. The curve shown in Fig. 37 was used as the limiting roll motion criterion in the Phase seakeeping operability assessments, which helped to confirm the choice of the notch as the primary Midway motions f'lx. Early in Phase, a joint NAVSEA / NAVAR CV Limiting Deck Motion Working Group was formed to perform a more extensive review of the roll criterion and also to reexamine all the other limiting motion criteria. The working group initiated a number of studies including the extension of efforts to collect motion data on carriers at sea, correlating said data with points in time when conditions for air ops were at or near the limit for safety in order to establish improved limiting motion criteria. A ship motion recorder had been installed on CV 41 in early January 1987; an additional recorder was later installed on USS Constellation (CV 64). n parallel with the early efforts of the working group, NA- VAR convened an Air Department Workshop on March At this workshop, a large number of Air Department personnel with fleet aircraft handling experience exchanged views on what were the pertinent limiting motion components for the 462 The USS Midway Blister Story

35 tal.j,. O iv- j CV 41 HULL X (11.1 SEC) PHASE CRTERON i l OTHB ~'S "PHASE CRTERON -- CV 41 HULL O (18.7 SEC) o ROLL PEROD (SEC) LATERAL, Ay=gsin~+l/2 411"20.X+ 411"----~2 ~2Y+ 4"n'2 ~Z t /T0' Te Te GRAVTY YAW ROLL TERMS TERM TERM 4~ 2 4T( 2 VERTCAL, Az?g_+ [~+ Tp2-- ~- ~X+ GRAVTY HEAVE PTCH ROLL TERM ACCEL TERM TERM = ROLL ANGLE TR = ROLL PEROD e" = PTCH ANGLE Tp = PTCH PEROD g = GRAVTY ACCELERATON h = HEAVE ACCELERATON DSTANCE RELATVE TO SHP CENTER OF GRAVTY X = FORE OR AFT Y = PORT OR STBD Z= UP OR DOWN Fig. 38 Acceleration equations, from ML-STD-1399 (Navy) Section 301 "Ship Motion and Attitude" Fig. 37 Limiting roll criteria for aircraft operations roll criterion as well as changes made in other limiting motion criteria by the Phase joint working group. two scenarios and the quantitative limiting values for each. The results of this workshop were brought into the working group effort. For big-deck carriers with natural roll periods over 20 sec, the Air Department Workshop concluded that the limiting acceptable roll amplitude is 3-deg SSA. The workshop also concluded that below 2 deg SSA, roll period is not a concern. When the natural roll period of an aircraft carrier drops below 20 sec, aircraft handling becomes increasingly more difficult. n the Phase Study, these increased handling difficulties were assumed to be related to aircraft skidding or tipping, which is influenced by lateral acceleration of the deck. Assuming that there is a threshold in the lateral acceleration needed to skid or tip an airplane, as the roll period decreases, the limiting roll amplitude has to decrease to maintain lateral acceleration at or below the threshold level. With this model, a limit curve was developed and matched to a 3-deg limit reported by a former aircraft handling officer on USS Vinson at 21.1 sec roll period, as previously explained. However, the workshop pointed out that roll period also limits the ability of the crew to move aircraft during heavy roll motion. At times rolling is heavy enough to require brakes and chocks at the maximum excursions but aircraft movements can be made between maxima. This allows about ~4 of the complete roll cycle (side-to-side times 2) for aircraft movements. The shortest feasible time to move an aircraft between braking and chocking is about 5 see (4~ sec absolute minimum). This corresponds to a natural roll period of 20 see (18 see absolute minimum). Also, judgment on when deck motion lulls will occur and aircraft can be moved more safely is impaired when the ship roll motions are quick. Thus NAVAR and NAVSEA agreed upon the roll limit curve labelled "Phase Criterion" in Fig. 37 for use in the final Midway Motions mprovement Project seakeeping operability assessments. This curve is based on a 3-deg limiting roll criterion for ships with natural roll periods over 20 see and a 2-deg limiting roll criterion for ships with natural roll periods less than 18 see. The curve is applied in both the aircraft handling scenario and the aircraft launch and recovery scenario. n the latter scenario, aircraft must be moved to be spotted for hook-up to the catapult, and aircraft must taxi away from the landing area. At the time work on the project ceased, the other efforts undertaken by the joint working group had not been carried far enough to influence the Phase limiting roll criterion, which was based entirely on the experience and judgment of the aircraft handlers assembled at the NAVAR workshop. Hard data to confirm or temper this judgment are badly needed. t should be noted also that extensive seakeeping operability assessments were made to evaluate the effects of proposed changes to the limiting roll and other motion criteria during Phase. t is regretted that space does not permit a review of this important work and the lessons drawn from it. Figures 6 and 23 of the paper reflect the effects of the difference between the Phase and Phase limiting Appendix 2 Ship motion correlations At the outset of the Midway Motions mprovement Project, concern was expressed by upper management as to the validity of the motions predicted for the blistered ship (Hull X) using SMP. After all, it was these predicted motions and how they related to the then-current limiting motion criteria and to the motions predicted for other air capable ships, especially LHA-4, upon which the decision to proceed with the 1986 blister had been based. Thus, during the first few months of the project considerable effort was expended attempting to validate SMP for Midway. This effort will be briefly summarized here. The effort was not fully successful, partly for technical reasons and partly because of the nature of correlation work, which requires time-- time for careful experimentation and time for careful analysis-- and is thus fundamentally incompatible with the crisis atmosphere of a top-priority "get well" project. n addition to a review of previous SMP correlations with model tests (some of which involved aircraft carriers) and full-scale trial data ( almost none ), the Midway Phase correlation work involved four specific tasks. For this correlation work, SMP was modified to permit arbitrary (measured) wave spectra to be input, to permit arbitrary (measured) roll damping to be input, and to include a gravity component [g sin (roll angle)] in predicted lateral accelerations. The first of these tasks was to compare SMP predictions with the motions measured on board CV 41 on the evening of 16 December 1986, during the abbreviated trial made to establish the ship's roll period. This effort was recognized to be high risk due to the large number of uncertainties, especially those related to the seaway present. Roll motions were measured at five headings at 10 knots ship speed. Wave conditions present at the time (2100 hours local time) were estimated (hindcast) using the Fleet Numerical Ocean Wave Model. The estimated seaway had a significant wave height of 8.7 ft, a modal wave period of 8.5 sec and was corrupted by a 13.9 sec swell component from a different direction. Figure 39 shows the resulting correlation. The dotted line connecting the measured trial data is strictly a flight of fancy by the second author. The data seem to indicate the existence of a heading error of about 40 deg. The correlation was not comforting to those seeking assurance regarding the validity of SMP predictions. The second correlation effort utilized the motion data recorded for Hull X during the fall 1985 seakeeping model tests. These tests were conducted in sea states 5 and 6, at ship speeds of 20 and 25 knots and at headings of 0 deg (head), 315 deg (port bow) and 105 deg (starboard quarter). Also, roll decay experiments The USS Midway Blister Story 463

36 i i i BUSTERED CV 41 TRALS (16 DEC) DUSTERED CV 41 SMP 3 -- ~ PRE-UUSTERED CV 41 SMP. - " "e-.., = == =.=. 1:.... O NORTH EAST SOUTH WEST TRUE SHP COURSE, DE6 Fg. 39 SMP--Midway trial correlation: ship speed = 10 knots, sig. wave height = 8.7 ft with the ship's 5-ft-width bilge keels were performed in calm water at speeds from 0 to 25 knots at 5-knot increments. The results of the roll decay experiments showed that SMP overpredicted the roll decay coefficient by about ~ at zero speed. The overprediction magnitude decreased as speed increased down to only a few percent at 25 knots. The motion correlations showed that SMP overpredicted pitch by about 7 percent on average, heave by about 6 percent on average, and underpredicted roll by about 21 percent on average. The third correlation effort involved tests of Midway Hull X (using the existing large, old model) in bidirectional seas. This effort was designed to validate SMP's ability to predict ship motions in cross seas. The tests were performed in the MASK facility at DTRC, which has wavemakers on two adjacent walls of the rectangular basin. Thus the bidirectional seas generated were always at right angles to one another. Various combinations of an irregular sea state 5 with a 9.2-see modal period and a regular wave (swell) with an 11.9 sec period were used. Ship speed was always 10 knots. Ship headings were: (1) one wave component on the bow and the other on the starboard beam, and (2) one wave component 30 deg off the port bow and the other 60 deg off the starboard bow. Much more space than is available here would be required to properly describe these experiments and their results. Very briefly, the motion correlations showed that SMP overpredicted pitch by about 4 percent on average, under- predicted roll by about 19 percent on average, and underpredicted lateral accelerations at the flight deck by factors greater than 2 on average. The fourth correlation effort involved roll decay tests for various bilge keel alternatives fitted to Midway Hull X (using the existing large, old model). The tests were conducted in calm water over a range of speeds. Four cases were examined: no bilge keels, current 5 ft-0 in. bilge keels, twin parallel 5 ft-0 in. bilge keels 'and 8 ft-o in. bilge keels. The tests showed that the twin bilge keels were ineffective; presumably they were close enough together so that the interference effects were large. The girthwise distance between the keels was set by the criterion that the added keel, above the existing keel which extended to the baseline, could not extend beyond the hull's maximum half-breadth. The roll decay coefficients measured for the other three cases were compared with the corresponding SMP predictions. The results showed that SMP substantially overpredicted the roll decay coefficients; an example is shown in Fig. 40. The overall conclusions from the brief Midway SMP validation effort were: Maximum differences between measured model pitch and heave and the corresponding predictions were within percent. On average, measured pitch and heave were about 5 percent less than SMP p, edictions showing good agreement. NO BLGE KEELS 5 FT BLGE KEEL 8 FT BLGE KEEL " ~ - ~... _ -., _--- o, o ~ ~ - - ~ ~ o. o ~ - ~ _ - ~ ~-. _~: ~ ~_~-~.~,~, ~,~ U 0.06~ :: T o. n r ~ - - i ~ ~ - ~ = ~ -_.,... ~,-~-=- :~---~- ~:~ u.u~f _ ~_-. -k~- --- ~ ~ 1 0, 0 5 ~ - ~ = ~ ~.~--~.'-^ :-~"6 "1 2" 3'" 4'=~5 ~" 6 ~ 7 81~'~0"~'1~2:--'~3~4:'~5 ~:6~7 : 8l~ " MEAN ROLL AMPLTUDE, ~, (DEGREES) Fig. 40 Roll decay coefficient comparison: February 1987 experiments versus SMP predictions, ship speed = 15 knots 464 The USS Midway Blister Story

37 On average, measured model roll amplitudes were about 20 percent higher than SMP predictions. Measured model roll decay coefficients were as much as 40 percent less than predicted. Measured model lateral accelerations at the flight deck were more than twice as great as predicted. Later in the Midway effort, when the results of the 20-ft model seakeeping tests became available, some hasty comparisons of SMP predictions and measured model motions were made. Examples are shown in Fig. 33 of the paper and in Fig. 41. Collectively, these comparisons were disconcerting and, again, they failed to enhance the credibility of SMP predictions in the eyes of top decision-makers. ROLL PTCH A ) UJ ll tu 9 W -- null A-$MP 0 D Hull A-EXP A /) 1 U,J LU CC (3 f~ 0.8! W D -- 1 ]1 A-5 ft Bilge geel-smp O Oxu t A-S ftdbtlge Beet-ZXp <<...J UJ _J-. 1 OO =:~ 1.0.5,-;- o Z r3 o j j J J J J! HEADNG << - 1 L 0.4 o " ~!!! los 120 1) io0 HEADNG Fig. 41 Model to SMP correlation--hull A; long-crested, sea state 5, ship speed = 20 knots Discussion M. MacKnnon, Member and R. S. Johnson, Member [The views expressed herein are the opinions of the discussers and not necessarily those of the Department of Defense or the Department of the Navy. ] We appreciate the opportunity to comment on such an extraordinary paper. The authors are uniquely qualified to have addressed the subject of the Midway blisters and are to be complimented on this very important contribution to our literature. There is a fundamental, underlying lesson here, familiar to all those who have performed design tasks under pressure of tight, unyielding schedules. There is a gestation period for any design. Acceleration of this period will invariably give a premature product, the success of which is heavily dependent on the complexities and risk of the project. When departures from the norm, particularly in the area of hydrodynamics and seakeeping, are involved, then history teaches us that we are far more liable to experience problems with such a premature product. The Midway story is reminiscent of a problem that goes back about 20 years. The U.S. Navy, to solve anticipated problems handling the Deep Submergence Rescue Vehicle (DSRV) in a seaway, designed a submarine rescue ship, the Pigeon (ASR-21) Class. The ASR-21 was configured as a' catamaran to enable handling of the DSRV between the hulls, close to the center of motion. This was thought to be clearly the best technical approach. Extensive model tests were initiated and the preliminary design proceeded with the designers concentrating on the anticipated structural problems associated with hydrodynamic loading in the cross structure, particularly in bending and shear. The model test reports, including those for seakeeping, were not carefully analyzed during contract design. These tests did show indications of synchronous motions in r011, pitch, and heave. The initial trials and operatiori of these catamarans showed very poor seakeeping, involving crossstructure slamming and severe corkscrew and ship whipping motions. The resulting scramble to analyze and fix the problem uncovered the model test clues that were there all along, but they came too late in the process to analyze and utilize. The "notch-in-the-blister" in this case was a foil up forward and between the hulls. n December 1987, the largest SWATH design in the world, the T-AGS (Ocean) or T-AGOS 23 was presented for release to contract design. The presentation pointed out that all the seakeeping and maneuvering requirements were not being met. As a result of the old Pigeon experience and, more point- The USS Midway Blister Story 465

38 edly, the more recent Midway experience, a hiatus was declared until the proper handle on the design could be obtained. An independent analysis was made that concluded that more model testing was needed, that the design criteria were stated improperly, and that the schedule being followed did not allow adequate time for the necessary analysis and model testing. As a result, the schedule was revised and proper analysis done, reducing the risk and raising the confidence in the ultimate performance of the design at sea. This paper has highlighted these situations for us in a very clear and relevant way. To paraphrase and expand on the authors' conclusions, the following is presented as a summary to these remarks. 1. When a new hydrodynamic geometry is being proposed, extensive analysis and testing must be part of the design process. n particular, we should be careful to look for fatal flaws. Such fatal flaws as synchronous motions will invariably be present for some sea conditions. We need to determine under what conditions they will occur and take the necessary steps to accommodate corrective action. 2. Considerable care should be taken to insure that the seakeeping and maneuvering requirements, as well as other requirements, properly reflect operational needs. We must communicate with the operators to determine their real needs. Then it is our responsibility to interpret those needs into meaningful technical requirements. 3. Hydrodynamic requirements should normally be stated in probabilistic terms because we are dealing with nondeterministic processes. We congratulate the authors for their efforts in describing this experience, which is of considerable benefit to the profession. We request that they offer their further views to expand on the lessons we should learn from these experiences. Bruce L. Hutchison, Member The authors and the U.S. Navy are to be commended for this paper reporting extensively on one of the most comprehensive and remarkable seakeeping study and design efforts ever undertaken. This paper is the more rewarding because the vessel and the problems are real and the solutions sought had to be practical and capable of development within a short time frame. Thus this paper addresses the real world of practicing naval architects. There is much in this paper that merits comment and additional discussion. will limit my remarks to two points and hope that other discussers will address further points among the numerous possibilities. n the conclusions, high priority is given to the development of improved ship motion criteria for the determination of ship operability in rough water. This is a crucial problem that have addressed in several papers and in comments presented on a number of occasions. Our present ability to predict and analyze ship motions exceeds our capacity to judge the results, once obtained. Further advance in the practical application of ship motion analysis is strongly coupled with our ability to develop improved motion criteria. Development of improved ship motion criteria will require a major effort. As the largest ship operator in the nation, operating a fleet of diverse vessels, large and small, with different missions, the U.S. Navy is in the best position to lead the way to improved motion criteria. n reference [17] (additional references follow some discussions) Dr. Jagannathan and proposed a general framework for the development of ship motion criteria. The specific audience for that proposal was the community of research vessel operators, but the proposed framework is general in character and could be applied to other vessels and missions. n that proposal it is recognized that motion criteria should be functional in nature; that is, they should relate to specific activities and functions. We recognize that different activities will be sensitive to different motion processes. Some may be sensitive to joint rather than pure processes. We believe that historically motion criteria have too much emphasized motion displacements and too little acknowledged the predominant role of accelerations, particularly local accelerations in vessel coordinates. t must also be recognized that human factors often enter into the determination of operability and any effort to develop improved motion criteria should include consideration of appropriate human factors. The second point wish to discuss is the recommendation that abilities be improved for predicting the local accelerations perceived in vessel coordinates by objects on the deck of a moving ship. This topic has long been of importance to The Glosten Associates due to the practical problems associated with cargo stowage on barges, and it has been an area of particular interest for this discusser. During the mid-1970"s developed a post-processor to compute the statistics of the local acceleration process in vessel coordinates, phase co-factors, cross co-spectral moments and induced forces and moments from transfer functions obtained from standard ship motion programs [ 18]. More recently we have modified this post-processor to accept transfer functions from the U.S. Navy's Ship Motion Program (SMP) and during the past year we implemented these same computations internally in SMP. Others, am sure, have independently implemented similar calculations of the total local accelerations in vessel coordinates. We agree that the total local accelerations in vessel coordinates are important ship motion processes. am tempted to declare that they are the single most important ship motion processes for most shipboard activities. To exploit the full significance of these processes one must also include information regarding the joint processes between the various acceleration components, for instance, the joint process between deck normal acceleration and acceleration in the plane of the deck. This joint process is, by way of example, essential to the determination of when an object will begin to slide across the deck. Such joint information is to be found, for instance, in the cross co-spectral moments [19] of the components of the total local acceleration process in vessel coordinates. The kinematics of the total local acceleration process in vessel coordinates are discussed in great detail in a recent Local Section paper [20]. Contained in this reference are some observations that could be of assistance in the formulation of "approximation" techniques referred to by the authors in their recommendations. Additional references 17 Hutehison, B. L. and Jagannathan, S., "Monohull Research Vessel Seakeeping and Criteria" in Proceedings, OCEANS ' Hutchison, B. L. and Bringloe, J. T., "Application of Seakeeping Analysis," Marine Technology, Vol. 15, No. 4, Oct Hutchison, B. L., "A Note on the Application of Response Cross Spectra," Journal of Ship Research, Vol. 26, No. 2, June Hutchison, B. L., "The Transverse Plane Motions of Ships," SNAME, Pacific Northwest Section, Oct. 1988; revised Aug The USS Midway Blister Story

39 R. A. Wilson, 9 Visitor [ The views expressed herein are the opinions of the discusser and not necessarily those of the Department of Defense or the Department of the Navy.] As Commanding Officer of the USS Midway (CV 41) from April 1987 to February 1989, had the opportunity to personally observe the ship's seakeeping characteristics in a variety of wind and sea conditions. Midway's roll and wetness problems had been identified and well-documented.prior to my arrival. Therefore, concentrated my efforts on establishing operational procedures which would allow the ship to continue to perform her mission of providing carrier presence in the Western Pacific and ndian Oceans while waiting for NAVSEA to develop solutions to these post-blister installation discrepancies. would like to begin my comments by reemphasizing the fact that most of the objectives of the 1986 blister installation were realized. Midway's hull was strengthened, damage stability characteristics were back in spec due to increased reserve buoyancy and directional stability had been improved by larger rudders and the centerline fixed fin. The increased buoyancy located well outboard also greatly enhanced the ship's performance during turns. Prior to the 1986 blister installation, Midway's bridge teams used a rule of thumb called the "Rule of 27" during turns to ensure the ship's heel was kept within acceptable limits of not greater than approximately 2 deg. For example, if the ship was making 20 knots through the water, a maximum of 7 deg of rudder could be used ( = 27); with 22 knots rung up, a maximum of 5 deg of rudder ( ) and so on. n Midway's post-blister configuration, bridge teams could use as much as 25 deg of rudder at speeds in excess of 20 knots without exceeding one half of a degree of heel. This improved seakeeping behavior significantly enhanced the ship's overall tactical capability both during aircraft handling evolutions and when performing defensive maneuvers such as those required for torpedo evasion. The primary discrepancy resulting from the 1986 blister installation was clearly the decrease in the ship's natural roll period from 18.6 sec to approximately 12.2 sec. f a plot is made of the probability of encountering waves or swells of various periodicities while operating in the many oceans of the world, it forms somewhat of a bell-shaped curve with a very low probability of encountering waves / swells with periods less than 2 sec or greater than 17 sec. Midway's post-blister roll period of 12.2 sec places her within the upper and lower bounds of this probability curve. The result of this is that anytime Midway operates in seas with 12.2 sec waves or swells approaching within 15 to 20 deg of either the port or starboard beam, her natural roll motion becomes excited and the ship begins to roll. This phenomenon occurs even in very low sea states. The roll quickly dampens out when the ship is either turned into, or away from, the beam seas. Flight deck respots and other aircraft handling evolutions could almost always be accomplished by avoiding a ship's course which would result in beam seas with a 12.2 sec periodicity. However, a major problem area did exist during aircraft launch / recovery operations whenever the local wind (and associated waves) was present in conjunction with a Rear Admiral, USN; Deputy Director, Unified and Specified Command J-6, Joint Staff, Washington, D.C. sec swell coming from a direction of 90 deg to the left or right of the true wind vector. Strict crosswind limits for launching and recovering aircraft dictate that the carrier keep the relative wind within 5 deg of the bow and angled deck for launches and recoveries, respectively. Flight operations were, in fact, canceled on several occasions during my tour as Commanding Officer because of excessive ship's roll during these conditions. would also like to add that under heavy sea states, the amplitude of the roll angle would continue to increase if the ship remained on a course which resulted in beam seas with a 12.2 sec periodicity. On one very memorable occasion, the ship's roll angle rapidly built from approximately 5 deg to greater than 23 deg within four successive roll cycles before the ship was maneuvered to eliminate the beam sea conditions. did not consider flight deck wetness to be a major problem during my tour as Commanding Officer. Removal of the forward scupper extensions and their boat guards significantly reduced the amount of spray generated on the forebody of the ship. However, the secondary bow wave generated by the hull shoulder did continue to cause wetness, particularly on the forward aircraft elevator located on the starboard side of the ship. This was primarily a problem when the elevator was in the down position (hangar deck level) at ship speeds above 20 knots. The proximity and size of the shoulder wave at these speeds frequently resulted in elevator spray and wetness in all but the calmest of seas. The operational work to get around this problem was to slow below 20 knots, conduct elevator moves as required, and then reestablish the desired higher speed. n conclusion, the roll problem can be corrected by increasing the ship's natural roll period so that it no longer lies within the upper bound of the aforementioned periodicity curve. This will eliminate the possibility of the ship getting into a natural resonance with the local seas. The remaining wetness problems are not of a magnitude which would justify the money required to incorporate the proposed fixes in the current austere funding environment. My congratulations are extended to the authors for a most professional and interesting paper. Donald N. McCallum, Member [ The views expressed herein are the opinions of the discussers and not necessarily those of the Department of Defense or the Department of the Navy.] This paper is so comprehensive in scope that it is almost impossible to comment on, in a coherent manner. But shall give it my best effort. As one of the NAVSEA participants in the Midway Motions mprovement Program can look back upon a time of in-depth engineering challenges conducted in an abbreviated time frame by highly motivated engineers. Truly, with apologies to Sir Winston Churchill, "Never in the course of NAVSEA's history... has so much... been accomplished... by such a dedicated few." n conjunction with the work described herein, NAV- SEA has made improvements in the procedures which govern the model test requirements, planning, transfer of information and instructions, data retention and model inspection. These procedures ensure that our model testing will be more disciplined in the future. These procedures were completed by NAVSEA and DTRC early in 1989 and are now being followed. The USS Midway Blister Story 467

40 would like to emphasize a basic finding of this paper: that the section area curves should be given much attention during the early stages of design. Figure 8 highlights the bulges in Hull X's section area curve which exacerbated the wetness problem. n our computer-generated lines efforts we must take the time to study section area, and not gloss over this critical, and basic, design step. Concerning wetness assessment, wetness merit factors, and such, although care was taken during the experiments, as evidenced by Fig. 16, still would caution that the results be taken with a grain of salt. n my opinion, the state of the art was being pushed too far too fast, with possible errors i n the interpretation of data. ssues of spray generation, wind effect, scaling and run time warrant further investigation. Large bilge keels of 8 ft-3 in. span are addressed on pages and in Figs. 26 and 33. The statement is made that there is "a significant technical risk associated with such bilge keels." would question seriously the technical risk. A 3.5-ft bilge keel fitted to a 400-ft frigate is equivalent to bilge keels of about 8 ft SPan on Midway, and the stresses due to rolling would be proportionately less on Midway due to its longer roll period. The increased wetness associated with the larger bilge keels, as indicated by model experiment, is questioned, considering the uncertainties of the wetness merit factor. wo.uld be remiss at this point ff did not acknowledge Mr. Peter Gale for his exemplary leadership during this project. Pete dedicated himself to living, breathing, inspiring, questioning, thinking, and solving the multitude of problems which have been addressed in this milestone paper. seriously doubt that anyone else could have led this project with s.uch thoroughness and professionalism. t was a high point in many of our careers to work under his direction. Thank you, Pete. Lewis Molter, Member [ The views expressed herein are the opinions of the discussers and not necessarily those of the Department of Defense or the Department of the Navy. ] The authors deserve considerable credit not only for this paper but for their dedication to the ship and this work. Their steady nerves under the pressure of this highly visible project are to be commended. t is difficult to make any serious criticism of this work. The project included a tremendous number of people and all had ample opportunity to voice an opinion, or recommend additional work, etc. Every comment, including several that could have easily been ignored, was given full consideration by the authors. would like to address the accuracy of some aspects of the work. The accuracy and correctness of almost every piece of information obtained throughout the project were questioned and checked very carefully. The motions predicted using the David Taylor Research Center Ship Motion Program (SMP) did not agree exactly with motions measured during the model experiments. The agreement between measured and predicted pitch and heave was better than for roll, but all were within the accuracy limits expected of the program. The program is not perfect and can always use improvement; however, the model experiment measurements are not perfect either. The primary purpose of SMP or any of the several other ship motion programs in existence today is to make relative comparisons of the seakeeping performance of ships. 468 The USS Midway Blister Story Ship motion criteria and criteria limits are the link that correlates the sailor's gut feelings about when to limit operations with the numerical values of ship response from predictions, model experiment or ship trials. The use of ship motion criteria to predict ship operability limits serves very well to put the thousands of numbers calculated by motion prediction programs into some perspective and to show the practical implications of these numbers. However, the operability prediction process introduces another source of error associated with criteria and criteria limits. t became obvious early in this project that the roll criteria limits were not accurate for Midway, and that a lateral acceleration criteria should be added. The roll criteria limits for Midway were improved considerably as presented in the paper. The lateral acceleration criteria were too involved to be developed in time to be of use for the Midway Motions mprovement Project. t is hoped that the lateral acceleration criteria and prediction capability to use it will be developed even though the concerns about Midway have diminished. The pitch criterion did not appear to be a limiting factor for Midway but doubt it is any more accurate than the original roll criterion. The accuracy of the natural roll period determination is worth considering. The authors presented the problems involved in arriving at the official value for natural roll period. Nobody kept a record of all the values and the accuracy of each value so that a proper statistical analysis could be made. Values were obtained from acceleration measurements, stopwatch readings by several people at different times, sallies in calm water, records from the ship gyro, and from ship videos at sea. As the authors said, the values ranged from about 10 to 13.1 see. An appropriate value was selected to be the official value but the actual value could easily be 0.5 see higher or lower. doubt if the natural period of any ship is known to any better accuracy or needs to be. The authors have experienced the limits of the ship design process first hand and have put together a fine summary of the process and a fine set of positive recommendations to improve the process in the future. William H. Garzke, Jr., Member The technique of adding additional beam to ships for the purpose of gaining additional buoyancy, stability, strength, and protection against damage is not a new concept, but one that requires much design work and model testing as the authors have described in this excellent paper. Bulging in battleships and aircraft carriers was done to achieve more buoyancy to support the additional defensive and offensive capabilities that new technology brings about. The one point not discussed in this paper is the measure of importance of bulging in the underwater defense of the Midway-class carrier. t is a sensitive issue and one that the authors could not discuss here. Based upon published sources [ 21,22 ], bulging does provide additional underwater defense against torpedo attack. One example of bulging was the French battleship ]jean Bart (see Figs. 42 and 43 herewith). The existing shell of]jean Bart was removed and a new teardrop-shaped bulge replaced in This was done to reduce the draft, which in her sister ship Richelieu had exceeded design requirements. t brought the armor deck to a prescribed distance above the design waterline of 10.0 m. This hull expansion also provided additional protection against torpedoes and bombs which had grown in explosive power during World War and had exceeded

41 B(~timent de ligne "R CH EL'EU" Coupe ou rno;ft,e /.,/&.///~. "Jean Bart" - Coupe au maltre Echelle: /00 " i~' =, Lt.-. ".. Jt ~ --7'ffm,~- _e~ i[ ~m'~mm~,! /- ~ \ L ~ -- -t -_.--- ;.:= ' 0 Fig. 42 design values. These bulges were so configured that there was actually a slight improvement in speed of knots. would like to add emphasis to one of the authors' conclusions regarding hard shoulders into the sectional area curve, which can lead to deck wetness. This problem is also prevalent in the owa-class battleships, which have good freeboard but are wetter from Turret aft than seems proper for ships of their size. This wetness was created by a hard shoulder in their sectional area curve, resulting from a sudden and extreme widening of the hull lines forward of Turret. The narrow bow was necessitated by displacement limitations and speed considerations. The hull sections outside Turret were a compromise between the turret's shell capacity and bow entrance requirements for speed and power. As a result, the ships are wet in rough seas of gale force or more. This paper has much fascinating data and information and will be a much sought-after reference in future studies of bulging ships. Additional references 21 Garzke, W. H., Jr., and Dulin, R. O. Battleships: Allied Battleships in World War, Naval nstitute Press, Annapolis, Md., Sims, P., "Bulging Warships," Naval Engineers Journal, Nov. 1989, pp (EDTOR'S NOTE: The remaining discussions were oral comments from the floor.) C. Graham, Member -[-The views expressed herein are the opinions of the discussers and not necessarily those of the Department of Defense or the Department of the Navy. ] 'd like to pick up on a comment that Admiral Ricketts made in introducing this paper: To err is human, but it's The USS Midway Blister Story [o:o',olo;olo _L.. t Fig. 43 against company policy. Of Course, we also know another saying, that a glass is half full or a glass is half empty depending on your perspective. And you know, you're darn right that to err is human, especially when you reach out into the unknown, extend the state of the art and take on a great challenge. And to keep a 45-year-old aircraft carrier operating effectively with modern aircraft and all the associated growth is reaching out into the unknown. As you have heard stated earlier, many of the improvements to which we reached out were successful. A few were not. And the question is now, do we consider that the glass is half empty or half full? would submit that it is well over half full. We've learned a tremendous amount about hydrodynamics due to the work of Admiral Ricketts and Peter Gale and his team; and when we embark on the next large ship design, whether it be an aircraft carrier or whatever, we will have considerably more knowledge to work with. So would submit that the glass is well more than half full and think we have a tremendous debt of gratitude to Admiral Ricketts and Peter Gale. also submit that we the company--the United States Navy--have to be more tolerant of errors ff we're going to reach out into the unknown or we will be overly conservative and never make breakthrough-type progress. Na Salvesen, Member The Navy deserves all the credit for having f'lxed this problem the second time. But we shouldn't forget about why it went wrong the first time. Seakeeping predictions are extremely complicated. We have model testing and codes and so on, and it's probably getting too complex for us sometimes. We're forgetting about the real basic thing because the roll period is the only thing you need to know. f we had known the roll 469

42 period, we would never have put the blisters on. Why didn't we know the roll period? t was mispredicted--the roll mass moment of inertia would be 36 percent! That's a large percentage. The added mass moment of inertia was included twice, and the roll radius of duration was assumed to increase in the same manner as the beam increased. And that was also wrong. Those two errors together resulted in 36 percent error in the mass moment of inertia. think we should address this. What can be done to prevent this from happening? think in the Navy with these fire drills that they have and these urgencies, you have to design this; you have to get it done so-and-so fast, you have these complex tools, and you can miss the fundamental thing that must be done correctly. What is the number one thing? n roll, the number one thing is the mass moment of inertia. We cannot afford to be off by 36 percent. And we shouldn't forget that because if you had done it correctly, you would never have put the blisters on. Rod Barr, Member The authors and at least two of the discussers have mentioned this question of the need for caution in action. 'd like to just mention that with regard to this idea, think it's very important. One of the things that could be learned from the Midway is to maintain a historical perspective. 'm not saying this incident 'm going to relate to would have prevented what happened or changed the course of events, but it might at least have led people to think a little bit more about it. 'd like to note that there was uncovered during the course of the Midway work a rather striking similarity or parallel between the problems that were encountered with the Midway regarding reduction of the roll period and the results thereto with the problems encountered by the U.S. Navy during the 1920's in building ton cruisers to try to meet the treaty restrictions that were imposed after World War. The beams were increased. Several classes of cruisers in this size were built and operated very successfully with regard to seakeeping in an attempt to, think in part, beat the rules. New classes were designed which had greater beams and in which the roll natural periods were reduced from about sec to about 12 sec--a very strong parallel with the present case. And those ships were found in general to be not very well liked from the standpoint of their seakeeping ability. They were found to roll rather severely in certain types of seas. As a result, certain actions were taken, including the use of larger bilge keels and free-flooding tanks, which was one of the concepts considered and ultimately not selected for the Midway. think that if we as a community, as an industry, were to pay more attention to historical perspective and keep fresh what has happened before, we'd find that these things do tend to reoccur. And if someone had remembered those incidents, it might have given some thought as to what was done, and perhaps the course of events might have changed somewhat. At least the implications of the actions might have been better understood before the fact, which would have probably considerably reduced the flap that occurred after the ship put to sea. Authors' Closure We shall respond to the written discussions first, in an order somewhat different than that in which they were given. First, Lew Motter. 470 The USS Midway Blister Story Lew was a key member of the Midway fix effort. He was our right-hand man at the model basin, and we'll always be grateful for the support he gave us throughout the whole thing. Lew made three main points. First, on the question of the validity of SMP, he said that neither SMP nor the model test results were perfect and that the pitch, heave, and roll correlations were all within the accuracy limits expected of the program. We can accept both of these statements but, nevertheless, we'd like to see improvements made in SMP's roll and lateral acceleration predictions. As pointed out in the Appendix to the paper, roll amplitudes were consistently and significantly underpredieted by SMP by about 20 percent on average. Lateral accelerations differed from model test results by factors of two or more. Discrepancies of this magnitude can be misleading and lead to faulty design decisions in the future. So we feel that improvements should be made in these areas. On limiting motion criteria, we agree with Lew that they're a vital link between ship response predictions and true ship operability limits. We agree that the limiting pitch criterion used in the Midway studies was probably no better than the initial roll criterion. The subject of limiting motion criteria is an area which needs a great deal of additional work, as was also emphasized by Mr. Hutchison. On natural roll period determination, we agree extreme accuracy is not required and that varying results were obtained for Midway using a wide variety of methods, some of dubious accuracy. However, we would point out that a careful deep-water sally and a careful statistical analysis of ship motion records yielded essentially the same result, significantly less than the in-dock sally result. Hence, our conclusion that restricted water effects can be substantial and should be avoided, especially ff the intent of the sally is to check a ship's Gill and hence vertical center of gravity. Admiral Wilson took command of the Midway in the middle of our fix efforts and we linked up with him shortly after. He since has had two years of operating experience with Midway. Also, he has an unusually fine understanding of ship motion principles for a naval aviator turned ship driver. We thank him for taking the time to comment on the paper. We appreciate his pointed reminder that the 1986 blister corrected critical strength and damage stability deficiencies. His discussion of the ship's greatly improved tactical capability resulting from the high Gill and hence the reduced heel in high speed turns was fascinating. This is another plus for the current blister which we failed to mention in the paper. Admiral Wilson makes the point that the most severe operational effects of the '86 blister are experienced during aircraft launch and recovery operations in the presence of a head wind and a resonant swell on or near the ship's beam. n this scenario the ship can't change heading and may have to cancel air ops ff the rolling becomes too severe. When visited the ship in December 1986, observed this exact situation. On that occasion, air ops were conducted with great difficulty due to a swell on the ship's starboard beam which could not be seen. The local wind-generated sea state on the port bow was moderate, 'd say about sea state 4. n the first paragraph on page 436 of the paper, the results of an analysis of Midway motions in multidirectional hindcast northwestern Pacific sea spectra are described which tend to corroborate Admiral Wilson's observations.

43 For example, significant single amplitude roll angles of 1.4 to 3.3 deg were predicted for the current ship in head seas based on the dominant wave direction with significant wave heights of 12 to 15 ft. These roll amplitudes were about three times greater than those for the pre-blister ship. We are honored to receive comments from the Navy's Chief Engineer and his principal Technical Deputy, both friends of long standing. As might be expected, they get right to the heart of the matter and focus on the lessons learned from our experience. This is a major contribution to the paper, whose conclusions and recommendations have a narrower technical focus. We wholeheartedly endorse the three lessons which they put forward. We would go further and suggest that they be carved in stone and bolted to the bulkheads in ship design and program offices in Washington and elsewhere. Our thoughts on additional lessons were requested. We would suggest lesson number four. "n areas like seakeeping, depart from traditional design guidelines with great caution." Too often these days computer outputs and the results of complex systems analyses and operability assessments are being seized upon to drive design decisions which fly in the face of design guidelines based on generations of hard-earned experience. A case in point is at the heart of the Midway motions problem, too great a GM. Many of us were taught years ago that too high a GM will result in an unsatisfactory ship with a snap roll and high lateral accelerations topside. We learned that the GM designed into real ships generally reflects a compromise between the low GM favored for good seakeeping and the higher GM favored for good intact and damage stability. Further, we learned that the GM-to-beam ratios for most ships will lie in the 2 to 5 percent range. Surface warships will have higher values due to their stringent damage stability requirements, normally in the 6 to 10 percent range. But these ships will have less than optimum seakeeping qualities due to their higher GM's. Values above about 12 percent are to be avoided like the plague. This has been the traditional design guidance. Midway's pre-blister GM-to-beam ratio was about 8 percent and currently post-blister is over 18 percent. The proposed notch would reduce her GM-to-beam ratio to below 12 percent. Why did we so readily accept a blister configuration which yielded such a high GM-to-beam ratio? The dominant reason in my view was that too much faith was placed in computer roll motion predictions, which turned out to be low by about 20 percent in amplitude and high in natural period by 10 percent or more, compared to a limiting roll criterion which turned out to be too high by a factor of about two and a half. Too little consideration was given to the traditional design guidance that GM-to-beam ratios over about 10 to 12 percent will likely cause big trouble. This is not to say that we shouldn't do computer studies to identify areas of potential improvement but rather to suggest that when an improvement is identified which lies outside of our experience phase, we should proceed with extreme caution. This gets back to the discussers' lesson number one-- extensively analyze and test a new hydrodynamic geometry. And lesson number two--be sure you understand the operator's criteria of acceptability. f there is not sufficient time to analyze and test an innovative geometry, one should retreat and stay within well-known territory. Finally, we propose lesson number five. f seakeeping behavior is a ship design issue or risk area, then a model with realistic topsides should be built and those principally responsible for the design should ride the carriage and observe the critical model tests firsthand. Too often the naval architects responsible for a total ship design delegate responsibility for the hull form design to a subordinate who in turn orders model tests and then reads about the results many, many months later in a dry report. This is a recipe for disaster. We are grateful for the historical perspective Mr. Garzke has brought to the discussion and for his making reference to the excellent recent paper by Mr. Phil Sims, which is recommended reading. First, to clarify semantics. A blister is added external to the existing shell plating of a ship while a bulge is added after the existing shell plating is first removed. Thus a bulge will add less weight to a ship than will a blister of the same configuration. Regarding the effect of blisters and bulges on torpedo defense capabilities, suffice it to say here that such additions do provide improvements, generally blisters more so than bulges since blisters add an additional layer of steel in addition to increasing the stand-off of the attacking weapon. n the case of Midway, these improvements were gratefully accepted but they were not the motivation for the blister. We were very much interested to learn of the hard shoulder in the owa-class battleship sectional area curve and the apparent linkage between this shoulder and increased wetness aft of Turret. As we saw with Midway, a hard shoulder creates a secondary wave which reduces local freeboard and, ff breaking, acts as a source of windblown spray. We have long admired Mr. Hutchison from afar through his published work. We thank him for his insightful comments and for the valuable references he has added to the paper. We are pleased Mr. Hutchison agrees that improved motion criteria are urgently needed and that accelerationbased criteria are of dominant importance. We agree that the U.S. Navy should play a key role in the development of improved criteria, but we feel that a broader-based effort would be appropriate, perhaps led by the cognizant panel of this Society. We concur with Mr. Hutchison's comments as to the importance of the total local accelerations in vessel coordinates. We are encouraged by his report of the progress made in predicting these accelerations, which is supported by similar recent comments we have heard from the experts at the model basin. We thank Mr. McCallum for his comments. He was a key member of the Midway fix team and stuck with us all the way through. We endorse his emphasis on the importance of the sectional area curve in the hull form development phase. We agree that further research into the subject of wetness assessment is needed. The approach adopted for Midway certainly needs to be tested against full-scale experience and other possible approaches for a variety of hull types before it could be generally accepted. One comfort was that the Midway approach gave merit factors which were repeatable and generally consistent with one's intuition. On the matter of the risk associated with very large span bilge keels, Mr. McCallum's view is that this risk is not significant. He might be right, but there is no way to know until such keels are put to sea. CO-5, concerned about the possibility that unforeseen problems might crop up, argued that installation should be deferred until at-sea experience had been gained with the notch itself. Then if further motion improvement was deemed necessary, the bilge keels would be a ready option. The USS Midway Blister Story 471

44 Finally, we would again like to thank all the Midway team members for their dedication and professionalism and also to thank the discussers, whose contributions have added greatly to the value of the paper. (Following are additional oral comments from the floor and authors' responses.) F. H. Sellars (Member, 2nd session moderator)-- wanted to add a couple of points on roll in general. Roll is a highly nonlinear response compared with others. Damping is well known to be nonlinear, but also the roll restoration, which means that the roll period is nonlinear. So even though you calculate a roll period carefully, in the real world you'll see other ones depending on how high this ship rolls. 've done model tests where in a sally test and a model test you generally displace the hull over 20 deg and let it decay. n the first few cycles, the period may be one or two seconds different than it is in the low-amplitude case, which corresponds to the calculation. So the question on the roll period is maybe the solution here was to vote on the right roll period, which probably is practical in a nonlinear case like that. Another point on the uses on SMP or misuses. The reality of today is that 'm working in a firm designing roll stabilization equipment, and we're seeing shipyards coming in with Navy specification requirements that specify SMP type-results we have to design to. t causes problems in some cases. So the code presently is being used as a design requirement. With that reality, think that the modification of the code should be expedited. lack Abbott (Member)--'ve been involved with naval ships for about 25 years off and on, and 've never yet seen one get lighter or have the GM increase over time. That's what prompted the 1986 blister. t seems to me that the team succeeded too well. Perhaps they had something up their sleeve. would be interested to know their perspective on whether this situation might simply go away as the ship is in the natural cause-effect and its additional weight added topside. P. Gale (author's reply)-- think the short answer to that is there's not enough time left. [ Laughter. ] There's not enough time left in Midway. That would happen think if we kept her around for another 20 years. But right now she's got lots of margin designed in. didn't respond to the other extemporaneous comments. appreciated Captain Graham's comments very much. Dr. Salvesen, ff understood him correctly, said that we had made a substantial error in the rotational mass moment of inertia, which is correct, and hence we were significantly off on our roll period estimate, which is correct. But then thought heard him say that if we had known what it really was, we would not have added the blisters to the ship. Well, that's not true. think if we had known exactly what it was, we would have added blisters with somewhat less waterplane inertia. But the ship needed blisters and should have them. (Unidentified commenter)--one of the key statements here was that unfortunately the computer program used to access the seakeeping operability could not handle multidirectional asymmetric wave spectra on the Midway assessments. To my knowledge, that has not been corrected. n other words, very few people are working on the ability to input actual sea data into these computer prediction programs, and we're asking for that to be done. P. Gale (author's reply)-- thought might have heard that at the model basin they have cranked that capability into SMP now. My impression was that one of the things that came out of this effort was the ability to crank in the multidirectional spectra with swell components and so on into the operability assessments. But agree with you 100 percent. f that hasn't been done yet, it should be done quickly. That's needed very much. But it's a fact that when we were crunching these numbers, the capability wasn't there. (Unidentified cornrnenter)--f might add, the Society's Seakeeping Research Panel has been prodded by this effort over the past year and is planning to organize a minisymposium to look at current state-of-the-art techniques in ship motions programs from the user's standpoint as well. And we hope to work closely with the folks at the model basin and get them involved in this and come up with perhaps enhanced capabilities and ways to interpret and input the codes and things like that. (Unidentified cornmenter)-- want to make one comment. For those of you who were not here yesterday, you know that everything is relative. Three rather extensive papers were presented as part of our minisymposia on the subject of stability and capsize problems and solutions. These in effect had to do with smaller vessels--fishing vessels--in which some of the standard ways of producing hydrodynamic response and so forth in smaller vessels have been inadequate to correct and predict the response of those vessels. So by comparison, this has been no problem. [ Laughter. ] F. H. Sellars~ want to thank the authors, the discussers, and all of you for a very excellent program and presentation and discussion. 472 The USS Midway Blister Story


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