Development of a Marine Propeller with Non-Planar Lifting Surfaces

Transcription

1 Development of a Marine Propeller with Non-Planar Lifting Surfaces Poul Andersen, Visitor, Technical University of Denmark, Lyngby, Denmark. Jürgen Friesch, Member, Hamburg Ship Model Basin, Hamburg, Germany, Jens J.Kappel, Member, J. J. Kappel A/S, Hillerød, Denmark, Lars Lundegaard, Visitor, Dampskibsselskabet NORDEN A/S, Copenhagen, Denmark, Graham Patience, Affiliate Member, Stone Manganese Marine Limited, Birkenhead, England. ABSTRACT The principle of non-planar lifting surfaces is applied to the design of modern aircraft wings to obtain better lift to drag ratios. Whereas a pronounced fin or winglet at the wingtip has been developed for aircraft, the application of the non-planar principle to marine propellers, dealt with in this paper, has led to the KAPPEL propeller with blades curved towards the suction side integrating the fin or winglet into the propeller blade. The combined theoretical, experimental and practical approach to develop and design marine propellers with non-planar lifting surfaces has resulted in propellers with higher efficiency and lower levels of noise and vibration excitation compared to conventional state-of-the-art propellers designed for the same task. Conventional and KAPPEL propellers have been compared for a medium sized container ship and a product tanker. In total nine KAPPEL propellers and two conventional propellers have been designed and models of all propellers have been examined with respect to cavitation and efficiency in the open water and behind conditions. Casting procedures, measurement procedures and stress analysis methods for the unconventional geometry of the KAPPEL propeller have been developed. Furthermore, the KAPPEL propeller has been applied in full scale to the product carrier investigated. Sea trials with the conventional propeller and the KAPPEL propeller have been performed and have proved an efficiency gain of 4% in favour of the new propeller. The improved efficiency was obtained at lower propeller-induced pressure fluctuations. The correlation between the theoretical, experimental and full-scale results is discussed.

2 INTRODUCTION The majority of propellers in service today are developed by means of propeller theories based on helical surfaces with straight generator lines. This geometry is a condition for the traditional method by Lerbs and for the optimum load distribution by Betz. Even if the modern propeller may be considered a remarkably simple propulsor, it has been refined and optimised to a high degree of sophistication over the last century. This in order to design a propeller which, at a given rate of revolutions, gives the ship a specified speed, while exciting low noise and vibration levels, has good cavitation properties and absorbs the least possible power. A successful propeller is a good compromise between these requirements. Limitation and control of propeller-induced noise and vibration levels are important criteria for modern ships and represent in many cases a formidable task, for instance in case of the modern, fast, large container carriers propelled with one single propeller absorbing in the order of 100,000 SHP. The importance of a low power requirement or a good propulsion efficiency is highlighted in times with high fuel prices, but low power requirement should always be important to the naval architect due to its beneficial secondary effects on ship design. An obvious method to achieve higher propulsion efficiencies is to use a large propeller diameter and a slow-turning propeller. As this choice may not always be possible a number of unconventional propulsors have been invented, like contra-rotating propellers, propellers with a vane wheel, pre- or post-swirl stators, etc. Some of these propulsors are quite complicated from a practical point of view. In particular contra-rotating propellers are mechanically complicated, however, achieving high propulsion efficiencies A different class of propulsors with enhanced efficiency is the propeller with non-planar lifting surfaces, i.e. a propeller no longer based on a helical surface with a straight generator line. These propellers are sometimes called tip-modified propellers, cf. Andersen (1999). This propeller type can be applied in lieu of normal propellers without requiring additional precautions by the shipbuilder. However, special consideration and effort will be required with respect to design and manufacture of the propeller in order to obtain blade geometries yielding better performance than a conventional propeller. Aircraft Wings The straight wing of an aircraft is to a certain extent comparable to the conventional propeller blade. The optimum load distributions for both were deduced in the 1920ties. Aircraft wings with non-planar lifting surfaces in the form of elliptical end plates were investigated by Prandtl and Betz (1927). Their model experiments showed improved lift/drag ratios significant at high loading and diminishing at lower loading due to the frictional drag of the end plates. See Fig. 1. Prandtl and Betz hinted in their report that the end plates should be designed as lifting surfaces rather than plates. Fig.1. Increase in lift for constant drag for wings with end plates and winglets, cf. Andersen, Kappel & Schwanecke (1992) Cone (1962) examined lift and minimum induced drag on non-planar lifting surfaces. In this theoretical work the effect of induced drag was determined for families of wings with lifting surfaces configured as circular and semi-elliptic arcs and more complex forms including fins and end plates. It was found that such a wing could reduce the induced drag and thereby increase the lift/drag ratio compared to a flat elliptical

3 plan form wing with equal span. The reduction of induced drag was independent whether the non-planar lifting surface was pointing towards the suction side or the pressure side of the wing. However, the non-planar elements also induced additional lift, positive when curved to the suction side and negative when curved to the pressure side, i.e. wings with elements pointing towards the suction side were found to be the most efficient by having the best lift/drag ratio. In the mid seventies, half a century after Prandtl and Betz, NASA reconsidered the idea of unconventional wing configuration and Whitcomb (1976) developed and tested aerodynamically shaped winglets fitted to the wingtips (see Fig.1). Apart from reducing the intensity of the wingtip vortex the winglets increased the lift/drag ratio with up to nine per cent at cruising wing load. Winglets are now used on many modern aircraft, from wide-body jetliners to competition gliders. Marine Propellers and Aircraft Wings The methods describing the flow over the aircraft wing can in many cases be applied to the flow over the marine propeller blade. This is despite complications arising from the more complex flow around the blades compared to wings. The effect of applying non-planar lifting surfaces, be it in the form of winglets, fins, curved elements or even end plates is to increase the total length of the lifting surface elements (seen orthogonal to the flow) for the same wing span or propeller diameter. This can be deducted from Cone (1962). In this way the induced drag and the intensity of the tip vortex are reduced. The parallel development of aircraft wings and marine propellers is illustrated in Table 1. According to Andersen (1999) four separate and distinct propeller designs have evolved, each seeking to enhance efficiency by unconventional propeller blade geometry, particularly in the tip region. The CLT propeller, previously known as the HEFA propeller or the TVF propeller, incorporates end plates attached to the blade tips extending to the pressure surface. The end plate of the first devices also extended to the suction side of the blade. Large increases in efficiency have been claimed for this type of propeller and a relatively large number of units are in service. The RUG propeller is characterised by twin end plates carefully designed as lifting surfaces and extending over both the suction and the pressure surfaces. The efficiency gain for the RUG propeller is claimed to be about 1.8 per cent according to model tests. The Groningen propeller is a further development of the RUG propeller having two-sided shifted end plates, which can be characterised as fins. Model tests indicate an efficiency gain of 4 per cent. Propellers with fins (bladelets) added to the tip of the blade are experimental devices, where the fin configuration is established by experiments. Model experiments performed by Goodman and Breslin with a fin to the suction side showed no gain, whereas Itoh in successive model experiments with one fin forward to the pressure side and one fin aft to the suction side has obtained and published results indicating an efficiency gain of about one to four per cent in the operating range. Itoh obtained no gain when fitting one fin (bladelet) to the pressure side. Finally, as mentioned earlier, the KAPPEL propeller features blades curved towards the suction side and thereby integrating the fin into the propeller blade. This is in contrast to separate end plates or fins, which are applied to the suction or pressure surface in the same way as winglets or end plates are fitted to aircraft wings. Model experiments and full-scale tests confirm a gain in efficiency for this type of propeller of 4-5 per cent when compared to an optimised conventional state-of-the-art propeller. TABLE 1 Development of marine propellers and aircraft wings with non-planar lifting surfaces Type Reference End-plates TVF CLT Rug Gomez et al Sparenberg De Jong Fins, Bladelets, Winglets Marine propellers Bladelet Winglet Groningen Itoh et al. Goodmann Breslin De Jong Curved wing or blade KAPPEL Andersen & Andersen Kappel % Gain 2 - *) Application Full-scale Model Full-scale Aircraft wings Reference Prandtl & Whitcomb Cone Jr. Betz Application Model Full-scale For details of references, application and efficiency see Andersen (1999). *) Large increase in efficiency has been claimed. % Gain is increase in efficiency in percent relative to a conventional propeller. Propeller loadings are different. THEORY AND DESIGN METHODS The first KAPPEL propeller was basically designed according to the classical Lerbs method. However, experiments performed by the Danish Maritime Institute (DMI) 1980 to 1982 with straight and curved fins to the suction side showed that traditional methods were inadequate for the calculation of propellers with 3

4 non-planar lifting surfaces. A method for computing the optimum blade load distribution for arbitrarily curved mid-chord lines was consequently developed by Andersen and Andersen (1987). Later an extension for tracing of the blade surface based on vortex lattice procedures was added (Andersen & Schwanecke (1992)). The optimisation of the load distribution over the propeller blade is a very important feature in the design system, particularly since the optimum load distribution is different from that of a conventional propeller, cf. Fig. 23 to 26. which one corresponds to the pitch of the blade and the other component can be defined as the nose tail inclination as indicated in Figure 2. After this calculation a preliminary geometry has been obtained and the optimum circulation over the entire blade specified. With these data a tracing of the blade surface is made, also on the basis of the vortex-lattice method. EXPERIMENTAL INVESTIGATIONS TABLE 2. Tests with KAPPEL propellers Year Model tank Type of vessel or test Blade no. No. of Load. Cth % gain 1980 DMI 4 5 R&D VWS R&D 1993 HSVA Cont. ves SVA Fast ro-ro 1995 DMI Cont. feed HSVA Cont. ves. Parameter 2001 HSVA Tanker Full-scale Fig. 2. Geometry of the KAPPEL propeller with definition of the orthogonal mid-chord line S o. The design method is essentially the same as for a conventional propeller, with due allowance for the differences in geometry. The numerical procedure is based on the vortex-lattice method. The first step is to find the optimum spanwise distribution of loading. The design point is defined by required thrust, rate of revolutions and inflow velocity, assumed to vary only with radius. Besides its main dimensions the propeller data must include the ordinates of the mid-chord line (x(s), r(s), θ(s)), where s is the arc length along the midchord line, cf. Figure 2. In the optimisation a variational approach is used with a restriction in the chordwise distribution of circulation. Data for the sectional geometry (chord length and thickness) are included in the calculation, making possible an estimate of the viscous forces and a very rough and preliminary evaluation of the cavitation properties. A more detailed outline of this procedure is given by Andersen (1997). For a conventional propeller the angle of incidence of the individual blade sections is described by the pitch angle. For the KAPPEL propeller this angle of incidence is broken down into two components, of The theoretical and mathematical tools were steadily improved and applied to the design of all KAPPEL propellers except for the previously mentioned initial variations tested at DMI. Several propellers were tested in European model tanks including a small series of propellers tested by Versuchsanstalt fûr Wasserbau und Schiffbau, Berlin (VWS), (Andersen et al. 1992) and a CP propeller for a fast single screw ro-ro vessel tested by Schiffbau-Versuchsanstalt Potsdam (SVA). Furthermore, a conventional and a KAPPEL propeller, both designed for the same vessel, were tested by DTU at DMI, Lyngby (Andersen 1997). All tests are briefly summarised in Table 2. However, it became evident that a more systematic approach was highly desirable. A programme to this effect was undertaken from 1997 to 2002 with the objective to further develop, design, manufacture and test a full-scale application of the KAPPEL propeller. This required among other efforts an improved basis for the selection of the geometric parameters describing the propeller. The main objectives, referring to comparison with a conventional state-of-the-art propeller, were defined as: Higher efficiency, lower fuel consumption Reduced noise and vibration levels Full scale demonstration of the new propeller Moreover, other problems such as methods for scaling of model test results and manufacturing of the full-scale propeller were to be solved during the project. 4

5 METHODOLOGY To investigate the feasibility of the KAPPEL propeller as fairly and correctly as possible it was decided to compare with a state-of-the art propeller for the given target ship, rather than with for instance a Wageningen B-series propeller. The comparator propeller was especially designed for the ship in question by an experienced propeller designer and manufacturer. As the target vessels were single screw, the propellers were wake adapted. Conventional and KAPPEL propellers with same diameter and number of blades, but different chord distributions - were designed for the same task and compared in model tests with respect to efficiency in open water and behind condition. Furthermore, in the cavitation tunnel the cavitation behaviour was observed and the pressure pulses measured. The model tests were planned and performed in order to obtain the best possible relative accuracy. The main particulars of the target ships and propellers are summarised in Table 3 in the next section. In the first part of the project the most important parameters of the propellers were changed stepwise and one by one. This parameter investigation included: Length of orthogonal mid-chord line Radius of curvature of non-planar element Skew distribution The parameter investigation was divided into two parts with an intermediate evaluation for selection of parameter steps. Based on the final evaluation of these results the final propeller designs were made. In total two conventional and nine KAPPEL propellers designs have been tested in the programme. FULL SCALE DESIGN CONDITIONS The stepwise parametric investigation was based on a 2900 TEU container vessel for which one conventional propeller (Comparator) and six KAPPEL propellers were designed and tested for the parametric investigation. The main particulars of the container vessel are shown in Table 3. However, during the project a change of the shipyard s newbuilding programme made it impossible to use this type of vessel for the final part of the project. Instead the shipowner D/S NORDEN joined the project and made a 35,000 tdw product tanker available for full-scale application of the KAPPEL propeller and comparative sea trials with the new and the conventional propeller. A model of the existing propeller was made and tested as Comparator together with three KAPPEL designs for this vessel. The final propeller design was chosen between these three. The main particulars for the product tanker are summarised in Table 3. TABLE 3. Main particulars of target vessels 2900 TEU containership 35,000 tdw product tanker Length bpp m m Beam mld m m Depth mld m m Design draft 11.6 m 9.80 m Max cont. rating MCR 28,880 k/w 104 RPM 7,986 k/w 129 RPM Normal cont. Rating NCR 25,992 k/w 103 RPM 7,187 k/w 128 RPM Propeller dia m 5.80 m Blades 6 4 Prop. weight approx. 40t 13.40t Model propellers tested MODEL TESTS Comparator: C1 Parameter: KAPPEL: K1, K1new, K2, K3, K4, K5, K7 Comparator: C2 Final optimisation: KAPPEL: K8, K9,K10 Basic considerations The conventional propellers and all KAPPEL propellers were compared in model tests with respect to efficiency in open water and behind condition. Furthermore the cavitation behavior was checked and the excited pressure pulses were measured. This was done for both types of vessels investigated during the project. All tests were performed at the Hamburg Ship Model Basin (HSVA). Resistance, open water and self- propulsion tests were mainly performed in the large towing tank and all investigations related to cavitation were carried out in HYKAT, the large Hydrodynamics and Cavitation Tunnel. To improve the relative accuracy between the experiments all propellers were tested in the same facility and set-up, each series was tested in the same time frame and wake, and great care was taken to obtain the same geometrical accuracy and roughness of the model propellers. However, in spite of all efforts certain time intervals between tests were unavoidable. In the towing tank self-propulsion tests were performed with the Comparator C1 in February and in July The deviations between the results of the two test series were negligible. In HYKAT the cavitation observations and pressure fluctuation measurements were also repeated for KAPPEL propeller K1 after several months. And again the results showed a very good agreement. Special care was taken to assure the exact geometry of the KAPPEL propellers. Because of the different geometry of this new propeller in the tip area, compared to a conventional design, a new method to manufacture and to check the propeller geometry was 5

6 developed. Difficulties were mainly seen in the curved areas near the tip (Fig.3). Fig. 3. Model propeller K8. Rough measurements in the tip region for the first propellers manufactured indicated deviations in the flow direction of a couple of degrees. Therefore, detailed measurements were performed at Kiel University, using a special apparatus (Renishaw probes) which made it possible to measure the propeller geometry more accurately in the very last tip region. Parallel to these investigations the procedure to manufacture the outer radii of KAPPEL propellers was improved and introduced from propeller K5. To check the influence of the deviations on the propeller performance, the KAPPEL propeller K1 was again manufactured using the improved methods for manufacturing and geometry measurements. The geometry of the second propeller K1 manufactured (K1 new ) was again checked, using a Renishaw-probe. Detailed comparisons of the geometry with the design data were performed. The result of these comparisons was that the model propellers manufactured according to the new procedure showed improved and acceptable accuracy throughout. Furthermore the model tests with K1 new was repeated proving that the geometrical inaccuracies of the first model propellers had only marginal influence on the test results. In total HSVA fabricated 12 model propellers (10 KAPPEL propellers and 2 conventional Comparators), all from brass within the programme. The container ship model propellers had a diameter of 270,91 mm (scale 27.5) and 6 blades, the model propellers for the product carrier had a diameter 250 mm (scale 23.2) and 4 blades. MODEL TESTS FOR CONTAINERSHIP The first series of model propellers in the programme was subject to the following model experiments in the parametric investigation: propeller open water tests in the towing tank and in the cavitation tunnel resistance and self propulsion tests in the towing tank cavitation investigations and pressure fluctuation measurements behind the entire hull model in HYKAT. The test results confirmed that the conventional propeller C1 (the Comparator) represented a good stateof-the-art propeller and could be considered a good compromise between efficiency and cavitation behavior and therefore was well suited as Comparator to all KAPPEL designs for this type of ship. Open Water Tests The open water tests were performed in the large towing tank. The method normally used at HSVA to predict the full-scale open water tests data is the Lerbs- Meyne-Method of the equivalent profile. This method implies a conventional propeller with an optimum load distribution and is therefore less suitable for a KAPPEL propeller where the load distribution deviates from that of a conventional propeller (cf. Fig.23-26). The frictional component and therefore the scale effect of a KAPPEL propeller is larger compared to a conventional design. Consequently a consistent method to estimate the scale effects was developed: A Surface Strip Method (scaling the frictional forces over the blade) was combined with the existing data of HSVAs experience with the Lerbs-Meyne-Method. The method has been tested experimentally by performing open water tests in the medium sized cavitation tunnel by varying the Reynolds number. The measurements confirmed the principle of the new extrapolation procedure. The results of the open-water tests showed an improvement of the open-water efficiency for all the six KAPPEL designs in the parametric investigation, compared to the Comparator propeller C1. Self Propulsion Tests The self-propulsion tests were also performed in the large lowing tank of HSVA (length 300 m, breadth 18 m and depth 6 m). The test results were analyzed and extrapolated to full scale according to the HSVA Standard Method, except for the propeller open-water characteristics as mentioned above. The tests were performed with a wooden model of the medium sized container vessel (scale 27.5) on even keel at a draft corresponding to 11.6 m of the real ship. All KAPPEL propellers obtained higher efficiencies up to 4 % compared to the Comparator C1 - as can be seen from the results given in Fig. 4. The rate of revolution for the 6

7 KAPPEL propellers investigated was slightly lower than the design value. The improvements were not only achieved by higher open water efficiencies, but also by higher hull efficiencies. However, slightly lower relative rotative efficiencies had a negative effect. measure the pressure amplitudes created by the cavitation behavior of the different propellers. The tests in HYKAT included: Tests to check the safety margin against the onset of face cavitation Erosion tests Estimation of the cavity thickness Measurement of the pressure fluctuations at different points on the hull above the propeller at different ship speeds Harmonic analysis of the results for blade rate and higher harmonics. Fig. 4. Results of propulsion tests for the parametric study of the container ship propeller. Cavitation Tests Cavitation tests were performed in HSVA s large cavitation tunnel HYKAT. The prediction of higherorder pressures and noise from cavitation by theoretical means is complex and as a consequence most prediction is done using model propellers operating in a cavitation tunnel. HYKAT allows the installation of the complete ship model and therefore model propellers are investigated in the full three-dimensional inflow. This simulation is important, especially when testing KAPPEL propellers, which interact more with the radial velocity component - in addition to the tangential and axial flow components - than conventional propellers. Model test results concerning cavitation, pressure fluctuations and noise gained in a large cavitation test facility using whole ship models correlate better with full-scale data, compared to small sized facilities. Correlation investigations for different ships have shown that also the higher harmonic components of the pressure values are in a good agreement with full-scale measurements. One of the reasons for this is the better modelling of the cavitating tip vortex. The tip vortex in HYKAT is more stable, and correlates well with fullscale video investigations. The comparison of different designs can be made with sufficient accuracy and improvements in the designs can be quantified. The cavitation behavior is recorded by means of video pictures, still pictures and sketches. Friesch (2000) describes the procedure of testing model propellers in HYKAT in detail. The ship model was additionally equipped with more than 10 pressure transducers, which were installed in the hull above the propeller. They were used to Fig. 5. Model installation in HYKAT Results of the cavitation observations The KAPPEL propellers designed for the purpose of the parametric investigation showed less stable sheet cavitation, compared to the conventional design. For several of the KAPPEL propellers the sheet cavitation started at the inner radii behind the leading edge ( R). The sheet became rather thick in the region of the curved tip. There the cavity volume was increased, compared to the conventional design. Cloudy cavitation could be observed on several of the KAPPEL propellers in the region of the curved tip and behind the blade. This type of cavitation may cause erosion. Paint tests confirmed this assumption on some of the KAPPEL designs in the parametric investigation. On the face of the propeller blades no sign of cavitation could be found and the safety margin against the onset of this type of cavitation was adequate. From the cavitation point of view, KAPPEL propeller K5 showed the best behavior. For more details on the cavitation behavior of the different KAPPEL propeller designs in the parametric investigation cf. Andersen et al. (2000). 7

8 Fig. 6. Examples of cavitation sketches from parametric investigation, propeller K7. The results of the systematic series of cavitation tests showed that the development of cavitation on a KAPPEL propeller is somewhat different compared with a conventional propeller design. It seems that, because of the special geometry of a KAPPEL propeller in the area of the curved tip, the behavior of the cavity is different and that also the behavior of the vortex-like cavitation and the cloud cavitation behind the blades looks somewhat different. It seems as if the interaction of the flow and the induced velocities in the tip area is responsible for this behavior. Additionally the interaction between trailing vortices in the tip region seems to be stronger than on a conventional propeller blade. Pressure fluctuation measurements The pressure fluctuations for the KAPPEL propellers investigated with respect to geometrical parameters indicated results from 15% reduction to 40% increase compared to the conventional propeller. The situation was less clear in the case of higher order pressures, where K2 and K5 showed some improvement. In Fig. 7 the pressure amplitudes measured for the different KAPPEL propellers are compared for the 1. order pressures (blade rate). MODEL TESTS FOR PRODUCT CARRIER A change in the newbuilding schedule of the shipyard made the application of a KAPPEL propeller to the 2900 TEU container vessel inconvenient. Instead, the results and experience from the parametric investigation for the six-bladed propeller for the container vessel were utilized to design the four-bladed KAPPEL propellers K8, K9 and K10 for a 35,000 product carrier that had become available for full-scale application and comparative sea trials. The conventional propeller originally designed for the product carrier was used as Comparator (C2). Models of the propellers (K8, K9, K10 and C2) were fabricated and tested. The test programme was fully comparable to the one described above; again open water tests, self-propulsion tests and detailed cavitation observations were performed as indicated in Table 4. Fig. 7. Result of pressure fluctuation measurements at blade rate of seven model propellers in the parametric investigation. Transducer P1 to P4 are in the centerline fore to aft; transducer P8 to P11 are in the propeller plane port to starboard. Distance between transducers is 40mm in both directions. Table 4. Model tests for Product Carrier Open water K9, K10, C2 Resistance, 9.8m draft HSVA hull model Propulsion, 9.8m draft K8, K9, K10, C2 Propulsion, 9.8m draft, K9, K10 two add. propeller pos. Propulsion, 11,6m draft C2 Cavitation K9, K10 aft K8, C2 Pressure fluctuations K9, K10 aft, K8, C2 8

9 Open Water Tests The open-water efficiencies for the different propellers are compared in Fig. 8. These results show an improvement of all KAPPEL-propellers relative to the Comparator C2 at full scale Reynolds numbers. The improvements were more pronounced for propeller K10 than for K8 and K9 at a C Th value around thrust and n from trial trip prognosis 3. w from trial trip prognosis, n from full-scale measurements 4. power and n from full-scale measurements Fig. 9 Open water full-scale performance, scaled model tests versus calculations. Fig. 8. Comparison of open water test data for propeller for Product Carrier. Calculated propeller performance Analysis of a propeller of given geometry was carried out by use of the boundary-element method which for the KAPPEL propeller is similar to the procedure for a conventional propeller as described in the previous section under theory and design methods. Results for the calculated open water efficiencies are given in Figure 9 and compared with results of model tests. It can be seen that results from the calculations deviate from those of the measurements. The shapes of the curves also suggest that greater accuracy could presumably be obtained by a more complete model of the blade wake. However, at the relevant thrust loading, C Th approx. 2, the theory predicts improvements in efficiency for the KAPPEL propeller of the same order of magnitude as the model tests. Calculations were also made for the propeller in the behind condition. Unfortunately, only a very primitive correction was used in transforming the measured wake field into the effective field. The calculations were carried out for the following cases: 1. w, t and n (rps) from trial trip prognosis In all calculations the relative rotative efficiencies from measurements were used. The values estimated on the basis of model tests for the wake fraction was approx higher for the KAPPEL propeller, whereas the thrust deduction was approx lower. The results of these calculations are somewhat mixed. However, increases in efficiencies from 1.5 to 3.8 per cent in favour of the KAPPEL propeller were obtained. In cases 1 and 3 the predicted thrust deduction coefficients were almost the same for the two propellers, respectively. In case 2 the wake fraction for the KAPPEL propeller was slightly lower than for the comparator in contradiction to the estimate from the model tests. In case 4 the wake fractions were almost identical whereas the thrust deduction coefficient for the KAPPEL propeller was 0.03 lower than for the comparator propeller. This not only indicates that greater accuracy for the calculated open-water propeller characteristics could be desired but also that the interaction between ship and propeller in the form of propulsive coefficients is different for the KAPPEL propeller. Calculations with the boundary element method coupled with a simple cavitation model using twodimensional section theory shows less accurate results as far as cavity extent and pressure signature are concerned (Andersen et al. (2000)). One of the reasons is presumably a strong three-dimensional flow effect on the cavity in the curved tip region of the KAPPEL propeller, as also mentioned under cavitation observations. The conclusion is that the theoretical tools in particular for design but also for analyses of KAPPEL propellers are necessary for obtaining a well-performing propeller. However, at present model tests and experience from previous designs and tests must complement the theoretical tools. 9

10 Self-Propulsion Tests Nearly all tests were performed at the design draught 9.80m. The resistance tests were followed by selfpropulsion tests with the different propeller models. The KAPPEL propellers K9 and K10 were also tested for a second propeller position. The propellers were shifted back 12.5 mm corresponding to 290 mm in full scale. Fig. 10 shows the differences in power consumption. All KAPPEL propellers showed lower power consumption than the conventional propeller at speeds higher than 12 knots. The best results were found with propeller K9 in the original propeller position and K10 in the shifted back position (K10 aft in Table 4 and Fig. 10). The improvements were between 2 % and 5.5 % at speeds between 14 to 16 knots. KAPPEL propeller K8 indicated slightly smaller gains. low values for blade rate, but compared to blade rate levels the higher orders (second and third harmonic) reached the same or even a little higher values for all propellers (including the Comparator) and at nearly all transducers. Fig. 10: Reduction in power for the three KAPPEL propeller designs for the Product Carrier. Cavitation Tests The KAPPEL propeller K8 showed the best performance in the cavitation tunnel tests indicating less tip vortex cavitation for this propeller compared to C2, K9 and K10. The cavitation appeared mainly in form of sheet cavitation between 0.8R and the tip of the blades. Mainly for KAPPEL propellers K9 and K10 this sheet became foamy close to the tip. The foaming cavitation extended into the water behind the blade tip. The area covered with foamy cavitation was the smallest for all investigated KAPPEL propellers so far (see Fig. 11 and Fig. 12) A comparison of the measured pressure fluctuations at blade rate for all propellers including the Comparator is given in Fig. 13. It can be seen that KAPPEL propellers K9 and K10 induce higher pressures compared with the results for the conventional design, while K8 shows an improvement at nearly all pressure transducers. All predicted full-scale values show rather Fig. 11. Cavitation behaviour of KAPPEL propeller K8 in different blade positions. Fig. 12. Sketches of the cavitation behaviour of KAPPEL propeller K8 in HYKAT 10

11 KAPPEL PROPELLER MANUFACTURE Fig. 13. Comparison of pressure fluctuation data for the Product Carrier at blade rate. The transducers P1 to P7 are in the centerline fore to aft and the transducers P8 to P12 are in the propeller plane port to starboard. The distance between transducers is 30mm in both directions. Selection of Product Carrier propeller design The conventional propeller C2 was considered a very good Comparator. Relative hereto the three versions of the KAPPEL propeller K8, K9 and K10 indicated improvements up to 5% in the upper end of the operating speed range. The KAPPEL propeller K8 showed the best cavitation behavior of all KAPPEL propellers within the two series K1 - K7 and K8 - K10. The tip vortex cavitation was decreased, the margin against the onset of face cavitation was still sufficient and the observed sheet cavitation was less foamy compared with all other KAPPEL designs. The extent of the sheet cavitation was limited to the area between 0.8R and the tip. Furthermore, K8 showed the smallest blade-rate pressure amplitudes (85% compared to the Comparator) and only slightly higher values for the higher harmonics. The two other designs K9 and K10 increased the pressure amplitudes at blade rate to 110% and 129% compared to the conventional design. Achieving an efficiency gain of 4% at design speed the propeller design K8 was slightly inferior to the designs K9 and K10 with respect to efficiency. However, the K8 design indicated better cavitation performance, wherefore this design was chosen for the full-scale propeller as the best compromise regarding reduction of power and propeller induced noise and vibration. The KAPPEL propeller, with its characteristic tip rake, presents a number of specific problems for manufacture. These can be summarised as follows: The geometry is difficult to define. The geometry is difficult to measure. The tip geometry lies outside existing standards. The unconventional tip shape presents potential casting difficulty. Traditionally the geometry of marine propellers has been defined about cylindrical sections - co-axial with the shaft axis - and this is consistent with most design procedures. Unfortunately KAPPEL propeller design does not utilise cylindrical sections. Furthermore, if cylindrical sections are taken through the outer part of the propeller, it can be readily appreciated that they would be of no practical value. A further problem arises in that the industry standard for geometrical tolerances, ISO484, is not only written around the concept of cylindrical sections, but it does not even include the region outside 90% radius wherein the main features of the KAPPEL propeller are situated For practical manufacturing purposes, therefore, it was necessary to combine the existing form of definition, using cylindrical sections, for the inner conventional shape of the blade, with a Cartesian and/or polar co-ordinate system utilised for the unconventional tip shape For measurement the development of a suitable technique, in particular for the tip geometry, must ensure that the manufacturer can demonstrate to the designer that the hardware is representative of the intended design shape. To this end alternative methods for the measurement of the complex three dimensionally curved surfaces were investigated including both contact and non-contact systems. This review revealed that, whilst a number of systems could be utilised for the purpose, the cost of these systems if not already being applied may not be justifiable always keeping in mind the need to apply a simple cost effective method for general application. The practical solution adopted was to employ templates, the exact form of which would be derived from interrogation of the blade surface contour from surface modelling techniques. When considering manufacture itself it should be noted that the methodology applicable here is the process used in the UK, the philosophy of which is based upon the production of as accurate a casting as is possible to minimise the subsequent manual finishing work required to achieve the specified geometry within the tolerance envelope. Nevertheless the solutions employed are considered to have general applicability. 11

12 Pattern-work For the case of the KAPPEL propeller the procedure adopted was, after forming both the pressure and suction surfaces of the blade excluding the tip in the conventional way, to generate the tip geometry in the following way: To form the tip pressure face using radial templates. Then to form the tip suction surface from a solid pattern. Moulding The moulding function is the most critical aspect to be considered for propellers of unconventional geometry. Not just because of its effect upon subsequent costs that will arise in the finishing process but, more importantly, because the primary objective is to obtain a quality casting. There are three main requirements: To define the mould with sufficient accuracy to suit the philosophy. To adopt an appropriate moulding medium. To promote a cast consistent with good foundry practice. Fig 14. Moulding the blades. Definition For practical purposes, the pattern-work considerations outlined above represent the simplest approach. The majority of the blade shape inside 0.9 radius can be defined and formed in the traditional way using existing technology, Fig. 14. For the tip region it then remains to ensure that the patterns and jigs are positioned with sufficient accuracy to ensure the end result. Medium The existing moulding medium is concrete. It provides a strong, easily formed, mould material with good heat retention. Additional advantages are its capability for recycling, its inert status from environmental considerations, and it lends itself readily to appropriate counter-measures in the event of metal run-out. It should prove equally suitable for the Kappel propeller. However, with a view to avoiding the potential of high residual stresses in the blade tips during solidification, the Furane system involving a resin activated by an acid catylist which breaks down with heat - was adopted for the mould tops. This allows the tip contraction greater freedom of movement. Casting In the casting process success is achieved by obtaining an appropriate balance between the conflicting requirements of metal flow and turbulence. This problem is exacerbated by the KAPPEL propeller with its rapid change in curvature at the blade tips. These problems were resolved by the combination of a number of adjustments to the traditional approach. These included: Additional pseudo-thickness at the blade tips. Increasing the metal temperature. Increasing the metal flow rate. Raising the temperature of the mould prior to casting. Finishing Propeller finishing in the UK involves removal of unwanted material by manually applied tools. No special techniques are required in the finishing process. From the foregoing it was concluded that modern foundry technology could be readily adapted to ensure the satisfactory casting of a Kappel propeller with excessive, localised rake change in the blade tip regions. The adaptations applied to the process, as outlined above, were applied in the production of the first KAPPEL propeller produced and it is to the credit of those involved both in the formulation of the methodology and its application that the propeller was successfully produced. FULL SCALE MEASUREMENTS Prior to the sea trials with the DWT Product Carrier NORDAMERIKA the vessel was dry-docked, the hull was cleaned and painted and the propeller polished to a surface roughness of less than 2 micron. Furthermore, observation windows and pressure transducers over the propeller were installed (see Fig. 15 and Fig. 16). Sea trials with the vessel powered by its original propeller (C2) were performed on Immediately after this first trial the vessel dry-docked again for installation of the KAPPEL propeller (K8) in lieu of the original. The comparative sea trials with the KAPPEL propeller (K8) took place on All trials were conducted off the west coast of Portugal between Cap Espichel and Cap de Sines (approximately 38 N, 09 W) during good weather conditions. Both sets 12

13 of trials included a comprehensive range of double run speed trials plus turning circles. During the speed trials and the first voyage of the ship the pressure fluctuation measurements were performed and the cavitation phenomena were documented on videotapes. Speed and manoeuvring measurements were performed using HSVA's DGPS-Trial Measurement System, which is based on the GPS/NAVSTAR satellite navigation system. The hardware consists of a DGPSreceiver and a portable computer. The software was developed by HSVA. During all tests the propeller shaft torque was determined with the help of strain gauges applied directly to the shaft. With the help of the continuously measured propeller speed, time histories of the shaft power were derived for all trials. Based on the speed measurements and the corresponding recordings of the propulsive power, the attainable speed of the vessel at different power rating was determined for both propellers under consideration. Beside this, the manoeuvring qualities of the M/T NORDAMERIKA equipped with the two different propellers were investigated by turning circle manoeuvres. Andersen, Friesch & Kappel (2002) give a brief description of the results of the full-scale measurements. filtered by a 2 khz low pass and a 1 Hz high pass to eliminate influences from ship motions. Four accelerometers with a range of ± 5g were additionally installed. All signals from the amplifiers were analogdigital converted and stored in a PC to allow subsequent harmonic and FFT analysis. Fig.15. Arrangement of observation windows and pressure pick-ups in full-scale. Photographing and viewing was carried out at night with artificial lighting and during daylight. A frame storage system was used to freeze the actual video frame between one revolution to the next. During daylight the camera could be operated in the shutter mode to provide still standing pictures as well. However, night observations with stroboscopic light result in a much better quality since the flashes are extremely short. Eight pressure pick-ups were installed in special inserts in the hull and located as seen in Fig. 15. The pick-ups were of the strain gauge type (Kulite XTM 190) with a diaphragm diameter of 3.7 mm allowing pressures up to 350 kpa and suitable for frequencies up to 50 khz. The signals were amplified by a Hottinger MGC plus system, which is suitable for frequencies up to 10 khz. In this case the signals were Fig. 16 Windows installation MT NORDAMERIKA Results of the Speed and Manoeuvring Trials The comparative speed-power analysis for the no wind and no sea conditions indicated a reduced power requirement for the KAPPEL Propeller K8 in comparison to the conventional C2 propeller. For the most interesting power rating of 90% MCR the advantage of the K8 propeller relative to the C2 propeller power requirement was found to be in the range of 4 % (at a speed of 15.5 knots), cf. Fig. 19. Therewith the findings of the corresponding model test investigations in general were confirmed by the fullscale measurements. Furthermore, a good agreement between measured and predicted propulsion power as a function of rate of revolution was determined for the C2 propeller. Within the power range of interest the final corrected values of the rate of revolution for the KAPPEL propeller was about 2 revolutions per minute slower than predicted. 13

14 A comparison of the ship s tracks measured during the different turning manoeuvres indicates a negligible influence of the two propeller designs on the turning capabilities of the vessel. Taking environmental effects into account, the ships tracks during the turning circles can be seen as comparable for the two propeller designs investigated. Cavitation and Pressure Fluctuations On the Comparator propeller mainly sheet cavitation could be observed. Extent and thickness of the sheet cavity was rather small, leading to low levels of propeller induced pressure fluctuations. Additionally a rather strong bursting tip vortex was found. A deformation of the lower rim of the sheet cavity could be seen when the propeller blade passed the wake peak and furthermore shedding of cloudy vortices starting at the leading edge was found. This type of cloudy cavitation may cause propeller erosion. rate, but with 1.5 kpa at 90% MCR both values are rather low. The observations of the KAPPEL propeller showed cavitation phenomena, which were quite comparable to those observed on conventional propellers. Extent and thickness of the sheet cavity were rather small, leading to low levels of propeller induced pressure fluctuations. Also for this propeller a deformation of the lower rim of the sheet cavity could be seen when the propeller blade passed the wake peak and also shedding of cloudy vortices starting at the leading edge was found. The cavitation leaves the blades in a cloudy vortex, which like the Comparator tip vortex - was bursting when passing the wake peak. This type of cloudy vortex may in certain cases cause propeller erosion. However, when the propeller was inspected after nearly one year of service no trace of erosion could be observed. The level of the measured pressure fluctuations was very low, for the first harmonic orders. Because of the cloudy character of the vortex, the 2 nd harmonic order was higher than blade rate, however, the level 1.3 kpa at 90% MCR was anyhow rather low. Fig.18. Full-scale cavitation behaviour KAPPEL propeller (90% MCR) COMPARISONS AND CORRELATIONS Fig.17.Full-scale cavitation behaviour, Comparator propeller (90% MCR) However, no erosion damages caused by cavitation has been found after 1.5 years of operation. The level of the measured pressure fluctuations was low, for the first harmonic orders. Because of the cloudy character of the tip vortex, the 2 nd harmonic order is as high as blade The full-scale measurements proved that the efficiency improvements of the KAPPEL propeller indicated in Fig. 19 were achieved without adverse effects due to cavitation and pressure fluctuations. Although the Comparator propeller did not induce undue noise and vibration, service reports from the vessel actually claim that the onboard environmental conditions are further improved after installation of the KAPPEL propeller due to reduced noise and vibration levels. 14