The San Francisco Mud Bottom Anchor Tests

Transcription

1 The San Francisco Mud Bottom Anchor Tests Conducted by NAV-X Corporation (manufacturers of the Fortress Marine Anchors) Approved to present by Brian Sheehan of NAV-X Sponsored by West Marine Anchor tests were conducted on April 27-29, 1990 by NAV-X Corporation of Ft. Lauderdale, FL, manufacturers of FORTRESS Marine Anchors. The tests were sponsored by West Marine Products of Watsonville, CA. Test results and procedures were verified by E.S. Maloney, Author of "Chapman's Piloting & Seamanship." The tests were also witnesses in part by Robert Danforth Ogg, Co-Inventor of the "Danforth" pivoting fluke anchor design: Robert Taylor, an expert on anchors and soil mechanics from the U.S. Naval Civil Engineering Laboratory, Port Hueneme, CA; Lysle Gray, Executive Director of the American Boat & Yacht Council representing the ABYC's Ground Tackle Committee; Gary Sable, Vice President and General Manager of Rule Industries, Inc., manufacturers of the "Danforth" brand of pivoting fluke anchors; Robert A. Smith, N.A., Author of "Anchors, Selection and Use;" Chuck Hawley, West Marine Product's Manager of Technical Information, and others. The tests were also witnessed at various times by writers and editors from several marine publications, and was documented on video tape by "Sailing Quarterly Video Magazine." PURPOSE: These tests were conducted in order to establish a reliable, controlled database for comparing the performance of various popular recreational marine anchors in soft muddy sea floor conditions. Of particular note during these tests was the introduction of three new designs from FORTRESS, Danforth, and Delta anchors. FORTRESS and Danforth in particular, for their new designs intended specifically to provide greatly improved performance in soft mud bottoms. These tests were "Part Two" of a series of anchor tests, conducted by NAV-X, which began in Miami, FL in February, 1990 with sandy bottom straight line pulls tests. The intent of this series is to establish a credible reference for the performance of anchors in the various bottom types and anchoring conditions the yachtsman can expect to encounter while cruising. DESCRIPTION OF SITE: These tests were conducted in the "Hunter's Point" area of San Francisco Bay. The site is located approximately 4 miles southeast of Candlestick Park, on the west side of the southern half of the Bay (Approx: 37º 41.5' N ' W). The tests were conducted in arbitrary lanes in North-to-South, South-to-North, and West-to-East pull directions. Water depths had been previously sounded by NAV-X and found to range from 17' to 26'. Samples of the bottom material were obtained and evaluated as being consistent soft grey-brown mud without apparent binder content, such as clay. The bottom was described by NAV-X as "Extremely Poor" holding ground. This area has been used for anchor testing by the U.S. Navy, and the consistency and low sheer strength of the sea floor was confirmed by the Navy's representative on board. Thumbnail Chart (not for navigation use) of Test Area. TEST VESSELS: The vessels included the twin-screw 60' (1400 Shp) commercial push-tug "Pacific Rose", which served as the main test platform. The 50' single screw tug "California Eagle" (400 Shp), served as anchor recovery vessel. A 39' Carver Motoryacht was chartered to provide shuttle service for guests and observers. The 25' workboat "Grizzly Bear" provided a platform for photography and video.

2 SCOPE AND ANCHOR RODE: Six prefabricated anchor rodes were prepared prior to the tests. They consisted of one fathom (six feet) of 3/8" High Test chain, and 100' of 7/8' three-strand nylon line. These in turn were shackled to a 14' length of 1-1/2" three-strand nylon line leading from the load measuring devices overboard to the main rode to insure that possible line-partings would always occur under water for the safety of the crew. Total rode length was therefore 120', resulting in a range of scope from 6.3:1 to 4.2:1 depending on water depth and the state of San Francisco Bay's average of 7'-8' tides. LOAD MEASURING: A redundant load measuring system was used: A 4,000 lb. calibrated "Dillon" Dynometer, with a certificate of accuracy traceable to the U.S. Bureau of Standards provided primary readings. A backup hydraulic cylinder and load gauge provided confirming readings. A closed-circuit video camera was permanently mounted on the "Dillon" gauge, with the monitor visible to observers on the bridge during the tests. The confirming hydraulic gauge was also mounted on the bridge. Verbal readings were called out by an observer on the test deck through a loud hailer, and transmitted by radio to 12 two-way headsets worn by various observers. The readings were recorded every second or so by West Marine representatives, and later confirmed by E.S. Maloney. ANCHORS TESTED: A primary grouping of 8 anchors, and secondary grouping of 7 anchors were tested. The primary group, illustrated below, represented commonly available anchors from the lb. size range. This size was chosen for testing because it represents the size often chosen for service aboard cruising boats in the 35' to 50' range. Generally speaking a given anchor's "performance ratio", or holding power per pound of anchor weight, can be used to determine the relative performance to be had from different sizes of the same design and model. All anchors were obtained randomly from the stocks of various marine distributors, including West Marine, Svendsen's, Lewis Marine and Sailorman. The secondary anchor group included experimental, prototype, or less commonly available anchors. These included the Creative Marine "MAX" anchor, the 30 lb. Davis "Sea Hook," a revised version of Rule Industries' T-4000 Deepset Hi-Tensile, the P-1800 Danforth Deepset Plow, and a 35 lb Plastimo brand plow anchor. Also, at the request of the Navy, larger FORTRESS FX-85 and FX-125 anchors were tested as well. LIMITS: No load limits were imposed on these tests, as the potential for damage to an anchor in such poor holding ground was unlikely. (Limits were imposed during the sandy bottom tests in Biscayne Bay.) TESTING PROCEDURE: The primary group of 8 anchors were tested in a revolving order. A ninth test position was reserved for testing the next anchor from the 7 member secondary anchor group, called the "wild card" anchor. Each anchor to be tested was connected to a rode and staged for deployment from the bow of the tug "Pacific Rose". After the tug had maneuvered into the proper test lane and direction, the rode was connected to the dynometer and the anchor was thrown into the water. The tug would then be backed down, allowing the line to slowly pay out. When a "set" was established, i.e.: when some load was registered on the load gauges, RPMs were increased at "Dead Slow" speed, while the loads were carefully observed and recorded, until the anchor either dragged or pulled out of the bottom, thus ending that test. Bearings taken on 8 buoys along the test course helped observers to determine the point at which an anchor began to drag. When a test was completed, the rode was disconnected, attached to a buoy, and tossed into the water for recovery by the tug "California Eagle". This enabled the next anchor to be tested immediately, allowing a new anchor to be tested about every 15 minutes. After being recovered from the bottom, the anchors were returned to the "Pacific Rose" for inspection and re-testing. TABULATION OF RESULTS: When the three days of testing were completed, and the results verified and compiled, each anchor's

3 performance was then scored and rated into two categories: (1) "Average Test Pull," or, all pulls added up and divided by the number of pulls to determine an average holding power for that particular anchor. (2) "Holding Power Per Pound of Anchor Weight" sometimes referred to as a "performance efficiency ratio" to illustrate how many times an anchor's actual weight it is capable of holding in a given bottom. The results of these categorical tabulations can be examined in the charts on the next page. A summary of the raw test data, as verified by Mr. E. S. Maloney, is following. TYPE WEIGHT Lbs. AVG. DEPTH MAX AVG MIN SLACK LINES HP/LB Bruce CQR Delta H T FX deg FX deg VSB WILD CARD GROUP FX deg FX deg FX deg FX deg Note: Depth, Max, Avg, Min, Std, and Slack are based on best 5 pulls out of 6. Standard deviation based on maximum reading of best 5 pulls.

4 CONCLUSIONS: The test was designed to test an area of anchor performance that has had little recorded data to date: straight line holding power in soft mud. Since many anchorages around the world have mud bottoms, this is an important aspect of anchor performance. The author realizes that there are many other aspects of anchor performance that need to be weighed before selecting an anchor for use. Those performance criteria include: sand holding power, structural strength, ease of setting, ease of retrieval, ability to reset after veering, ability to penetrate vegetation on the bottom, rock hooking ability, ease of stowage and overall holding power per pound of anchor weight. The tensions recorded in this test were substantially lower than those recorded in a similar test held in Miami with a sand bottom. This was expected, as the sheer strengths of the two bottoms were quite different. In comparing results from the two tests, anchors held only 11% to 33% as much tension in the mud bottom compared to the sand bottom in Miami. The two anchors which are optimized for mud bottoms did substantially better than the traditional designs; the Nav-X Fortress FX-37 at 45-degree fluke angles, and the Rule/Danforth VSB These anchors held roughly four and two times the tension, respectively, of the other anchor styles. The manufacturers of both of these anchors warn that their "mud" anchors will not reliably set in firm bottoms such as sand/clay or sand. These anchors must only be used in soft, mud bottoms. The convertibility of the FX-37 by changing its fluke angles makes it appropriate for both sand or mud when correctly assembled.

5 1990 DATA