A computer simulation of a boat and drogue in 20-foot storm waves with a wave length of 200 feet indicates that the drogue will experience a cyclic velocity variation between 10 ft/sec in the direction of the wave to 2 ft/sec opposite the wave direction every 6 seconds. Thus, if the storm lasts 10 hours, the drogue will be subjected to 6000 load cycles.
To simulate this type of service, a 5.5-foot section of a full-scale series drogue, which included three sailcloth cones, was mounted in the dynamic rope testing machine at the U.S. Coast Guard R&D Centre, Groton, Connecticut. A schematic of the test sample is shown on Figure 17.
The rope testing machine is provided with a hydraulic ram with a maximum stroke of 36 inches and a maximum linear velocity of 2.5ft/sec. For this test, the ram was programmed to give a sinusoidal motion with an amplitude of 18 inches and a frequency of 0.3 cycles/sec. The motion of the ram was multiplied by a factor of 4 by a pulley system so that the test piece moved with a stroke of 6 feet and a peak velocity of 10 ft/sec. The ram pulled the test piece forward and a 5/8 inch diameter shock cord pulled it back. The sample drogue was submerged in a tank of water that was 17.5 feet long, 1 foot high, and 1.3 feet wide. A drawing of the test set-up is shown on Figure 20.
The individual cones were made of 1.5 oz. ripstop Dacron sailcloth material in the flat with a single line of stitching and then turning the cone inside out. The three 3/4inch nylon tapes were sewn to the cone material before the axial seam was fastened. The fore and aft edges of the cone material were left as cut, i.e., no tape or hem was applied.
Several methods of splicing the nylon tape to the 3/4-inch diameter braided line were investigated before the fatigue test. For this test two of the cones were attached with a double pass splice and one of the cones with a single pass splice. As shown on Figure 18, an overhand knot was tied in the free end of the tape to prevent it from pulling out.