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A study of the moving
pictures of breaking wave strikes in this test series and the tests
reported in reference 1 shows that the model is often brought up to wave
speed before the load in the drogue builds up. The horizontal component of
the buoyancy force as the model rides up the wave face and the impact
force of the breaking wave crest are of such a magnitude that the model is
accelerated up to wave speed in a very short distance, too short to allow
the drogue to develop much load. Then, as the model moves with the wave
crest, the towline tightens and the drogue takes up the load. Initially,
the largest portion of the load is the inertia load associated with
decelerating the model from wave speed. To this must be added the
horizontal component of the buoyancy force which acts until the boat is
pulled over the top of the wave crest. From then on most of the drogue
load results from the boat being dragged backwards through the surf ace
water which is still moving at a speed close to wave speed behind the
crest.
A computer model of the
above sequence of events will be discussed in a separate section. It is
not possible to directly relate the computer simulation with the actual
load measurements since the velocity of the test wave at the model
location was not accurately determined. On Figure 6 the computed loads at
two assumed wave velocities are plotted together with the measured drogue
loads.
It should be noted that for
these tests the elasticity or spring rate of the drogue towline was not
properly simulated. A true scale model of a 3/4-inch diameter nylon
towline would have a spring rate about 1/10 that of the model towline used
there. This would reduce the peak loads.
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