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.