An investigation of the use of a drogue to prevent breaking wave capsize must consider two distinct operating conditions: (1) the long time exposure to regular storm waves, and (2) the infrequent breaking wave strike. The tests described in this section relate to the first condition, in which the boat/drogue system rides for the duration of the storm, possibly 10 to 20 hours, in waves with a height of 15 to 20 feet and a wavelength between 150 to 300 feet. The object of these tests was to study the transient load in the drogue towline and to evaluate the motion of several types of drogues with particular reference to the vulnerability to fouling or tangling, and to mechanical failure from fatigue or wear.
In regular waves, the particles of water near the surf ace of the wave move in a more or less circular path, forward at the crest and backward in the trough, with relatively little net motion in the direction of travel of the wave. If the boat is small in comparison with the wave, i.e., has a waterline length less than a quarter of the wavelength, and if the boat is not moving through the water, as is the case with the drogue deployed, the motion of the boat will be similar to the motion of the water particles in the wave surface. The boat will move in a more or less circular path in the vertical plane. Superimposed on this circular path will be a relatively small drift to leeward resulting from the force of the wind on the hull and rigging and from the drift which occurs in the surface water of storm waves.
To study the motion of the drogue under these conditions it is necessary to impart the same oscillating motion to the drogue towline as it would receive from the boat. It was determined that a close approximation to the horizontal velocity variation of the boat is a simple sine function plus a constant drift velocity. For the tests described in this section, the horizontal velocity variation was provided by attaching the drogue towline to the end of a rotating arm and the drift velocity wave obtained by adjusting the velocity of the water in the flow channel.