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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.
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