The size of the drogue (the
effective drag area) is the most important design decision. If the drogue
is too small the boat will broach and capsize when struck by a breaking
wave. if the drogue is large the maximum load will be high. Thus we wish
to select the minimum size that will do the job. In the model testing
reported in Reference 1, it was found that for a small sailing yacht with
a displacement of 7500 lbs. a cone or parachute drogue with a diameter of
4 feet or an equivalent series drogue would generally prevent capsize even
when the model was struck by a very large breaking wave. Tests with a
2foot diameter drogue showed the model to be capsized on approximately
half of the wave strikes. With no drogue the model would be violently
capsized on all the wave strikes. Based on these tests it was concluded
that small sailing yachts require a drogue at least four feet in diameter
or an equivalent drag device such as a series drogue.
The above discussion applies
only to a drogue deployed for the stern. A sea anchor deployed from the
bow would have to be much larger, 2 or 3 times the diameter of a stern
drogue, in order to hold the bow into the wind and sea.
If a 30foot boat displacing
7500 lbs needs a 4foot diameter drogue, direct scaling would result in an
8foot diameter drogue being required for a 60foot boat displacing 60,
000 lbs. However, the incidence of breaking wave capsize decreases rapidly
with displacement and it is a rare occurrence for a yacht over 60 feet to
be capsized by a breaking wave. It is reasonable to believe that a drogue
with a diameter less than 8 feet would be adequate for a 60,000 lb. boat.
The drogue, however, should be large enough to prevent the boat from
surfing down the face of a breaking wave and plunging into the preceding
wave, an event that has been documented on a number of occasions. This
criterion leads to the requirement of a 5.5foot diameter drogue rather
than an 8foot diameter drogue for the 60foot, 60,000 lb. boat. The
calculations supporting this selection are presented in Appendix C.
Figure 25 presents a plot of
the recommended drogue size vs. displacement for parachute and cone
drogues and for equivalent seriestype drogues.
Top
