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 2-foot 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 30-foot boat displacing 7500 lbs needs a 4-foot diameter drogue, direct scaling would result in an 8-foot diameter drogue being required for a 60-foot 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 5.5-foot diameter drogue rather than an 8-foot diameter drogue for the 60-foot, 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 series-type drogues.