Differences in the effects that rough water has on similar sailing yachts has been one of the intriguing puzzles that sailors, designers, and researchers have long tried to understand, It is not uncommon that two yachts whose performance is equal in smooth sea conditions will have their speed or point­ing ability reduced by different amounts when encountering waves, To investigate the causes of such behavior, it is important to have a rational procedure to analyze how changes in hullform, weight distribution, rig, and other design features affect the speed and motions of sailing yachts.

This paper discusses the relationship of wind to rough water and of motions and added resistance to wave length and height, It then describes a procedure to predict motions, sailing speed, and speed-made-good to windward in realistic windward sailing conditions, The procedure utilizes results of heeled and yawed model tests of twelve meter yachts in oblique regular waves to predict performance in a Pierson-Moskowitz sea state corresponding closely to the equilibrium true wind speed.

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