A new theory and computer program to predict the resistance of a sailing yacht with keel and winglets ace presented. While established ship-motion and added-resistance theories have been shown to give reasonable results for conventional ships, they do not satisfactorily predict the rough water behavior of a yacht sailing to windward. This is partially due to an inability to accurately account for the effects of the keel and other lifting appendages, including winglets as found on most modern International Twelve-Meter Class Yachts. Theories for evaluating both steady and unsteady lift, and the associated thrust and drag, due to body motions and waves must be included in order to obtain useful results. These theories have been incorporated into existing motion and added resistance prediction programs and can be used to determine the behavior in waves of a yacht sailing to windward at a specified angle of heel and yaw. Computer results for yachts with and without winglets are presented and compared with experimental results, which include the recent model experiments conducted by Gerritsma in Delft.