This paper describes an experimental investigation into the effect of riblets on the aerodynamic forces generated by sails. Riblets are known to reduce the viscous drag on flat surfaces and on airfoils, but their effect on the drag — or on the lift — of a highly three-dimensional, very thin lifting surface such as a sail is unknown. In this work, we show that riblets have little effect on the drag of a sail, but they increase the lift by about 3%. The result of this increase in lift is to increase the force in the direction of boat motion by up to about 5%. This is significant, because the sails provide the only locomotive force available to a sailboat. In addition, the greatest improvement is found for the case of riblets on both sides of the sail rather than on only the pressure or the suction side. This is, fortunately, the most practical arrangement for use on a real sailboat.