Dynamic Lift Coefficients for Spade Rudders on Yachts

The loss of a rudder is a dangerous situation for any vessel, and with the increasingly higher aspect ratios in current sailing yacht rudder designs, a better understanding of the forces on a rudder are required. While many failures have been caused by impacts with objects, a large number have failed due to underestimation of sailing loads. While larger aspect ratios increase the lift-to-drag ratio, they also increase the bending moment about the rudder’s root. Combined with thinner airfoil sections to reduce drag, modern rudders are highly stressed. Traditional design methods normally assume that the maximum lift coefficient is constant for all aspect ratios. This project combined computational fluid dynamics (CFD), finite element analysis (FEA) and the tank testing of a 1/5-scale yacht to determine suitable design lift coefficients for spade rudders of cruising and racing yachts. Two rudders of different aspect ratios were tested at various speeds, heel angles and wave conditions in the tank at the Naval Surface Warfare Center – Carderock Division. The rudders were equipped with strain gauges to determine the strains at various positions along the stock and blade. The strain profile was compared against FEA results that used a CFD prediction of the pressure profile. Through back-calculation the lift coefficients in still water and waves were derived. The results indicated that these lift coefficients are not constant.