An engineering method to design daggerboards and rudders for small racing yachts

Abstract

The variety of shapes and sizes among daggerboards and rudders of small racing yachts indicates the lack of a comparative design method and that some aspects may be neglected in the designs currently proposed. In particular, a literature survey showed that: (i) the conventional design approaches are exclusively based on upwind conditions, disregarding any downwind contribution to the race, (ii) the detrimental effects related to the typically low Reynolds numbers involved are marginally considered, and (iii) there are ambiguous indications about sweep angle effects on foil performance. In this paper, a design method based on a Velocity Prediction Program compares different appendage planforms for a yacht, identifying the ones minimizing the overall racing time, sum of upwind and downwind contributions. The method is applied to a 4.6 m skiff and the performance of the daggerboards analysed are compared with xflr5 simulations, validated against experimental tests. The results quantify the time gains of longer foils and the similar performance of elliptic and rectangular geometries with equal drafts but different aspect ratios. Swept foils are investigated as well, showing lower lift curve slopes compared to straight geometries for the high aspect ratios typically used on small yachts.