Numerical investigations of the effects of winglets on IACC yacht drag components

A computational study of an America’s Cup Class keel has been carried out to improve the understanding of the effects of the winglet geometry on yacht drag components. A potential flow panel code together with a coupled thin turbulent boundary layer method was used. Systematic variations of the geometrical parameters were first carried out for chord length, span, taper ratio, dihedral and sweep angles. The calculations covered a wide range of leeway angles in upright and heeled conditions for each parametric variation. The study was extended to analyze possible interactions between the winglets and the free surface and its influence on the optimum winglet geometry. The largest impact on the efficiency was associated with the chord length, span and taper ratio which to a large extent influence both the induced and friction drag. The calculations indicate that optima with respect to the varied parameters can be found for a given side force. In most cases a reduction of the induced drag involves a frictional drag increase. The positive effects of the winglets are easier to achieve at higher keel loadings when the induced drag is significant. Free surface interaction is more distinct at large heel angles and may affect the winglet design. The study shows that a very detailed analysis is necessary to design winglets that will improve a yacht’s overall performance. The design can be supported efficiently by the presented numerical methods, preferably supplemented by optimization routines to reduce the effort and time spent on processing the results.