Wind-Tunnel Investigation of the ILCA 7 MKII Sail in Downwind Conditions

Abstract

Aerodynamic coefficients from wind-tunnel tests are presented for the ILCA 7 in all six degrees of freedom. Two apparent wind speeds are considered: a light wind case corresponding to 4 m s-1 at full scale and a strong wind case corresponding to 7 m s-1. The light wind tests comprise five apparent wind angles, seven sheet angles, three kicker trims and three heel angles. For the strong wind case the dinghy is assumed sailing upright and only dead downwind, but with four sheet angles and three kicker trims. The scale is 1/7, which gives a reasonable blockage in the wind-tunnel. Blockage corrections are obtained using CFD. Correct scaling of the sail stretch, and mast bend is achieved with the full-scale sail cloth and a solid stainless-steel mast exposed to a wind speed √7 times larger than at full-scale. However, the Reynolds number is √7 times smaller. The main result of the investigation is the comprehensive data set, which is available on a public server. This data can be used in a VPP to optimise downwind sailing in smooth water. Several interesting conclusions can also be drawn directly from the data. Thus, for the light wind case the best sheet angle is 90° regardless of the course sailed. However, for the strong wind case (dead downwind), the best angle is 80°. In all cases the tight kicker trim produces the largest drive force and in the light wind case also the smallest side force.