Aerodynamic Analysis of the Bi-Radial MKII Sail for the Olympic Sailing Class Dinghy ILCA 7

Detailed data for the downwind aerodynamics of cat-rigged dinghies are scarce. In most Velocity Prediction Programs (VPPs) the aerodynamic input comes from established relations for perfectly trimmed sails, and there is no information on sheeting angles or kicker settings. Further the effect of heel is approximate. In the present thesis, aerodynamic coefficients for the ILCA 7 in all six degrees of freedom are reported from wind-tunnel tests at Chalmers. Two apparent wind speeds are considered: 4 m/s and 7 m/s at full scale. The light wind tests comprise five headings, seven sheeting 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 sheeting angles and three kicker trims.
The scale selected for the sail is 1/7, which gives a reasonable blockage in the wind-tunnel at Chalmers. The blockage effects cannot be estimated from standard wind-tunnel corrections, since the massively separated sail is too far from the streamlined shapes usually tested, so comprehensive CFD computations of the blockage were carried out and used to correct the data. It is shown in the thesis that correct scaling of the sail stretch and mast bend is possible with the full-scale sail cloth and a solid stainless-steel mast exposed to wind speed \(\sqrt{7}\) times larger than at full-scale. The Reynolds number is however \(\sqrt{7}\) times smaller.
The main result of the investigation is the comprehensive data set, which also is uploaded and publicly available. This data can be used in a VPP to optimize downwind sailing in smooth water. Several interesting conclusions can also be drawn directly from the data. Thus, for the light wind case the best sheeting 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 best drive force and in the light wind case also the smallest side force. There is little support in the data for "sailing negative", i.e., with the wind moving from the leech towards the mast at small sheeting angles. The conclusions from the tests should be of interest also to sailors of other cat-rigged dinghies.