Performance of wing sail with multi element by two-dimensional wind tunnel investigations

Following the 33rd America's Cup which featured a trimaran versus a catamaran, and the recent 34th America's Cup in 2013 featuring AC72 catamarans with multi-element wing sail yachts sailing at unprecedented speeds, interest in wing sail technology has increased substantially. Unfortunately there is currently very little open peer-reviewed literature available with a focus on multi-element wing design for yachts. The limited available literature focuses primarily on the structures of wings and their control, rather than on the aerodynamic design. While there is substantial available literature on the aerodynamic properties of aircraft wings, the differences in the flow domains between aeroplanes and yachts is significant. A yacht sail will operate in a Reynolds number range of 0.2 to 8 million while aircraft operate regularly in excess of 10 million. Furthermore, yachts operate in the turbulent atmospheric boundary layer and require high maximum lift coefficients at many apparent wind angles, and minimising drag is not so critical. This paper reviews the literature on wing sail design for high performance yachts and discusses the results of wind tunnel testing at the Yacht Research Unit at the University of Auckland. Two wings with different symmetrical profiles have been tested at low Reynolds numbers with surface pressure measurements to measure the effect of gap geometry, angle of attack and camber on a wing sail's performance characteristic. It has been found that for the two element wing studied, the gap size and pivot point of the rear element have only a weak influence on the lift and drag coefficients. Reynolds number has a strong effect on separation for highly cambered foils.