Abstract
A yacht often rolls very violently when it is sailing down-wind. This divergent oscillation may be dangerous enough to capsize the yacht. This study models a yacht as a sail-ballast system rolling in a wind. A mathematical model is presented to account for this phenomenon. Experiments have also been made for systems with triangular and elliptic sails. Analysis of the nonlinear governing equation implies that the instability occurs owing to coupling effects between aerodynamic and righting moments: in particular the large change in the side force under a small change in the angle of attack. The experimental results support this argument regarding interaction, as well as the existence of three distinctive types of rolling motion: divergence, divergent oscillation, and oscillation with weak damping. Estimated and measured periods of oscillation are in fairly good agreement. Evidence supports the idea that vortices behind the sails are not the main cause of the instability.
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