Dynamic stability assessment of yacht downwind sailing in regular waves

A comprehensive approach for assessing the dynamic stability of yachts sailing downwind in regular waves is presented, with the focus on course keeping phenomena and in particular on surf-riding and broaching-to incidents. It is based on an aero/hydro-dynamic modular mathematical model of yacht motions in 6 degrees of freedom and consists of two coupled solvers, one for the hull and another for the sails. In the hull solver, inertial, hydrodynamic reaction and wave loads are accounted for and they are calculated up to the instantaneous wetted surface. The sails solver is based on aerodynamic RANS and structural FEM pre-calculations leading to the creation of large interpolation tables of sail force coefficients. A systematic simulation campaign, with parameters the wind velocity, the wave length, the wave steepness and the desired heading, has been carried out for a YD-40 type yacht, for realistic ranges of values of these parameters. Post-processing of the simulation results led to creation of stability diagrams that can be used directly for evaluating the yacht's course-keeping capability in downwind sailing, with respect to incidents of surf-riding, broaching-to and capsize.