In order to create capability for analyzing course instabilities of sailing yachts in waves, the authors are at an advanced stage of development of a mathematical model comprised of two major components: an aerodynamic, focused on the calculation of the forces on the sails, taking into account the variation of their shape under wind flow; and a hydrodynamic one, handling the motion of the hull with its appendages in water. Regarding the first part, sails provide the aerodynamic force necessary for propulsion. But being very thin, they have their shape adapted according to the locally developing pressures. Thus, the flying shape of a sail in real sailing conditions differs from its design shape and it is basically unknown. The authors have tackled the fluidstructure interaction problem of the sails using a 3d approach where the aerodynamic component of the model involves the application of the steady form of the Lifting Surface Theory, in order to obtain the force and moment coefficients, while the deformed shape of each sail is obtained using a relatively simple Shell Finite Element formulation. The hydrodynamic part consists of modeling hull reaction, hydrostatic and wave forces. A Potential Flow Boundary Element Method is used to calculate the Side Forces and Added Mass of the hull and its appendages. The Side Forces are then incorporated into an approximation method to calculate Hull Reaction terms. The calculation of resistance is performed using a formulation available in the literature. The wave excitation is limited to the calculation of Froude - Krylov forces.

Measurements of the inertia parameters (Gregory, 2006) of a keelboat hull using a bifilar suspension (Newman and Searle, 1951) are described. Bifilar yaw moment measurement normally entails accurate measurement of the length l and spacing 2d of the suspension, and of Ty the period of pure yaw oscillation (Miller, 1930). The primary difficulty with a bifilar suspension is avoiding unwanted modes of oscillation, specifically sway when measuring yaw. However, for an athwartships suspension, the sway motion is that of a simple pendulum of period Ts and observation of the combined motion allows the yaw gyradius ky ≡ kzz to be determined as ky = (Ty/Ts)d. Thus only the ratio of the periods and the suspension spacing need to be measured. Measurements of the normal mode periods of the double pendulum motion (Rafat, Wheatland et al., 2009) when the hull is displaced in surge allow for the pitch gyradius kp ≡ kyy and the height l2 of the center of mass to be determined. The latter can be confirmed by measuring the incline angle of the hull when a weight is suspended from the stern and/or the bow. Repeating yaw measurements with the hull tilted, and then with the bifilar suspension fore and aft to measure the roll gyradius, kr ≡ kxx, allows for the angle ψ of the inertia ellipsoid (Wells 1967) principal x axis to the hull x axis to be calculated. Although the present keelboat measurements were made using ultrasonics (Daedalon, 1991) and photogates (Pasco, 2000), such measurements can now be more easily made using MEMs gyros, such as that in the iPhone (xSensor, 2010). This is illustrated by the measurements on a model keelboat.