Prediction and Measurement of the Roll Motion of a Sailing Yacht at Zero Forward Speed

The roll motion of a yacht at zero Froude number is investigated, motivated by limitations of existing theoretical models of roll motion when applied to bodies with large appendages. A time domain single degree of freedom roll model has been developed in order to identify the dominant excitation and damping sources. The canoe body forces were determined from a wave diffraction program whilst the appendages were treated as fully submerged flat plates. Calculation of the forces acting was based on a stripwise Morison formulation. A series of full scale validation experiments has also been conducted, in calm water and in ocean waves. The results show that the keel, rudder and sail dominate the damping, whilst the canoe body contributes very little. The hydrodynamic damping is non-linear with respect to wave amplitude, but the overall damping with a sail hoisted is only weakly non-linear with respect to wave amplitude because the sail dominates the damping, particularly in a wind field. The numerical model predicts a strong influence of wave heading which is not borne out by the full scale trials.