A time-domain simulation for predicting the downwind performance of yachts in waves

Yachts racing in many of today's high profite races, such as the America's Cup and the Volvo Ocean Race, spend much time sailing downwind in following seas. The development of a method for predicting the performance of yachts sailing downwind in waves would therefore provide a valuable design tool for racing yacht designers. This paper describes the development of a timedomain simulation for predicting the performance of yachts sailing in irregular seas for apparent wind angles between 90 and 180 degrees. The simulation output may be used to either directly compare different designs or augment existing polar plots for the effect of a following sea. The simulation is comprised of three main modules: the wave induced longitudinal force, the resistance force and the sail aerodynamic force. The resistance and wave force modules have been validated through semi-captive model experiments. Results from the complete simulation have been compared with those obtained from free running model experiments. Numerical experiments on a number of hull and rig configurations have been conducted using the simulation. Results are presented with conclusions being drawn on the effect of hull form and environmental conditions on downwind performance.