Your clients invest considerable resource in acquiring a yacht which is luxurious and safe. Part of their dream is to be able to anchor in a secluded bay surrounded by nature with 5 star luxury on board. The reality can turn into a stomach churning nightmare if the vessel starts to roll - exit your next commission. Roll motion is a nuisance on both motor yachts and sailing yachts for the following reasons: • it causes sea sickness • crew and passengers may fall and hurt themselves • embarking and disembarking become difficult and possibly dangerous • noise is generated through water slap on the hull and motion of inadequately secured objects • some on board equipment will not perform adequately All yachts roll to a greater or lesser extent when subject to waves. When the vessel is on passage and travelling at reasonable speed the roll motion can be controlled by the use of fin stabilisers. Roll motion reduction in excess of 40% is readily achievable using this well proven technology (Haywood et al. 1995). However, when the vessel is moving slowly or is at anchor, fin stabilisers do not work because they require water to flow over the foil at high speed in order to generate the roll-reducing forces. A different solution is required when the vessel is not moving through the water.

Yachts racing in many of today's high profile 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.