Wind tunnel experiments using a Real-Time Velocity Prediction Programme to investigate the optimal trim of a VO70 model under various simulated true wind speeds are reported. The results illustrate that the decision made depend upon the particular apparent wind direction and true wind speed. It is suggested that these can be sub-divided into three broad bands: low wind speeds where the total drag is minimised and the trim that provides the maximum thrust coefficient is chosen, moderate wind speeds where the heel angle has a strong effect and the optimum choice includes a reduction in lift coefficient and centre of effort height and strong winds where the heel angle and hence heeling moment is limited to the maximum acceptable value and the optimum loading distribution is strongly constrained by this limit. Extended Lifting Line Theory is used to further investigate the detailed loading distribution on an AC90 mainsail. The result illustrate the way in which the optimal distribution changes with varying conditions.