The need to reduce green-house gas emissions from shipping has reborn the interest in wind propulsion for commercial cargo vessels. This places new requirements on the tools used in ship design, as well as the methods usually applied in sailing yacht design. A range of design tools are used by designers at various stages in the design of wind-assisted ships and for different purposes. One important tool is the steady-state velocity prediction program (VPP) which is typically used to predict the speed of the vessel when sailing in a range of wind directions and wind speeds. Steady-state VPPs can be very efficient and fast and may be used to rapidly assess a large number of design alternatives. However, steady-state VPPs are not able to consider dynamic effects such as unsteady wave forces on the hull which may require the rudder to be active to control the heading and course of the vessel. This, in turn, leads to different mean forces than those predicted by a static VPP and therefore the sailing performance may be reduced compared to the predictions of a static VPP. Another effect of the ship’s motions in a seaway is that the angles of attack of the sails fluctuate, which can lead to different optimum sheeting angles and possibly a loss of performance. This study uses an unsteady 3D fully nonlinear potential flow hydrodynamic model coupled with an efficient lifting-line aerodynamic model to investigate the differences in sailing performance of a vessel sailing in steady conditions to the performance when sailing in a seaway and gusty wind based on a spatio-temporal wind model. The analysis shows clearly that the unsteady wind model affects the predicted performance. This is especially the case when sailing close-hauled and on a beam reach, where large changes in the local sail angles of attack can be observed.