Resonant behaviour in the gap between two adjacent bodies on a free surface is investigated numerically. The study is done within the framework of two-dimensional potential theory. It is of common conception that traditional potential theory overpredicts the gap elevation around resonance due to neglect of viscous effects. In this study we consider empirical formulas for skin friction, eddy damping and bilge keel damping, and include the effects by a free surface damping model. A non-physical free surface damping model is also considered, and the damping effects from the two models are compared. Some brief comments are given on three-dimensional effects.