A model is under development to study the hydrodynamics of a flapping foil 1i beneath water waves. Cavitation effects and large separation phenomena are ruled out but nonlinearities due to moving and free boundaries are fully retained. In this framework, the flow field, sketched in fig. 1, can be qualitatively described in terms of inviscid-rotational fluid mechanics in which a thin vortical layer mimics the hydrofoil wake W. A suitable unsteady Kutta condition provides the mechanism for vorticity generation.