Flapping foils have been increasingly studied and applied as thrusters. Their use to propel Unmanned Surface Vehicles (USV) with ocean waves is a solution to overcome endurance and range limitations. To support experimental evidence and concept development, many studies have been carried out, employing methods based on linear, non-linear theories and numerical approaches. This work presents an analytical model intended to provide a better understanding on the physics affecting foil propulsion when oscillating in incident waves that can be used as an engineering tool to improve this concept. The formulation of the problem is introduced and the theoretical model is described. Several simulations are carried out, for different combinations of parameters and results are compared with data available in the literature. Results suggest that the simple model captures with enough precision oscillating foil lift and thrust forces for the conditions and ranges of interest. However further development is required, namely to include free surface effects and to study flow separation. A practical example of a vehicle propelled by a foil oscillating in waves is given.
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