Oscillating foils located beneath the ship's hull are investigated an unsteady thrusters, augmenting the overall propulsion of the ship in rough seas and offering dynamic roll stabilization. The foil undergoes a combined oscillatory motion in the presence of waves. For the system in the horizontal arrangement the vertical heaving motion of the hydrofoil is induced by the motion of the ship in waves, essentially ship heave and pitch, while the rotational pitching motion of the foil about its pivot axis is set by an active control mechanism. In previous works, a potential-based panel method has been developed for the detailed investigation of the effects of free surface in harmonic waves, and the results are found to be in good agreement with numerical predictions from other methods and experimental data. Also, it has been demonstrated that significant energy can be extracted from the waves. In the present work we examine further the possibility of energy extraction under random wave conditions using active pitch control. More specifically, we consider operation of the foil in head waves characterized by a given frequency spectrum, corresponding to specific sea states. The effects of the wavy free surface are taken into account through the satisfaction of the corresponding boundary conditions. Numerical results concerning thrust coefficient are shown, indicating that significant efficiency can be obtained under optimal operating conditions. Thus, the present method can serve as a useful tool for the preliminary design, assessment and optimum control of such systems extracting energy from sea waves and augmenting marine propulsion.
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