Expressions for the unsteady lift and moment acting on an oscillating hydrofoil submerged under a free surface are derived by an extension of classical unsteady thin-airfoil theory. The method of images is employed to account for the free surface, but the additional contribution resulting from surface waves, considered small, is neglected. The results of flutter computations are presented for a hypothetical example, using a simple representative station analysis, for ratios of depth of submergence to semi-chord of 2 and infinity; it is found that the lower flutter speed corresponds to infinite depth of submergence.

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