Longitudinal Motions and Stability of Two Hydrofoil Systems Free to Heave and Pitch in Regular Waves

Experimental measurements were made of the motions of two hydrofoil configurations free to heave and pitch in regular waves. The two configurations consisted of 1) a dihedral (V) foil forward, flat foil aft, and 2) dihedral (V) foils both fore and aft, and a flat foil near the center of gravity. The motions were measured in both head and following seas for a variety of wave characteristics and two towing velocities. The experimental results were compared with quasi-steady linear and nonlinear theory, and also unsteady linear theory. In general, agreement with the quasi-steady linear theory was satisfactory, although some improvement in the correlation was obtained by considering the effects of unsteadiness. Nonlinearities had little effect on the amplitudes of the oscillatory motions. The major effect of the nonlinearities was to produce a steady downward component of heave. The measured component was in most cases greater than that derived from analog computer solutions of the nonlinear equations. Experimental investigation of the inherent stability of both configurations in smooth water indicated that the damping was greater than predicted by the quasi-steady linear theory.