Lift and Drag on Surface-Peircing Dihedral Hydrofoils in Regular Waves

Experimental measurements were made of the lift and drag of a restrained surface-piercing dihedral hydrofoil in regular head and following seas. Two velocities and a variety of wave lengths and amplitudes were used. The oscillatory lift was predicted with fair success using linearized theory as developed by Ogilvie. The experimental forces were observed to contain harmonic distortion, and comparison of the second harmonic component obtained from nonlinear quasi-steady theory indicated that the calculated values were too low. No theory was available for comparison with the oscillatory drag forces. Tests with a restrained tandem dihedral configuration in smooth water indicated that the performance ratio of the aft foil could be considerably increased for a particular separation of the foils. The optimum separation increased with increasing velocity. A brief series of tests were made in head seas using the optimum foil separation, and little difference between the tandem foils and single foil was noted for the oscillatory lift component.