Experiments with actively pitch-controlled and spring-loaded oscillating foils

• A horizontal foil with harmonic heave motion and steady forward speed was studied.
• Forced harmonic pitch, active pitch control and spring-loaded pitch was tested.
• The phase between heave and pitch motions is important for the foil thrust.
• The spring-loaded foil was superior to the pitch-controlled foil at low speed.
• The reason for this is believed to be the phase angle between heave and pitch.

Experiments with a horizontal foil undergoing harmonic heave motion while traveling at a steady forward speed were performed. The objective was to study different pitch strategies for maximizing the forward thrust. An oscillating foil can replace the rotating propeller as ship propulsor, or the foil can be mounted to the ship in order to create auxiliary thrust in waves. The latter was the motivation behind the experiments presented in this paper. An actively pitch-controlled foil was tested, where two vanes measured the inflow angle to the foil and a motor pitched the foil based on a specified control algorithm. The effect of varying the parameters in the pitch-control algorithm was tested in practice and is discussed here. A spring-loaded foil was also tested. The pitch motion of the spring-loaded foil is shown to produce higher thrust than that of the actively pitch-controlled foil tested. It was also found that the efficiency with spring-loaded pitch motion was higher than the efficiency with forced harmonic pitch motion leading the heave motion with a 90° phase angle.