Performance of heaving and passively pitching hydrofoils

Bio-inspired ship propulsion systems often consist of a hydrofoil in combined heaving and pitching motion. In order to reduce the number of actuators from two to one, the pitching motion in our design happens passively under the influence of the hydrodynamic moment on the hydrofoil, and is counteracted by a joint with adaptable stiffness. Since passive pitching is a relatively new development in bio-inspired ship propulsion, a proof of concept is needed. Further, it is not well known whether resonance is good or bad for the performance. The scaled prototype of a new, bio-inspired, heaving and passively pitching ship propulsor was built and tested. A new mechanism for passive pitching, that allows adaptation of the natural frequency was developed. The thrust and lift forces, as well as the pitching angle were measured for a range of heaving and natural frequencies. This article presents the design of the propulsion system and the results of a series of measurements. Resonance is possibly beneficial for maximal thrust production, but it would happen at a natural frequency outside the possibilities of the experiment. The highest thrust-to-lift ratio, a measure for the effciency, occurs at high heaving and low natural frequency, far away from resonant behaviour. Hence, passive pitching works, and the optimal natural frequency depends on the goal of the design: maximal thrust or maximal effciency.