Application of a Wave Devouring Propulsion System to Ocean Engineering

A WDPS (wave devouring propulsion system) is a device that generates thrust directly from wave power and, at the same time, generates large damping forces. This phenomena is same as the thrust generation by the oscillating hydrofoil which are commonly used as the animal propulsion system such as the sea mammals or birds or fish propulsion. Relative flow acting on the hydrofoil generates thrust and damping force respectively. It is quite simple and consists of hydrofoils positioned in front of the bow. If the WDPS is installed in a ship hull, it can drive the hull (even against waves). One example is shown in Photo 1-1, the small WDPS vessel named Mermaid II, which succeeded in a historical trans-Pacific voyage from Hawaii to Japan (about 7800 km) in 2008 using only wave power. It proves that the WDPS may have some potential in the field of ocean engineering. In this paper we will propose two WDPS applications. One potential application is a midsized floating-type ocean wind turbine generator, which is composed of a single wind turbine on a catamaran hull with a set one-point mooring system. For this application, a new hull form and WDPS are developed based on Mermaid II. The hull needs to be stabilized because the ocean wind energy absorption efficiency is affected by the hull’s motion. Additionally, the mooring forces acting on the hull need to be reduced to keep the construction and power generation costs down. Therefore, the WDPS is installed in front of the hull to reduce/overcome the wave drifting force and the wind drag force. Another anticipated function of the WDPS is motion stabilization, particularly for pitch and roll motions. A suitable hull form is determined from wave tank experiments and is discussed herein. The other proposal is a multi-function WDPS that functions as both a wave energy absorption device and a wave thruster. Many hydrofoil-type wave energy conversion systems with forward propulsion have been proposed, but no advanced speed-type systems with a multi-function WDPS. A newly designed forced heave-pitch oscillator is also introduced, and tests are performed in the wave tank. The optimum hydrofoil control method in waves is discussed, and the thrust control is also tested and discussed.