A windmill ship, wind energy conversion system ship or wind energy harvester ship propels itself by use of a wind turbine to drive a propeller. They use wind power through a mechanical or electrical transmission to the propeller. Where transmission is electric, storage batteries may also be used to allow power generated at one time to be used for propulsion later on. Windmill ships should not be confused with rotor ships, which instead rely on the Magnus effect for propulsion.

Points of sail

Equipped with a wind conversion device that can rotate 360° no matter in which direction the ship is heading, a windmill ship can sail in any direction.

Types

Several types can be made; these include wind-turbine-only ships as well as hybrid ships which store wind power from the turbine when the ship does not need to be propelled. To reduce the energy required to propel the boat, windmill ships are often equipped with low-friction hull designs, such as multihulls, or they are hydrofoils. Boats without low-friction hulls or hydrofoils can be equipped with wind turbines, but often the force generated by the turbines alone is not sufficient to propel the craft. In this case, the turbines only provide supplemental force to conventional sails or other propulsion systems. At present, research is ongoing and the best types of bladed rotors still need to be determined. For example, high horizontal axis windmills are proven to make the ship less stable. Therefore, vertical axis wind mills (e.g., Savonius turbines) are sometimes preferred. Also, the wind mill needs to be highly durable as marine environments tend to degrade windmills more quickly than what is common on land.

Current ships

Few windmill ships have been built to date; these include: The film Waterworld starring Kevin Costner featured a trimaran powered by a vertical-axis Darrieus wind turbine.