Using the magnus effect, a flettner or rotor ship can propel itself with the help of the wind.
The Magnus effect is a force that acts on a spinning body in a moving airstream, which acts perpendicularly to the direction of the airstream. In addition to the familiar principle of backspin imparted to increase range in ball sports, the Magnus effect was also employed in the bouncing bombs developed by Barnes Wallis.
Rotor ships typically use rotor sails powered by a motor to take advantage of the effect. German engineer Anton Flettner was the first to build a ship which attempted to tap this force for propulsion.
Typical expected fuel reductions in ships using this technology range in the 10-30% range.