Abstract: A propulsion system for a vessel comprises a conventional screw and rudder arrangement and includes one or more rotor sails in the form of an elliptical cylinder upstanding from the deck of the vessel formed from a rigid outer surface which is rotatable about a central longitudinal axis. The rotation by the Magnus effect develops a propulsion force transverse to wind currents crossing the vessel. The speed of rotation, direction of rotation and a position of park of the cylinder can be controlled. The position of park allows the major axis to be aligned with the longitudinal axis of the vessel or with the direction of the wind for feathering or transverse to the longitudinal axis of the vessel to act as a conventional sail downwind.