PROPULSION DEVICE

A wingsail for a powered watercraft comprising a first aerofoil element, a second aerofoil element, and a third aerofoil element, wherein each of the aerofoil elements is rotatable about an axis and has an aerofoil section with a centre of area which is closer to its leading edge than its trailing edge, and the aerofoil elements are movable to a configuration in which flow from the trailing edge of one of the first or third element is directed toward the leading edge of the second element, and flow from the trailing edge of the second element is directed toward the leading edge of the other of the first or third element.