Abstract: A propulsion system for a water craft having at least one shaft extending rearwardly form the transom of the hull of the craft and a drive system for driving the at least one shaft. A surface piercing propeller is mounted on the at least one shaft, proximal to the trailing edge of the hull, so that the propeller operates in a transom cavity created behind the hull, in use of the craft. The ratio (S/C) of the spacing of the blades to the blade cord for each propeller is at least 2.0 along substantially the whole length of the blades. The propulsion system has a high operational efficiency particularly suited for high speed marine craft.