The wind does not power the rotor itself, which must have its own power source. Like other sailing ships, rotor ships often have a small conventional propeller as well, to provide ease of manoeuvrability and forward propulsion at slow speeds and when the wind is not blowing or the rotor is stopped. In a hybrid rotor ship the propeller is the primary source of propulsion, while the rotor serves to offload it and thus increase overall fuel economy. Rotos sails have been reported to generate 5-20% fuel savings.