Flettner rotors are vertical cylinders which spin and develop lift due to the Magnus effect as the wind blows across them. Flettner rotors must be mechanically driven to develop lift and propulsion power, and maneuverability is restricted by wind speed and direction. Working on a ship, the force created will generate thrust. On board vessels, such rotor propulsion are often called Flettner rotors after the German innovator who was the first to install such a system on board a ship at the beginning of the 1920s. Flettner rotors may reduce the energy consumption of a ship, but they cannot be used as main propulsion.