The Magnus effect is a force acting on a spinning body in a moving airstream, which acts perpendicular to both the direction of the airstream and of the rotor axis. A Magnus rotor used to propel a ship is called a rotor sail and is mounted with its axis vertical. When the wind blows from the side, the Magnus effect creates a forward thrust. Thus, as with any sailing ship, a rotor ship can only move forwards when there is a wind blowing. The most common form of rotor sail is the Flettner rotor.