A rotor ship is a type of ship designed to use the Magnus effect for propulsion. The ship is propelled, at least in part, by large powered vertical rotors, sometimes known as rotor sails. German engineer Anton Flettner was the first to build a ship that attempted to tap this force for propulsion, and ships using his type of rotor are sometimes known as Flettner ships.
The Magnus effect is a force acting on a spinning body in a moving airstream, which produces a force perpendicular to both the direction of the airstream and the axis of the rotor.
Principles of operation
A rotor or Flettner ship is designed to use the Magnus effect for propulsion. 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.
Due to the arrangement of forces, a rotor ship is able to sail closer to the wind than a conventional sailing ship. Other advantages include the ease of control from sheltered navigation stations and the lack of furling requirements in heavy weather.
However if the ship changes tack so that the wind comes from the other side, then the direction of rotation must be reversed or the ship would be driven backwards.
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. Rotor sails have been reported to generate 5-20% fuel savings.
The German engineer Anton Flettner was the first to build a ship which attempted to use the Magnus effect for propulsion. Assisted by Albert Betz, Jakob Ackeret, and Ludwig Prandtl, Flettner constructed an experimental rotor vessel; October 1924 the Germaniawerft finished construction of a large two-rotor ship named Buckau. The vessel was a refitted schooner which carried two cylinders (or rotors) approximately 15 metres (50 ft) high, and 3 metres (10 ft) in diameter, driven by an electric propulsion system of 50 hp (37 kW) power.
The Buckau sailed from Danzig to Scotland across the North Sea in February 1925. The ship could tack (sail into the wind) at 20–30 degrees, hence the rotors did not give cause for concern in stormy weather. The ship was renamed Baden Baden after the German spa town and on 31 March 1926 was sailed to New York via South America, arriving in New York Harbor on 9 May.
The ship had proved inefficient on these voyages, with the power consumed by spinning 15m tall drums being greatly disproportionate to the propulsive effect when compared with conventional propellers. As the system could not compete economically, Flettner turned his attention to other projects and the rotors were removed.
Interest in rotor sails revived in the 1980s, as a way of increasing the fuel efficiency of a conventionally powered ship.
Enercon launched the hybrid rotor ship E-Ship 1 on 2 August 2008. From 2010, it has been used to transport the company's turbine products and other equipment. Enercon claim "operational fuel savings of up to 25% compared to same-sized conventional freight vessels."
In 2007, Stephen H. Salter and John Latham proposed the building of 1,500 robotic rotor ships to mitigate global warming. The ships would spray seawater into the air to enhance cloud reflectivity. A prototype rotor ship was tested on Discovery Project Earth. The rotors were made of carbon fibre and were attached to a retrofitted trimaran and propelled the vessel stably through the water at a speed of six knots.
In 2009, Wärtsilä proposed a cruiseferry that would utilise Flettner rotors as a means of reducing fuel consumption. The Finnish ferry operator Viking Line adopted the idea, with MS Viking Grace built in 2011–2012, initially without rotors. A rotor system was retrofitted in 2018.
In 2014 and 2015, Norsepower installed twin rotor sails on Finnish shipping company Bore's RoRo vessel M/V Estraden. In May 2018, the 1996 built cargo ship Fehn Pollux of the German based Fehn Shipmanagement (Leer) was fitted with an 18m long Flettner rotor of the EcoFlettner type at the front.
In 2018, Norsepower started testing the rotor concept with the world's biggest shipping company, Maersk. The Maersk Pelican, an LR2 class tanker, has been fitted with two Norsepower Rotor Sails in readiness for trials. 
In 2021, Norsepower announced to install a tilting rotor sail to allow maneuvering below bridges.
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