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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 vertical rotors, sometimes known as rotor sails. German engineer Anton Flettner was the first to build a ship which attempted to tap this force for propulsion, and the ships 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 the direction of the airstream. This is used in backspin to increase range in ball sports, and also the bouncing bombs developed by Barnes Wallis.  As described by Lloyd Bergeson (Naval Architect) who fitted a Rotor to the 42 Ft MV Tracker , Rotor ships take advantage of this same effect by spinning a large vertical cylinder, typically using an electric motor, and using the resulting force for propulsion. Due to the arrangement of forces, rotor ships are able to sail closer to the wind than conventional sails. Other advantages described by Bergeson are the ease of control from sheltered navigation stations and the lack of furling requirements in heavy weather.
The rotor ship should not be confused with the similar-looking turbosail. The turbosail also uses a solid vertical "sail", but uses airflow within the hollow interior to generate a force conventionally, instead of using the Magnus effect on the outside of the sail.
A rotor/Flettner ship is a type of ship 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 largely perpendicularly to the direction of the airstream. Rotor ships typically use rotor sails — spinning bodies that are essentially vertical cylinders — powered by a motor to take create the effect when there is sufficient wind available.  These types of propulsion cylinders are now commonly called Flettner rotors. Rotor ships have unique points of sail.[clarification needed]
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.
Following completion of its trials, the Buckau set out on her first voyage, from Danzig to Scotland across the North Sea, in February 1925. The rotor ship could tack (sail into the wind) at 20-30 degrees, while a vessel with a typical sail rig, cannot tack closer than 45 degrees to the wind; hence, the rotors did not give cause for concern in stormy weather.
Despite having completed trouble-free crossings of the North Sea and Atlantic the power consumed by spinning 15m tall drums was vastly disproportionate to the propulsive effect when compared with conventional screws (propellers). As the Flettner system could not compete economically Flettner turned his attention to other projects, such as his rotor aircraft.
Several types of rotor ships can be distinguished. Rotor sail-only ships exist, as do rotor sail-assist (hybrid) ships. Wind Ship Development Corporation has two types of sail-assist designs, for use with different sizes of ships. In practice, most rotor ships have a system with an electric motor that allows the initial start and eventual stop of the rotor by crew. Rotor's rotational speed (i.e., RPM) and direction of spin can also be controlled.
There has been renewed interest in the rotor ship concept in the later 20th century.
The German wind-turbine manufacturer Enercon launched and christened a new rotor ship, E-Ship 1 — see opening image, this article — on 2 August 2008. The ship is being used to transport turbines and other equipment to locations around the world; the maiden delivery of turbines for Castledockrell Windfarm arrived in Dublin Port on 11th Aug 2010. On 29 July 2013, Enercon provided a press release claiming a potential for "operational fuel savings of up to 25% compared to same-sized conventional freight vessels." after 170,000 sea miles; actual performance figures were not provided.
The THiiiNK Holding organization describes an improved version of the Flettner rotor that is retractable and supplemented by an additional control surface. The page claims that "The system has been developed and tested both in tank tests and in full-scale sea trials." The design improvement claims include improved rotor performance (by 50% or more), and an improved internal rate of return (IRR) compared to a standard rotor (up to 55%).
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 successfully propelled the vessel stably through the water at a speed of six knots. The focus of the experiment was based on the ability for the boat to move emissions-free for a specialized purpose, leaving it unclear[according to whom?] whether or not the efficiency of the rotors was on parity, inferior to, or superior to conventionally propelled vessels.
In 2009 the Finland-based maritime engineering company Wärtsilä unveiled a concept for a cruiseferry that would utilise Flettner rotors as means of reducing fuel consumption. This concept has been developed in association with the Finnish ferry operator Viking Line, (the ship, M/S Viking Grace, was built 2011–2012, without the rotors).
In 2014 Norsepower, a Finnish clean technology and engineering company pioneering the generation of renewable wind energy for the global maritime industry, announced that it will bring to the market the "Norsepower Rotor Sail Solution" which is a completely modernized version of the Flettner rotor. Later in 2014 Norsepower installed the first Norsepower Rotor Sail on Finnish shipping company Bore's RoRo vessel M/V Estraden, and in the end of 2015 Norsepower installed a second similar unit on the same ship. In the beginning of 2016 it was released by Norsepower, that based on the successful sea trials on board M/V Estraden, the technology has potential for fuel savings of up to 20% for vessels with multiple, large rotors traveling on favourable wind routes.
After Norsepower's success with the new design, also Viking Line returned with the rotor concept for their next planned newbuilding. The first image of a new 63,000 GT vessel shows large Flettner rotors which could help the ship to reduce fuel consumption with up to 15%. Letter of intent was signed for the ship in November 2016, awaiting final agreements signing in early 2017. According to a Finnish newspaper, the rotor concept of Viking Line is based on Norsepower Rotor Sails. The latest news from Norsepower is that they have agreed with world's biggest shipping company, Maersk, to start testing the rotor concept in Maersk ships starting beginning of 2018. The managing director of Norsepower, Tuomas Riski, promises their innovation to cut 7-10% of fuel cost leading up to 300000€ savings in big tankers.
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