What are Flettner rotors?
Reviving a concept first employed in the 1920s but soon forgotten as improvements in engine and propeller technology appeared to remove the advantage they gave, Flettner rotors are now enjoying renewed interest. The first vessel to be equipped with them in the modern era was the 2008-built E Ship 1 owned by renewable energy firm Enercon. More recently a Finnish organisation Norsepower has fitted them to and has orders for other ships including an 8,000dwt general cargo ship. The concept is often described as making use of wind power but this is somewhat misleading as the motive power that Flettner rotors can provide is not wind power in the conventional understanding but exploitation of a phenomenon known as the Magnus effect named after German scientist Gustav Magnus who first described the concept in 1852. To take advantage of the effect the rotors must be powered and capable of moving in forward and reverse directions depending on the wind direction. Thus they will consume power but the effect can outweigh this. The principal behind the effect is that rotating any round object such as a sphere or a cylinder that is moving through a fluid or air affects its trajectory and speed in several ways, in the case of a rotor ship, the wind passing over the surface creates suction. Suction is greatest on any part of the surface that does not move with the wind. Thus, if the forward surface of a rotor ship’s cylinder is made to move into the wind – ie clockwise into a starboard wind, counter-clockwise to a port wind – the suction will be strongest on that forward surface and the ship is drawn ahead. Under some wind conditions, the rotors will not confer any advantage and will need to be stopped. The fact that wind speed and direction is constantly changing also means that a much more sophisticated operating system than was possible in 1924 is needed to gain the best results. Modern sensors and computer technology can adapt the speed of the rotor almost instantly when in use but even so under some circumstances, the wind will be of a strength and direction that the system will be of little use.