Rotor sails put a new spin on Flettner
With the growth in wind power, it is time to take another look at Flettner and the Magnus effect and their influence on shipping today.
As shipping tries to reduce its carbon footprint with the use of wind power, one increasingly popular method has been the fitting of rotor sails, more usually associated with the inventor Anton Flettner.
Indeed, vessels as large as the 325,000 dwt bulker Sea Zhoushan are being fitted with such devices. Yet the first application of the concept took place around 100 years ago. Though successful, it was not pursued further at that time.
The Magnus effect
Sailing rotors use a physical phenomenon first demonstrated in an experiment carried out in 1852 by German physicist Heinrich Gustav Magnus, although Isaac Newton reported the phenomenon as long ago as 1672.
Magnus was a German experimental scientist who initially specialised in chemistry but his later research was mostly in physics; and he would eventually publish 84 research papers.
In the Magnus effect, a spinning cylinder in a moving airstream creates a lateral force perpendicular to the direction of the airstream which, when used on ships, propels the ship forward.
Diagram of the Magnus effect, with streamlines and turbulent wake (Credit: Rdurkacz. Creative Commons license.)
Anton Flettner: From aviation to shipping
German engineer and inventor, Anton Flettner (1885-1961), first used this principle for ship propulsion at the beginning of the 1920s. He was more famous as an aviation engineer, having worked on pilotless planes and the development of the trim tab during the First World War.
After the war he turned his attention to the rotor sail concept, apparently realising how the Magnus effect had potential for sail propulsion while at the beach with his wife and flowing sand over his rotating hand, to describe it.
His initial idea was to produce the propulsion force by using a belt running round two cylinders. Later, he decided that the cylinders would be better rotated by individual motors. He applied for a German patent for the rotor ship in September 1922 and had an experimental vessel equipped with two large cylinder rotors converted at the Friedrich Krupp Germania shipyard in Kiel.
Successful prototype
At the time, the 455 gross ton Buckau, a former schooner, caused a big stir and in several trial runs successfully proved that the rotors were efficient. The vessel was fitted with two cylinders (or rotors) each about 15m high, and 3m in diameter, driven by a 50hp electric motor at 140 revs/min and fitted with end plates that improved efficiency. The vessel was also equipped with a 200hp main engine as the rotors were seen as providing additional power.
The Buckau’s maiden voyage was in February 1925 from Danzig to Scotland across the North Sea. The rotors did not give the slightest cause for concern in even the stormiest weather, and the rotor ship could tack (sail) into the wind at 20-30°, while the vessel with its original sail rig could not tack closer than 45° to the wind. On 31 March 1926, the Buckau, now renamed Baden Baden, set sail for New York via South America, arriving in New York harbour on 9 May.
Second vessel: The Barbara
In 1926, a larger ship with three rotors, the Barbara, was built by the AG Weser shipyard in Bremen. She was a single-screw ship of 2,077 gross tons, using two six-cylinder diesels totalling 1,060hp for a designed speed of 10 knots. The rotors were 16.6m tall with a diameter of 4m and spun at a speed up to 160 revs/min.
However, the rotor system could not compete economically with diesel engines, so Flettner turned his attention to other projects and the rotors were dismantled. Baden Baden was destroyed in a Caribbean storm in 1931 while the Barbara had the rotors removed at a later stage.
Climate change revives interest
More recently, with increasing concerns about climate change and the need to cut carbon emissions, interest has resumed in the Flettner rotor, with a number of vessels successfully equipped with the system as an aid to propulsion.
Magnus stabilisers
The Magnus effect is also used in a special type of ship stabiliser consisting of a rotating cylinder mounted beneath the waterline and emerging laterally. By controlling the direction and speed of rotation, strong lift or down force can be generated.
The results of model tests comparing fins and Magnus-effect rotor stabilisers indicate that, in general, they have a similar performance at zero speed. At low speeds the rotors demonstrated better performance than the fins, while at high speeds the opposite was observed.
The largest deployment of the system to date is believed to be on Russian billionaire Roman Abramovich’s 162.5m long yacht Eclipse.
John Barnes is a journalist and author and former editor of Marine Engineers Review.