January 10, 2021

The concept of the Flettner rotor was first devised by Finnish inventor and architect Sigurd Savonius in the 1920s, concurrently with German aviation engineer Anton Flettner. The concept was based on the Magnus effect.
The Magnus effect is an observable phenomenon that is commonly associated with a spinning object moving through air or another fluid. The path of the spinning object is deflected in a manner that is not present when the object is not spinning. The deflection can be explained by the difference in pressure of the fluid on opposite sides of the spinning object. The Magnus Effect depends on the speed of rotation.
The German physicist Heinrich Gustav Magnus described the effect in 1852.
On the early 1920’s Flettner installed two vertical cylinders 15 meters high and 2.75 meters in diameter on a 52 m dismasted sailboat, the Buckau. The cylinders were driven in rotation by a small engine. The experience was conclusive as the ship moved faster than before under sail. In 1926, Flettner embarked on the Hamburg-New York crossing via The Canaries, with his then renamed Baden-Baden. Left on April 2, he arrived on May 9.
The rotating sails provided a net energy gain, but before they could be widely adopted the Great Depression struck, followed by World War II. The Flettner rotor would be abandoned for almost a century in favor of burning fossil fuel.
The Hamburg Amerika Line then placed an order for ten rotor ships. But only one copy was built. 92 m long and equipped with 3 rotors. The Barbara was placed on the Hamburg-Italy line, carrying 3,000 tonnes of goods. After six years of use, the cylinders were dismantled and the ship, fitted with conventional propulsion.
In 1980, the Japanese launched the small tanker Shin Aitoku Maru. It has two masts carrying a rigid sail, adjustable and hydraulically orientable. The system looked promising and it was a plan to equip17 other vessels.
The small British bulk carrier Ashington was fitted in 1988 with an auxiliary propulsion system, a rigid wing mounted on a turntable at the top of its chimney. The overall look, unsightly, was computer-controlled and saved 10% in terms of the hold.
Almost 100 years after the original project of Anton Flettner, the Finish entrepreneur Tuomas Riski started to search for a new potential cleantech concept. He happened to meet the famous naval architect, Professor, Dr. H.C Kai Levander from Finland, who had an idea of bringing a modernized Flettner rotor to the market. From this union of well-known principles and new innovation, Rotor Sail technology was born. The company Norsepower was founded in late 2012 with headquarters in Helsinki, Finland.
Norsepower Rotor Sails are modernized versions of Flettner rotors based on the Magnus effect. The original basic engineering solution had a limited degree of sophistication, but Norsepower has created various new improvements for which several patents have been granted. The Norsepower Rotor Sail technology is around ten times more efficient than a conventional sail, because more lift is produced with a much smaller sail area. Due to its simplicity, it requires no reefing or crew attention when in operation. It is “push-button wind propulsion” from the bridge.
Tuomas Riski, CEO and partner of Norsepower, said: “Our vision is to set the standard in bringing sails back to ocean transportation, and empower shipping towards reaching the goal of zero carbon emission. Our entire team is strongly motivated by our mission to reduce the environmental impact of shipping with our Rotor Sails.”
Today the tanker Maersk Pelican, the ferry Viking Grace, the MV Estraden and MV Copenhagen are equipped with Norsepower rotol sails. The SC Connector will be the next to use the Norsepower technology.
London based, Anemoi Marine technologies is also developing rotor sails. In November 2020, the company signed a joint development project with Lloyd’s Register and Shanghai Merchant Ship Design and Research Institute (SDARI) to develop a series of new energy efficient vessel designs equipped with Rotor Sails.
(Source: Norsepower/Scientific American/Anemoi – Image: Buckau ship fitted with rotor sails/DNV GL)