Flettner Rotor: Sailing Ships Without Sails

In 1926, a 2,000-ton steel-hulled schooner named Buckau made an extraordinary crossing across the Atlantic. Although the Buckau was technically a sailing ship, it had no sails—at least, not conventional ones.

Rather than thin masts and billowing sheets of white, the Buckau had two huge cylinders that rose from its deck and spun. By a physical phenomenon called the Magnus Effect, the spinning poles generated a propulsive force that carried the ship forward. It’s the same force that footballers use to bend a corner kick, cricketers use to swing a delivery, and a tennis player use to hang the ball just a little longer in the air.

The vertical columns on this ship are not smokestack. They are rotating sails. Photo: Alan Jamieson/Flickr

It was German aviation engineer Anton Flettner who was among the first who realized that the Magnus Effect could be used for purposes other than sports. As the director of the Institute of Aerodynamics at Amsterdam, Flettner filed an application for a German patent in 1922 on a ship propelled by vertical circular cylinders rotating in the wind. In 1924, he purchased the Buckau and aided by the famous aerodynamicist Ludwig Prandtl, among others, he converted it to the first rotor ship at Kiel, Germany.

The ship had two rotating cylinders, 10 feet in diameter and 50 feet high, driven by two 15-hp electric motors with power supplied by a 45-hp diesel generator. The rotors were spun such that the side facing the front of the ship turned in the same direction as the wind. The surface of the rotating cylinders drag air with it causing the air to move faster across the front surface and slower behind, where the counterwise turning of the rotor surface, with respect to the direction of the wind, created a high pressure zone just behind the rotor. The pressure difference between the front and the back surface of the rotor pushed the ship forward. Flettner reported that the Magnus effect created a propulsive force that is approximately ten times greater than the force against a sail having the same cross-section as the cylinder. Besides, the rotors were lighter than the masts and rigging they replaced, despite being constructed of heavy gauge steel.

Graphics adapted from Science Mag.

The Buckau performed magnificently, moving at nearly twice her former speed. She could also tack to the wind at a shallower angle compared to conventional sailing ships. In other words, she could sail closer into the wind and still derive motive power from it, where a conventional sailing ship would stall.

In February 1925, after about sixty trial voyages around Kiel, the ship was put into commercial service hauling lumber from Gdansk, Poland to Leith, Scotland through the North Sea. She handled the stormy weather exceptionally well because she had no sails. In 1926, the ship was renamed the Baden Baden after the German spa town, and in 1926 she sailed across the Atlantic to New York in less than six weeks. The Baden Baden continued to haul timber in American waters for several years, before she was converted to conventional engine and propeller propulsion because it was more energy efficient. Fossil fuel, after all, was cheap and abundant. She eventually sank in the Caribbean Sea in 1931.

The Buckau. Photo: Wikimedia Commons

Anton Flettner’s invention remained forgotten for half a century, until concern for environmental pollution due to burning of fossil fuel and rising cost of fuel provided shipping companies an incentive to look for a more efficient method of propulsion.

Most sea-going vessels burn the dirtiest kind of fuel called Heavy Fuel Oil or bunker fuel. It’s the component of crude oil that is left after all useful products have been distilled and extracted, leaving a tar like residue. Heavy Fuel Oil is cheap but it generates toxic chemicals when burned. According to one report, the 15 biggest ships in the world emit more sulphur oxide than all the world’s cars put together. This is concerning since ships carry more than 90 percent of the world’s goods over the ocean. In 2019, nearly 800 million standard-sized shipping containers were moved across the ocean, with the Chinese port of Shanghai alone seeing more than forty thousands containers pass through.

The difference between diesel and bunker fuel. Diesel is transparent while bunker fuel is thick and black.

In 2008, Enercon, a wind turbine manufacturer launched the first hybrid rotor ship called E-Ship to transport the company's turbine products and other equipment. The ship’s primary propulsion is the propeller, but when wind conditions are favorable, the rotor sails are set into motion. This allows the main engines to be throttled back, saving fuel and reducing emissions. Enercon claim the rotors have improved fuel savings by up to a quarter.

In 2012, a group of Finnish engineers started a company called Norsepower, manufacturing rotor sails of different sizes. In 2015, Norsepower installed twin rotor sails on Finnish shipping company Bore's RoRo vessel M/V Estraden. In 2018, they fitted the cargo ship Fehn Pollux with an 18-meter-long Flettner rotor. The same year, they started testing the rotor concept with the world's biggest shipping company, Maersk. In the trial, two sails were positioned aboard the Maersk Pelican, a ship with a gross tonnage of more than 60,000. The sails resulted in fuel savings of 8.2 percent, or about 1,400 tons of CO2.

The rotorship "Barbara" in the city of Barcelona (Catalonia) in 1926. Photo: Wikimedia Commons

Unlike Flettner’s all-steel rotors, Norsepower uses modern materials such carbon and glass fibers that cuts weight by a factor of three. That means less electricity is required to spin the rotor, which translates into more efficient propulsion.

“Our largest rotor sails can provide forward thrust equivalent of up to 3 megawatts of main-engine power while drawing less than 90 kilowatts of electricity,” said Tuomas Riski, CEO of Norsepower Oy Ltd. Furthermore, ships can easily be retrofitted with rotors and activated by an on/off switch, and its rotation controlled automatically by software.

In the last few decades, shipbuilders are incorporating different wind-propulsion technologies to cut back on emission and improve efficiency. One such example, beside rotor sails, is the kite sail. But Flettner rotors are getting the earliest adoption, says Sofia Werner, a naval architect who is studying their performance.

Norsepower Rotor Sails onboard Maersk Pelican. They are the largest Flettner rotors in the world. Photo: Wilsca/Wikimedia Commons

Flettner rotors at the bow of the German RoLo cargo ship E-Ship 1. Photo: Carschten/Wikimedia Commons

References:
# Wayne L Neu, Flettner rotor ship, McGraw Hill
# H. O. Herzog, More Facts About the Flettner Rotor Ship, Scientific American
# At last, rotor sails are back, Engineering at Sea
# Katherine Kornei, Spinning metal sails could slash fuel consumption, emissions on cargo ships, Science Mag
# Lynn Freehill-Maye, Rotating Sails Help to Revive Wind-Powered Shipping, Scientific American