On 19 November 1924, a ship slowly manoeuvred through the Port of Kiel. On board were Ludwig Prandtl and Albert Betz from the Aerodynamic Research Institute (AVA) in Göttingen – a predecessor of today's DLR – along with engineer Anton Flettner. The ship, named Buckau, had been a sailing ship for many years, but by now its sails were no more. Instead, to the astonishment of many curious onlookers, two large, rotating cylinders towered over the ship's hull. How could a ship be propelled without any sails, and what were the cylinders for?
To answer these questions, we need to go back in time. In 1852, German physicist Heinrich Gustav Magnus demonstrated that when a rotating cylinder is exposed to an airstream, it is subjected to a 'transverse force'. This same force causes a 'spinning ball' in tennis or table tennis to veer off its straight trajectory, with a slice or topspin shot. Magnus himself was unable to adequately explain the phenomenon – now known as the Magnus effect – as the field of fluid dynamics was still in its infancy.
Wind tunnel investigation of a rotating cylinder at the AVA, around 1923
Research into the Magnus effect was carried out at the Aerodynamic Research Institute (AVA) in Göttingen.
A little over half a century later, Ludwig Prandtl was investigating flows around rotating cylinders in a water channel in Göttingen. Although these investigations initially received little attention, Prandtl revisited his findings after the First World War. At the same time, one of his colleagues, Carl Wieselsberger, was repeating measurements on rotating cylinders in an attempt to uncover the secrets of the Magnus effect. The breakthrough came when Albert Betz – then deputy director of the AVA – developed small, high-speed 'three-phase' electric motors, adding end plates to either end of the cylinder.
The three-masted schooner Buckau before its rotor conversion
Engineer Flettner commissioned the Germania shipyard in Kiel to refit the Buckau sailing ship according to his specifications.
The findings were published by the AVA and caught the attention of Anton Flettner – a prolific engineer with numerous patents to his name – who then came up with the idea of using the Magnus effect to propel ships. Traditional sailing vessels common until then required a large crew and were comparatively expensive to maintain, due to the cost of regularly replacing the rigging. After constructing a small model ship with rotating cylinders and testing it on Berlin's Wannsee lake, Flettner approached the AVA and asked for systematic wind tunnel tests to be conducted.
The AVA's measurements confirmed the potential of rotating cylinders as a source of propulsion, leading Flettner to commission the Germania shipyard in Kiel to refit the Buckau sailing ship according to his specifications. Rather than sail masts, it now had two cylinders measuring 2.8 metres in diameter and soaring about 18 metres above the deck. Each cylinder was powered by a 7.5-kilowatt electric motor.
The rotor ship Buckau in the Port of Kiel, 1924
The ship caused quite a stir during the sea trials.
The first voyages of the rotor ship took place in mid-October 1924. On 12 November, Ludwig Prandtl and Albert Betz were able to verify for themselves that the ship's propulsion system worked. The press followed the construction and test voyages of the Buckau with great interest and credited Anton Flettner with the invention, without mentioning the AVA's significant theoretical work on the Magnus effect. This led to some disgruntlement at the AVA, especially since Flettner had not informed the research institute that he'd already filed a patent for the application of the Magnus effect in ship propulsion. This prompted AVA researcher Jakob Ackeret – who had been involved in the wind tunnel tests in Göttingen – to publish 'Das Rotorschiff und seine physikalischen Grundlagen' [The Rotor Ship and its Physical Principles]. The book explicitly credited Flettner with the development of the rotor ship, while clearly attributing the preliminary studies to the AVA.
In 1926, the Buckau became the property of Flettner-Rotorschiff GmbH and was renamed the Baden-Baden. After two years, the Baden-Baden was converted back into a three-masted schooner. It sank in a storm in the Caribbean in 1931.
Buckau sea trial
Ludwig Prandtl (second from left) was on board for the test drive.
Interestingly, Flettner's rotor ship has enjoyed something of a renaissance over the past decade. The rotating-cylinder propulsion system now supplements conventional engines in modern container ships and ferries – reducing their fuel consumption. The container vessel E-Ship 1, for example, which entered service in 2010, has four Flettner rotors each measuring four metres in diameter and 27 metres high. The rotors operate in tandem with the ship's diesel engines, reducing its fuel use by a quarter.
