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.
The effect can be applied to all manner of objects from cricket or baseball balls through to aircraft and ships. 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, the forward surface of a rotor ship’s cylinder being made to move into the wind – ie clockwise into a starboard wind, counter-clockwise to a port wind – suction is strongest on that forward surface and the ship is drawn ahead.
It is fitting that Buckau the first vessel fitted with Flettner rotors in 1924 was initially built as a sailing vessel that was later fitted with a small 160hp auxiliary engine. To accommodate the rotors the ships original sailing rigging was removed to be replaced by a pair of 15.6m high, 2.8m diameter Flettner rotors each driven by an 11kW electric motor, the cylinders rotated at 150rpm. The electric motors were reversible, so the vessel could manoeuvre according to the wind direction. During trial runs, a speed of 7.5kt was achieved using only the main engine and with the rotors not turning. With the rotors running and the main engine stopped, the Buckau achieved 8.5kt.
Observations of the action of the Magnus effect as applied to the Buckau showed that optimum exploitation of wind force was achieved when the wind was at an angle of 100–130° to the course of the vessel. Even if the wind came from astern, the rotor drive was able to create sufficient power. Long tacks against the wind were reported to have increased speed. At the same time, it was found necessary to permanently optimise the relation of rotor speed to wind speed.
The seeming superiority of the system on Buckau over its performance when driven solely by its motor disappeared as engines and propellers evolved to become more efficient themselves. 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.