Catching up with the world
by Edwin E. Slosson
In: Colliers. Vol. 78, no. 8 (August 21, 1926) p. 32.
How Rotors Move Ships
The Flettner rotor is the first application to the practical affairs of a scientific law, discovered by Magnus seventy-five years ago and neglected ever since, except as baseball pitchers employed the principle unconsciously to give a curve to the ball. This principle is simply that a rotating sphere or cylinder in the wind receives a pressure that shoves it sidewise as well as in the direction of the wind.
Flettner was enabled to make this discovery because he is a sailor-scientist. As a youth he sailed before the mast to Australia, and spent his vacations working on the Rhine steamers of his father's company. He was a peaceful teacher of mathematics and physics when the war broke out and he was called to figure out some way of making airplanes more stable. He invented the Flettner rudder, which he later applied to ships. This is a small auxiliary rudder which utilizes the current to bring the big rudder into position.
When the Germans lost the large part of their coal lands, they turned their attention to wind power, the earliest motive power of man. Flettner found that sails had been practically perfected in the six thousand years that men had been using them. So he dug up the forgotten formula of the Magnus effect and worked it out, first mathematically on paper, then experimentally in the wind tunnel of the Aërodynamical Institute at Göttingen. So completely did he solve the novel problem by this scientific method of approach that the first ship he equipped with the new motive power, the 600-ton Backau, now called the Baden-Baden, crossed the North Sea to England, and then the ocean to America.
Last April a larger rotorship was launched, the Barbara of 3000 tons, with three rotors instead of two, each 55 feet tall and 13 feet in diameter. These are hollow cylinders of aluminum, turned by a small engine at the rate of 140 revolutions per minute. The weight of these cylinders is carried on ball bearings, set two thirds of the way up.
The rotors look like tall and awkward smokestacks, but they are really lighter and less top-heavy than sails and masts. Safer too, because in a hurricane all sails have to be furled in a hurry, while the only effect of high wind on a rotor is to speed up the ship. Herr Flettner anticipates that the chief employment of the rotors will be as auxiliary motive power in steamers to steady the ship and economize on coal when the wind is favorable.
The best wind is abeam; that is, at right angles to the direction of motion of the ship. When such a wind strikes the revolving rotor the air current is speeded up and hurried past on the side of the cylinder that is running with the wind. This creates a partial vacuum, into which the rotor is sucked, and so the vessel is moved forward. On the opposite side of the rotor, where the cylinder is turning against the wind, the air is retarded and condensed, and this pushes the vessel forward. The wide flanges or disks at the top and bottom prevent the outside air from leaking into the vacuum on the side and the compressed air from escaping on the other.
Flettner has already applied his principle to stationary wind-power plants. The new windmill, like his seagoing rotor, needs no "turning off" in a high wind.