On the rough and icy waters of the North Sea in 1925, an unusual vessel plowed its way from Danzig, Poland, to Leith, Scotland, marking a first in maritime history. This maiden voyage was historic not for its distance but for the vessel’s ingenuity: Its simple design even impressed Albert Einstein, who later wrote an essay dedicated to its significance.
“Denuded of all sails, masts, and riggings,” wrote G. B. Seybold, reporting for Popular Science, the 177-foot-long steel schooner was propelled by nothing more than “two strange cylinders, resembling giant smoke-stacks. But no smoke was pouring from them and no engine noise was heard. Like a ghost ship, it moved mysteriously through the water with no apparent means of propulsion.”
Several months later, on Boston’s Charles River, two U.S. naval officers, studying at MIT, launched their own modified version of the same strange vessel. “This American boat,” wrote Popular Science in September 1925, “was the first actual demonstration in this country of how a revolving metal tower can replace canvas sails.”
By the late 1920s, orders for the strange new “rotor ships” began to surge. These hybrid vessels—combining oil- or coal-fired engines with tall rotating cylinders—promised to cut fuel consumption in half. Such savings were not just theoretical. In 1926, the first rotor ship, the Buckau, which had made the North Sea trek, was rebuilt as a hybrid and renamed the Baden Baden. It sailed from Germany to New York via South America, a 6,200 nautical-mile voyage that used only 12 tons of oil compared to the 45 tons it would have required without rotors. A new, more efficient shipping age seemed imminent.
But just as momentum gathered for the novel wind-ship technology, the stock market crashed. The Great Depression followed. Fuel prices plunged. The economic advantages of rotor sails vanished almost overnight, and with it the promising technology. A century later, however, as the shipping industry confronts volatile fuel costs and climate change, rotor sails are making a comeback.
The math teacher who invented a new kind of ship sail
The idea for rotor sails belonged to Anton Flettner, a mathematics teacher and self-taught engineer, who patented his novel invention in 1922. His design relied on a well-known aerodynamics principle, first described in the 19th century by German physicist Gustav Magnus.
Baseball fans know the Magnus effect well: It explains how a curveball bends. When a spinning object moves through air, or any fluid, its rotation alters air pressure—air moving with the spin flows faster, and air moving against the spin flows slower. The result creates a force that pushes the object sideways. It’s how Coco Gauff hits a shot with topspin or Tarik Skubal throws a curveball. Airplane wings also get lift from the same principle.
Flettner figured out that if he could keep a vertical cylinder spinning on the deck of a ship, it would harness the Magnus effect, catching the wind and pushing the ship forward or backward, depending upon the direction of rotation. Unlike conventional sails, which heel away from the wind, making them susceptible to capsizing, the Magnus effect pushes rotor sails in the opposite direction, which forces them to lean into the wind, making them surprisingly stable in stormy weather. But unlike conventional sails, which require no power other than the wind, rotor sails require something to keep them spinning.
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Rotor sails’ rise and collapse
In Boston, naval officers Joseph Kiernan and W. W. Hastings, put Flettner’s invention and theory to the test. When they raced their homemade rotor ship against a yacht in the Charles River in 1925, losing by a small margin, TIME magazine marveled at the vessel’s mechanics: “A beamy 35-foot Navy cutter was moving steadily by, showing neither smoke nor sail and emitting a ‘put-put-put’ altogether too faint to be coming from a gasoline motor proportionate to the craft’s size. Men on the deck were observing a smokeless stack that rose amidships, a cylinder three and a half feet in diameter and nine and a half feet high. The stack was revolving. The vessel was a U.S. rotorship—the first.” The faint “put-put-put” was the sound of the one-and-a-half-horsepower gas engine Kiernan and Hastings used to keep their cylinder spinning and steer the ship.
When the bottom fell out of the fledgling rotor ship industry during the Great Depression, Flettner moved on. At the time, he was already a prolific inventor. He had previously invented the adjustable trim-tab steering system, or servo tabs, still used by planes and ships today. A small, hinged surface, Flettner’s servo tabs use the power of passing air or water to move a much larger control surface with minimal effort, essentially acting as “power steering.” In the 1930s, Flettner adapted his rotor design to helicopters, developing intermeshing rotorcraft (sets of rotors that rotate in opposite directions for stability) that influenced later designs.
In 1961, Flettner died. Despite a life of prolific innovation in aeronautics, marine, automobile, and energy industries, Flettner—who had started out as a high school math teacher and became Managing Director for the Institute for Aero and Hydro Dynamics in Amsterdam, as well as a consultant to the U.S. Office of Naval Research after World War II—did not live to see his signature marine invention widely adopted.
The century old invention’s modern comeback
Today, the shipping industry transports more than 80 percent of the world’s goods, but in doing so generates roughly three percent of global greenhouse gas emissions. In 2023, the International Maritime Organization set an ambitious target of net-zero GHG emissions by 2030 for the marine industry. Ports have begun tightening emissions rules, pressuring ship owners to find and implement greener propulsion systems. With volatile oil prices and the urgency to reduce emissions, Flettner rotors have been making a comeback.
Finland-based Norsepower, founded in 2012, has equipped 22 ships with rotor sails as of June 2025 with 17 more under contract. While that’s a tiny fraction of the more than 100,000 cargo ships plying Earth’s oceans, the economics—up to 25 percent or more savings in fuel and GHG emissions—are compelling.
Whether they will remain compelling enough for widespread adoption remains to be seen. But a century after Einstein marveled at their simple design, Flettner’s rotors might finally catch on.