The Magnus Effect Explained: Why Spinning Balls Curve and Rotor Sails Push Ships (2026)

Drop a basketball off the top of a 126-meter dam and it falls almost straight down. Drop a second one with a gentle flick of backspin and something unsettling happens: the ball starts to swoop away from the wall, gathering sideways speed as it falls, and splashes down far out from where it was released. The trick-shot team that tried it in Tasmania had no idea it would happen. The force that carried their ball away is the same one that bends a curveball, dips a topspin forehand, curls a free kick around a wall and, this month, is helping push one of the largest wind-assisted cargo ships ever built across the ocean.

The short answer: the Magnus effect is a sideways force on any spinning object moving through air or water. The spinning surface drags a thin layer of fluid around with it. On one side that layer moves with the oncoming air and on the other it fights it, so the airflow leaves the ball bent toward one side. Push the air one way and, by Newton’s third law, the air pushes the ball the other way. Backspin lifts, topspin dunks, sidespin swerves. Spin a big cylinder on a ship’s deck in a crosswind and the same push becomes forward thrust.

How the Magnus effect works

Start with a ball that isn’t spinning. Air splits around it evenly, the flow peels away from the back into a messy wake, and the forces on the two sides cancel. The ball slows down from drag but flies straight.

Now give it backspin. Air touching a solid surface sticks to it, a property engineers call the no-slip condition, so the surface of the spinning ball drags a thin boundary layer of air along with it. Across the top of a backspinning ball, the surface moves in the same direction as the air streaming past, so the boundary layer clings on longer and gets carried around the back before it lets go. Across the bottom, the surface runs against the oncoming air, and the flow separates sooner. The wake behind the ball ends up pointing downward.

That bent wake is the whole story in one picture. The ball has thrown air downward, so the air throws the ball upward. NASA’s Glenn Research Center puts it in a single line on its page about rotating cylinders: "All that is necessary to create lift is to turn a flow of air." You can tell the same story with pressure instead: air moves faster over the side where the surface helps it along, pressure drops there, and the ball is pushed toward the low-pressure side. Both descriptions are true at once, the way they are for an airplane wing. Spin does not push the ball. It makes the ball push the air, and the air pushes back.

For a long spinning cylinder, the kind found on a rotor ship, there is a tidy formula. The Kutta–Joukowski theorem, worked out separately by Martin Kutta in Germany and Nikolai Joukowski in Russia, says the lift on each meter of cylinder equals the air’s density times the wind speed times a quantity called circulation, which grows with how fast the surface spins. Double the spin or double the wind and you roughly double the push, at least until the flow gets messy. NASA’s page is careful to note that the neat version leaves out viscosity, the very thing that makes the effect happen, so real cylinders have to be measured, not just calculated.

From Newton’s tennis ball to rotor sails: a short timeline

1672: Newton watches tennis. The first written explanation came from Isaac Newton, buried in his famous letter on light and colors to the Royal Society. Wondering whether light rays might curve because they spin, he remembered the tennis courts at Cambridge.

I remembered that I had often seen a Tennis ball, struck with an oblique Racket, describe such a curve line. For, a circular as well as a progressive motion being communicated to it by that stroak, its parts on that side, where the motions conspire, must press and beat the contiguous Air more violently than on the other. (Isaac Newton, letter to the Royal Society, 1672)

He was right about the ball, though he decided light did not behave that way.

1742: Robins and the wandering musket ball. The British mathematician and gunnery engineer Benjamin Robins argued in New Principles of Gunnery that spin explained why musket balls swerved unpredictably. He hung a wooden ball from a twisted double string and let it swing; as the string unwound, the spinning ball’s swing drifted sideways, in a direction set by which way it turned. For a while the effect was known as the Robins effect.

1852: Magnus spins a brass cylinder. The Berlin physicist and chemist Heinrich Gustav Magnus, studying why artillery shells strayed from their aim, mounted a rapidly rotating brass cylinder in front of a blower and showed that it was pushed sideways, toward the side where its surface moved with the air. His paper on the deviation of projectiles gave the effect the name it has carried ever since, according to the Wikipedia summary of the history.

1924: a ship without sails. Engineers at the Aerodynamic Research Institute in Göttingen, including Ludwig Prandtl and Albert Betz, measured how much force spinning cylinders could make. The engineer Anton Flettner saw a use for it and had the Germania shipyard in Kiel strip the masts from the schooner Buckau and fit two rotating cylinders, each 2.8 meters across and about 18 meters tall, driven by 7.5-kilowatt electric motors, according to the German Aerospace Center (DLR). Prandtl and Betz watched the propulsion work in November 1924.

1926: across the Atlantic, then forgotten. Renamed Baden-Baden, the ship sailed to New York via South America in 1926. A larger three-rotor freighter, Barbara, followed. But cheap diesel fuel and the Great Depression killed the market, and NASA’s own history note sums up the verdict of the day: the idea worked, but the motor would have pushed the ship harder if it had simply turned a propeller. Baden-Baden was converted back into a schooner and sank in a Caribbean storm in 1931.

2010: E-Ship 1. The wind turbine maker Enercon put its four-rotor cargo ship E-Ship 1 into service to carry turbine parts. Each rotor is 4 meters across and 27 meters high, and DLR says they cut the ship’s fuel use by about a quarter.

2018–2019: a tanker proves the numbers. Norsepower fitted two 30-meter rotor sails to the Maersk Pelican, a long-range product tanker. Over a year of normal trading, from September 2018 to September 2019, the ship used 8.2% less fuel, about 1,400 tonnes of CO2 avoided, a result checked by Lloyd’s Register, Norsepower reported.

2026: six rotors on one ship. In October, Norsepower finished installing six 35-meter-tall, 5-meter-wide rotor sails on Spirit of Toulouse, a new roll-on/roll-off ship built in China for LD Armateurs to carry Airbus aircraft sections between Europe and North America. The company says it is now the largest RoRo vessel with wind propulsion and the vessel with the most mechanical sails in the world. Marine Log reported on November 9 that the ship had completed sea trials.

Watch it happen: the basketball off the dam

The clearest demonstration on the internet is still this one. The trick-shot team How Ridiculous dropped basketballs from the Gordon Dam in Tasmania, 126.5 meters high, and Veritasium’s Derek Muller explains why the backspinning ball flew away from the wall instead of falling straight down.

Notice that the backspin ball does not curve right away. The Magnus force grows with speed, so the ball drifts only a little at first, then bends harder as gravity speeds it up. That is also why the effect matters so much more for a 90-mph pitch than for a lazy game of catch.

Spin does not push the ball. It makes the ball push the air, and the air pushes back.

The Magnus effect in sports

Almost every ball sport is, at some level, a contest over who controls spin. Physicist Alan Nathan of the University of Illinois, who measured the Magnus force on baseballs fired from a pitching machine at speeds of 50 to 110 mph and spin rates of 1,500 to 4,500 rpm, wrote that it is "responsible for much of the subtlety in the battle between pitcher and batter" in his 2008 paper in the American Journal of Physics.

Baseball: the curveball, the slider and the rising fastball

A curveball is thrown with topspin, so the Magnus force points down and adds to gravity; the ball drops more than a hitter’s eye expects. A slider has spin tilted sideways and breaks across the plate. A four-seam fastball carries heavy backspin, so the Magnus force points up and holds the pitch higher than gravity alone would, which is why hitters swear it "rises." It does not actually climb; it just falls less. Home run hitters exploit the same thing in reverse, undercutting the ball so backspin keeps a long fly ball in the air.

Soccer: Roberto Carlos and the spiral free kick

On June 3, 1997, in Lyon, the Brazilian defender Roberto Carlos struck a free kick from about 35 meters. It started so far right of the French goal that a ball boy standing meters wide of the post ducked, then hooked left into the net while the goalkeeper Fabien Barthez made no move for it. For years people called it a fluke. In 2010, physicists at the École Polytechnique in France fired small spinning plastic balls through water with a slingshot and found what they called the spinning ball spiral: as drag slows the ball, its spin keeps going, so the curve gets tighter and tighter instead of staying a gentle arc. A kick struck hard enough, from far enough out, bends most at the very end. Carlos’s distance was not a coincidence, the researchers wrote, but a necessary condition.

Tennis, table tennis, golf and cricket

Topspin is why a tennis forehand can be hit hard and still dip into the court. In table tennis, the ball is so light that spin dominates, and good players can make it swerve, kick and die on the table. A golf ball’s backspin gives it lift and extra carry, while a tilted spin axis, from a club face that is open or closed at impact, turns that lift sideways into a slice or a hook. In cricket, spin bowlers use the Magnus force to make the ball drift and dip, though the swing of a seam bowler is a different effect, caused by the raised seam and a rough and a smooth side rather than by spin.

When it runs backward

There is a twist. Smooth balls spinning at certain speeds can feel a sideways push in the opposite direction, because the boundary layer turns turbulent on one side and stays smooth on the other, changing where each side lets go of the air. Researchers call it the inverse Magnus effect, and a 2014 study in the Journal of Fluid Mechanics mapped when it happens. It is one reason a very smooth or unusual ball can move in surprising ways.

Why ships are spinning cylinders again

A rotor sail is a tall, smooth cylinder with an electric motor that spins it, usually with a disc on top. It is not a wind turbine; the wind does not turn it. The ship spends a little electricity to spin the rotor, and when the wind blows across the deck, the Magnus force pushes at right angles to the wind. If the wind comes from the side, that push points forward, and the main engine can throttle back. When the wind swings to the other side, the rotor simply spins the other way.

Why it is back now. Flettner’s ships lost to cheap oil. A century later, fuel costs and pressure to cut shipping’s carbon emissions have changed the arithmetic, and modern rotors use light composite materials, automatic controls and, on many new installations, folding or tilting mounts so they can be laid down to pass under bridges and cranes. They are added to ordinary ships rather than replacing the engine.

How much fuel they save. It depends heavily on the route and the weather. The Maersk Pelican’s 8.2% was measured over a full year, and Norsepower’s own simulations put the potential at up to 12% for that installation in average global winds. The operator Sea-Cargo, which added three rotor sails each to two short-sea ships early in 2026, says the wider upgrade, including solar panels and batteries, cut CO2 per tonne carried by up to 35% on those vessels, according to Norsepower. Ships on windy, steady routes such as the North Atlantic and the trade-wind belts gain the most.

The side effect engineers have to fight. A rotor does not only push forward; it also shoves the ship sideways. On Spirit of Toulouse, Germany’s Becker Marine Systems fitted folding underwater fins, called daggerboards, each with a surface area of 8.8 square meters, to counter that drift so more of the wind’s push turns into forward motion, Marine Log reported. Airbus says its three new ships are expected to cut annual CO2 emissions from its transatlantic shipping by more than half by 2030 compared with 2023.

Bigger and bigger. Rotor sails now ride on ferries in the Baltic, ore carriers chartered by the mining company Vale and some of the largest bulk carriers afloat. In late 2025 Anemoi Marine Technologies fitted four folding 35-meter rotor sails to the 208,000-deadweight-ton bulk carrier Berge Mer, Dry Bulk magazine reported.

Try it yourself

The paper-cup glider. Tape the bottoms of two paper cups together, wrap a few rubber bands around the middle, hold the free end of the bands under the cups and pull back like a slingshot so the cups roll off your hand with backspin as they launch. Instead of falling, they swoop upward and loop.

The table tennis ball. Brush down the back of a ping-pong ball with a paddle and watch it float; brush over the top and it plunges. A light ball makes the Magnus force easy to see.

The beach ball. Throw a beach ball with heavy backspin and it hangs in the air far longer than one thrown without spin. A soccer ball kicked off-center with the inside of the foot shows sideways swerve.

The bottom line

The Magnus effect comes down to a ball or cylinder that drags air around with its own surface, bends the flow behind it, and gets shoved the other way. Newton saw it on a tennis court in the 1600s, gunners cursed it for a century, a German physicist gave it his name, and a German engineer tried to sail ships with it in the 1920s. It lost out to cheap oil then. Now, with six spinning towers on a single cargo ship and rotor sails on ore carriers and ferries, the same push that curls a free kick is back at work at sea. If you liked this, the physics of spinning things goes deeper in our pieces on Euler’s Disk and the tippe top and rattleback.

Related on aglena: Why You Can Run on Oobleck: Non-Newtonian Fluids Explained (2026), The Nut That Flipped in Orbit: The Dzhanibekov Effect and the Tennis Racket Theorem

Further reading