The Flettner Rotor Makes a Comeback!

The new hybrid ship designs were pioneered in Scandinavia where ecological hydro-power has long been the standard for utilities. The public demand for more clean power use has led to the introduction of hybrid propulsion in several new ferries and short-haul cargo vessels. So I expected this to be the “gold standard” for green shipping for several years until some new technology like the fuel cell was perfected.

Boy was I wrong! On January 25, a news release from Viking Cruises of Helsinki, Finland really upped the ante in the “green ship” stakes. It announced that the M/S Viking Grace, an LNG-fueled cruise ferry and one of the most environmentally-friendly cruise ships in the world, will be adding “renewable power” to its credentials and its marketing, thanks to the addition of a Norsepower Rotor Sail.

When it resumes cruising in 2018 through the pristine archipelago between Turku (Finland) and Stockholm (Sweden), Viking Grace will be the third modern ship and first passenger vessel in the world to harness the wind via a simple law of physics known as the “Magnus Effect.” This radical concept in marine technology may sound like a “spin off” from advanced aeronautical research, but it is actually something we are all familiar with, since it causes balls to rise when hit with backspin, creating a surprising amount of lift in sports from golf to soccer.

Solving the Mystery of the Magnus Effect

When the wind is on the beam (60-130 degrees) and reaches a pre-determined strength, the 75 feet high and 12 feet diameter tubular Norsepower rotor mast will automatically begin spinning. This will extract lift from the wind to supplement the ship’s engines and reduce its emissions and fuel cost. This is based on research that began in the 1600’s when Sir Isaac Newton observed the unexpected motion of tennis balls.

A century later, the object in motion was the musket ball and the scientist was another Englishman named Benjamin Robins. He tried to predict the range of the British army’s standard musket in winds of different strength and direction. In the 1860’s, this problem was carefully studied and described by Gustav Magnus, a German professor and a scientific genius who actually made far more important discoveries in chemistry electro-magnetics etc. (Sadly, he is only remembered for some minor experiments involving the trajectory of spinning iron balls.)

Anton Flettner—Seaman, Engineer and Scientist

Other than making guns more accurate, none of these learned gentlemen seemed to have any more constructive uses for the rotor effect. But Anton Flettner (1885-1961) was a German-born engineer who came from a long line of seafaring men. His first dreams of invention began when, as a boy, he made a voyage to Australia before the mast in his father’s ships.

As a young engineer in the Zeppelin Works he made a close practical study of aerodynamics. He went on to become director of the Institute for Aero and Hydro-Dynamics in Amsterdam. At the end of World War I, the age of commercial sail was practically dead. Germany’s great fleet of steel square riggers built to carry nitrates from the Chilean desert around Cape Horn was stranded in ports all round the Americas and could no longer compete with the latest steam ships.

This was seen as a cultural disaster by traditional seafarers, and many retired captains pondered ways to modernize sailing ship and preserve this iconic symbols of a bygone era. Flettner himself was inspired to try a brief experiment with a metal sail around 1920. It almost ended in disaster, but he was widely quoted at the time stating it was far more efficient than a fabric sail—as the last America’s Cup clearly demonstrated!

Now his thinking led him in a completely opposite direction-away from sails entirely. Some colleagues were investigating the Magnus effect in aircraft, and he was intrigued by the idea of applying the concept to ships. He took the next logical step and used the institute’s wind tunnel to study this phenomenon. He proved that a vacuum was created ahead of the spinning cylinder and an area of high pressure behind, resulting in forward propulsion. His tests appeared to show that a rotating cylinder could extract up to 15 times as much energy from the wind as the same area of sail.

The Amazing Rotor Ship

In 1924, he successfully tested a prototype of his rotor mast on a small yacht and visualized a future where ships would be powered by rotor power. So he took the ambitious step of obtaining the funds to acquire a 156’ schooner to fully demonstrate the ability of his discovery. (If you think wingsails are revolutionary today, imagine what mariners must have felt when they saw Flettner replacing the two masts and rigging with two hollow cylindrical towers

The cylinders were over 60 feet high and 10 feet in diameter, made of riveted and rolled sheet steel, and sat on bearings so they could be rotated. Not by a few degrees like a wingsail, but at 120 rpm by small electric motors driven by a 50 hp diesel engine! The total weight of the complete mechanism–towers, engine and motors–was given as 15,000 pounds, just one-fifth the weight of the discarded sails and rigging. This must have been a dramatic sight in a time when traditional trading ketches still sailed on the Baltic Sea and most aircraft were bi-planes.

The next year, 1926, he renamed his ship the Baden-Baden and successfully crossed the Atlantic to South America. According to the New York Times, when the rotor ship arrived in New York, “He received a welcome such as is accorded only to celebrities of the very first rank. Here was a man who had truly revolutionized the art of harnessing the wind, who had replaced a device, used in essentially unaltered form for thousands of years, the canvas sail, by a modern machine, ten times as efficient and requiring no crew for handling.”

No less a mind than Albert Einstein pronounced the rotor principle of great practical importance. Popular Science Monthly jumped on the bandwagon with more praise for Flettner and his invention. Blue Coal” was now the slogan the inventor used for his wind machine that would harness wind-fuel for the good of mankind. “It is wonderfully cheap”, he tells us, “and it is available to the world in billions of horsepower.”

Flettner claimed that each tower produced 500 horsepower of thrust and that this was safer because “reefing” was accomplished by simply slowing the spinning. Two students in naval architecture at the Massachusetts Institute of Technology, were so impressed they decided to build their own rotor. themselves. They managed to complete the new rig from salvaged parts and step it on an abandoned 30′ navy cutter. (No report on how it really sailed and fuel consumption—a common feature of the coverage of Flettner’s “miraculous discovery.”

The German government showed their faith in him by building a 3000-ton 3-rotor passenger ship, the Barbara–then the story reaches a dead end with no explanation. Some skeptical marine engineer must have found the fuel log and discovered the rotors were a net loss–using more fuel than they were supposedly saving. The whole episode seems to have been viewed as a useful but unsuccessful experiment; the rotors were scrapped, the ships returned to their former roles, and the rotor ship seemed as dead as the clipper ship.

That would appear to be the end of the matter. (I wonder if the Germans have an expression for our No such thing as a free lunch.”) But in Berlin, one minister had predicted that “the time is not far off when forests of windmills will be centralized in various parts of the country to supply power and light to nearby cities and factories.” (History intervened in Germany, but this prediction really came true in the 21st century.

Flettner continued to do research and made many more design breakthroughs. He went on to invent and mass produce the famous Flettner rotary ventilator that ran without any electric input in buses, boats etc. During World War II he engineered new machinery for one-man helicopters, then was picked up by the occupation forces and brought to the USA as a consultant to the office of Naval Research along with many other aviation pioneers.

He started the Flettner Aircraft Corporation which tried to develop new helicopters for the U.S. military. His greatest contribution may have been the servo rudder for aircraft, which was incorporated into many self-steering vanes for yachts. He died at 76 years of age in New York City in 1961.

Cousteau’s Alcyone and the Turbosail

Fifty years after the rotor ship totally disappeared, the oil price started its climb in the 1970’s. Here was the opportunity for armchair sailors to once again expound on simple, cheap effective ways to propel ships by free, renewable energy. Ship owners had to show some interest in this energy-saving trend to keep their shareholders happy, but none of the Quixotic 1980’s solutions like kites and wingsails seemed any more feasible than those from the 1920’s.

The only serious effort came from Jacques Cousteau, who saw his windship project as an elegant statement of intent and marketing to his ecologically conscious fans. The Alcyone is a research vessel powered primarily by twin engines with a beamy 100′ aluminum hull.

Cousteau and his scientific team designed and patented a system that could assist the twin engines when the wind was fair. It steps two 34′ fixed ovoid cylindrical Turbosail masts, each with a movable full-height foil flap that worked like an aircraft wing flap and can be rotated around the back of the mast to tack. To delay the stalling point of the airflow, a large fan at the top of the Turbosail pumps air up the mast and out through thousand of little vent holes to allow the air to escape on the lee side exposed by the flap’s movement.

When the Alcyone visited Portland in the mid 1980’s, I fought my way through the crowds filling the dock and managed to talk my way on board. Jean Michel Cousteau gave me a tour of the windship and let me climb into the mast through the access door. I remember there was a ladder up the inside that looked very tempting, and large floppy discs in their computers, my first sight of the digital world.

According to one of the Alcyone’s more recent skippers, Bernard Deguy, who went around the world, results were not what they hoped for. “ First we never use the fans atop the masts. Because the noise, it is impossible for the crew to sleep inside the boat. Secondly, we need a good breeze coming from amidships to notice that the Turbosails are producing power. In fact, we are motoring most of the time !”

Finally—an American Rotor Yacht

That was probably the conclusion reached from the serious experiment on the 42-foot yacht Tracker in Martha’s Vineyard, Mass. In the early 1980’s. A reporter was impressed by the rotor 24 feet high and 42 inches in diameter as it revolved at up to 600 rpm, powered by a lawn-mower sized engine driving a hydraulic pump and motor. “Under rotor power alone, the 42′ Tracker reached a maximum speed of 6.1 knots in an 18.4 knot wind and a true wind angle of 122 degrees,” he wrote. (Of course, that begs the question how fast would it have gone if the engine was simply connected to a marine propeller?)

Flettner Rotor Gets Another Spin

In 2010, German wind-turbine manufacturer Enercon launched a 426 foot hybrid cargo vessel called E-Ship 1 with four 88-foot tall Flettner rotors. The ship was designed to transport wind-turbine components and has made several voyages to Uruguay. This route passes through the NE and SE tradewinds, which should provide near-perfect sailing/rotoring conditions. However, this was never cited in the report Enercon provided In 2013.

This stated that the the rotors had added two knots to the ship’s speed of 12 knots with the wind on the quarter near the Brazilian coast. The company, which is a major player in the turbine business, continues to claim fuel savings from 10% to 35% are possible.

It’s hard to avoid the impression that they are “cherry picking” their data by highlighting the rare days when the wind is on the beam and ignoring the rotors fuel use by using exhaust heat to run a small steam turbine, so it is more-or-less free energy anyway! But hope springs eternal in the attempt to capture the power of the wind for ships–even if does not use sails.

In 2015, the first Norsepower rotor was installed on the Finnish vehicle ferry Estraden, which operates between the Netherlands and the UK. Weather conditions were mainly calm throughout the study, but the results overseen by Finland’s VTT Technical Research Centre confirm fuel savings of 2.6%. So a second rotor was installed in 2016 and the fuel savings jumped to 6.1% with a peak of 10% while sailing.

This led to Norsepower receiving a $4 million investment from a clean technology venture fund to develop largest ever Flettner rotor with a height of 100 feet. It should have a maximum equivalent power output of more than four megawatts (MW), and the company forecasts savings of 20% for vessels on favorable wind routes. (Well they would, wouldn’t they?)

Windship believers look to a brave new world where new technologies like hydrofoils and weather satellites could be applied to shipping. This has certainly resulted in two amazing recent passages under sail: French singlehander Thomas Coville raced around the world non-stop in 49 days on a 103′ trimaran, while a crew of six of in a 105 footer followed soon after and returned to France in just 41 days!

I wasn’t expecting those records to fall for a decade or more, so I admit I have no idea how much more performance the high-tech Flettner rotor can deliver. Maybe we will soon see them spinning atop the state ferries as they go for a “spin” on the sound.

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