Shipping Company Takes Rotor Sail for a Spin
Tom Lombardo posted on June 14, 2015 |

The Magnus Effect is frequenly used by baseball pitchers, even if they didn’t study physics in school - it’s the principle that causes curveballs, sinkers, and risers to confound opposing hitters. For golfers, the Magnus Effect is a double-edged sword. When applied correctly, a good backspin gives a golf ball its lift, as shown below. (In the picture, the ball is moving from right to left; the backspin gives it an upward thrust.) With a bad swing, however, the Magnus Effect causes a golf ball to hook or slice, sometimes resulting in the “Profanity Effect” as the golfer tries to find his ball in the rough.

Here’s the physics of Magnus Effect in a nutshell: a rotating body moving forward creates high and low pressure zones on opposite sides of the object, which produces thrust in the direction of the low pressure zone. If you want the gory details of that, here’s a nice long paper about the Magnus Effect.

Beyond the sporting aspect, the Magnus effect is also the principle behind the rotor sail, which is now being used to supplement conventional propulsion on large shipping vessels, potentially reducing fuel costs by up to 20%.

Hardly a new concept, the rotor sail (shown above) was conceived by engineer Anton Flettner nearly a century ago. A motor provides the spinning motion which, coupled with the blowing wind, produces a net thrust perpendicular to the wind. That thrust helps propel the ship.

At the time Flettner’s rotor ship was seen as a novelty, with just a few small sailboats having rotor sails to provide their propulsion. In recent years, some large ship manufacturers have adopted a hybrid technology, using rotor sails not as main propulsion, but to assist the primary engines whenever the wind conditions are right. Norsepower, a maker of rotor sails, recently conducted successful testing of its technology on a Bore vessel, the Estraden, a 9700 DWT (dead weight tonnage) ship that sails the North Sea between the UK and the Netherlands.

Details of the testing procedure, which was conducted by VTT Technical Research Centre of Finland, are in the press release, so I’ll just briefly summarize them. A small rotor sail was added to the Estraden vessel. Whenever wind conditions were favorable, the rotor sail was engaged and the main engines were throttled back (the experimental condition). At other times when the wind conditions were favorable, the rotor sail was not powered and the engines ran at full throttle (the control condition.) Control and experimental conditions were varied at random, and the results clearly showed that the rotor sail reduced fuel consumption by 2.6% without affecting the speed of the ship. Norsepower believes that 5% could be achieved with a full sized rotor sail, and 20% savings are possible with the addition of four rotor sails. With those numbers, the rotor sails will pay for themselves in about four years. I’m guessing that those are optimistic estimates, but even if you double the payback period, it seems like a wise investment. And if fuel prices increase (what are the odds of that happening?) the payback could be even shorter.

I wondered how the rotor sail, which is powered by an electric motor, produces more thrust than simply attaching the motor to a propellor. Norsepower representative Jarkko Väinämö gave this explanation:

“When the motor rotates a propeller in water the thrust power generated is: Motor power reduced by different efficiencies (losses) like shaft and propeller i.e. it beings less propulsion than the motor power. When the same motor rotates a Norsepower rotor sail the propulsion power is on average roughly 10 times higher than the motor power, depending of course on the average long term wind speed and direction distribution of the ship’s route. This means the extra power should be always directed to the rotor sails when there is in an appropriate direction and magnitude of wind available. Down below you can see one example of a polar diagram showing the propulsion power a rotor sail system on a ship as a function of wind speed and direction – so you get an idea.”

So there’s no violation of the Laws of Thermodynamics - the thrust comes mostly from the wind, with the motor power allowing the vessel to take advantage of the wind power.

It’s interesting to see old technology suddenly finding itself useful in the modern world. The global shipping industry burns about 300 million tons of low-grade, high-sulphur fuel every year. The rotor sail, like other wind-based propulsion, has come full circle and now has the potential to put a significant dent in the pollution caused by heavy cargo vessels. I hope the rotor sail industry catches a good tailwind.

Images courtesy of Norsepower

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