^ Flettner Rotors @ the Buckau in 1925 – Image Courtesy of the U.S. Library of Congress at https://en.wikipedia.org/wiki/File:Buckau_Flettner_Rotor_Ship_LOC_37764u.jpg
Back from the Dead
Perhaps the one thing that separates the common from the extraordinary is the undying relevance of the work of the latter. German aviation engineer and inventor Anton Flettner is one such outstanding individual.
Way back in the 1920s, he devised the rotor sail and built two fullscale rotor scales – the Buckau and the Barbara. These were perhaps the earliest applications of wind assisted ship propulsion i.e. ships wherein wind propulsion usually plays a largely subordinate role.
Sadly, these failed to capture the shipping industry’s imagination. Back then, the industry was engrossed with the more economical diesel propelled ships. And when a storm sunk the Buckau in 1931, the technology lost its bid for wide acceptance. Or did it?
Rising emissions and their destructive role in Global Warming and Climate Change have brought all fossil fuel powered machines under the regulatory scanner. Shipping might be among the least polluting modes of transport, but does discharge sizable amount of wastes.
If shipping were a country, it would be the sixth largest emitter. Between 2007 and 2012, the global shipping fleet gulped about 250-325 million tons of fuel and emitted 3.1% of the total carbon dioxide and around 2.8% of the annual global greenhouse gas emissions.
Fuel prices will not maintain their current low levels for long. Emission norms are closing in and so are commercial pressures. The same economic factors that made the rotor ship unviable a century ago are now aiding its rise.
Flettner Rotor is a long and smooth cylinder with discs plates at the end of its length. When air rotates it along the longer axis, it generates an aerodynamic force in the third dimension on account of Magnus Effect coming into play.
Rotor Ships make use of the Flettner Rotor by placing the cylinder vertically. This generates a horizontal thrust in the forward direction. Rotor Aircrafts on the other hand have these cylinders in the horizontal plane to generate vertically upward force.
German physicist Gustav Magnus first identified the force in an experiment in 1853. A rotating spherical or cylindrical body placed in flowing fluids experiences a force. The direction of this force is perpendicular to the direction of fluid flow and of the axis of rotation.
Such direction of the force also enables the Flettner Rotor to play ship stabilizer in violent seas – you can generate upward or downward thrust by suitably modifying the speed and direction of rotation.
Illustrations of the Magnus Effect are not limited to rotor ships and aircrafts alone. You might have observed that a served tennis ball deviates somewhat from its expected straight-line trajectory. So does a driven golf ball and a fired artillery shell.
This departure from the expected straight line is on account of the spinning body triggering changes in the velocity of the moving fluid. This automatically brings about pressure differences on opposite sides of the rotating body.
In order to appreciate the creation of thrust from pressure differential and velocity changes, we need to understand the famed Bernoulli’s Principle – the one on which Magnus Effect is premised.
Swiss mathematician Daniel Bernoulli derived the theorem that came to be known as the Bernoulli’s Principle. He published the same in his book Hydrodynamica in 1738.
It states that the total mechanical energy of a flowing, incompressible fluid – the sum of its pressure energy, kinetic energy of motion, and potential energy of elevation – is constant.
If therefore a fluid is flowing horizontally i.e. there is no change in its potential energy, then any decrease in pressure will be accompanied by a proportionate increase in its pressure and vice versa.
Assuming there are no flow losses, the mathematical expression for Bernoulli’s Theorem is given by:
Bernoulli’s Theorem is in fact the extension of the Principle of Conservation of Energy – you cannot create or destroy energy, but only change it from one form to another – applied to flowing fluids.
Let us now analyze the Magnus Effect within the broader context of the Bernoulli’s Principle. Take for example a ball spinning in the air. Friction between the ball and the flowing air will ensure that one side of the ball drags air in the direction of flow while the other side does the opposite.
Airflow will speed up on the side that drags air in the direction of flow and slow down on the side that drags air in a direction opposite to the direction of flow.
Because the mechanical energy is constant, any velocity drop will be accompanied with a rise in pressure provided elevation does not change. Pressure will therefore rise on the side that drags air against the flow and drop on the side that drags air with it.
Thrust will then act from the side of higher pressure to the side of lower pressure. This is precisely the force that causes tennis balls to deviate from straight line trajectories. And this is also the force that provides the forward thrust to rotor ships and upward thrust to rotor aircrafts.
In the figure above, friction between the flowing air and rotating ball slows down air on the lower side of the ball while speeding it up on the upper side. Pressure on the lower side is therefore greater and thus the upward thrust – from the high pressure region to the low pressure one.
Around seven decades after Gustav Magnus propounded the Magnus Effect, in 1922 to be precise, another German named Anton Flettner filed for the German patent of a rotor ship.
By 1924, Flettner and his cronies built the first experimental rotor ship with two rotors / cylinders, the Buckau. A 37 kW electric propulsion system drove these rotors of 15 m height and 3 meter diameter. They completed building the second experimental ship, the Barbara, in 1926.
Now, the Buckau could sail into the wind at 20-30 degrees. Conventional sail rigs of the time could not do this below 45 degrees. Stormy weather was therefore not considered a challenge for the rotor vessel.
Although she successfully completed trials and voyages, she lost out to the more economical diesel engines of the time. Her end came in 1931 when a storm in the Caribbean destroyed her. Perhaps, an indication of the incompatibility of the technology with the era.
Now, its time has come. The need to cut down emissions, deal with commercial pressures, and cut down fuel expenses has brought the technology back in limelight.
Lest you doubt the importance of slashing fuel costs in an era of cheap fuel, please note that fuel prices will soon start their upward march. It is better to prepare now. The present offers a splendid opportunity to invest the savings accrued from low fuel costs into such green technologies.
Some of the best present and upcoming applications of the Flettner Rotor include:
- Eco Flettner
- E-ship by Enercon GmbH
- Rotor Sail by Norsepower that won the Energy Efficiency Solution Award at Ship Efficiency Award 2015
- MS Estraden
Eco Flettner: currently under development, the Wind Hybrid Coaster will be a completely automated sailing system. It will use two Eco Flettner rotors on its main deck. These are based on the rotor ship design of the Buckau.
With fifteen maritime partners involved in the project, the Wind Hybrid Coaster promises to address and exceed all the expectations viz. reducing fuel use, emissions, and commercial compulsions.
Its main engine consists of 5 x 300 kW generators that provide a 12 knot service speed. The cargo capacity is 4,000 dwt. The ship transports 3.3 tons per kW when only the engine drives it. This rises to 26.7 tons per kW when only the rotors drive it.
Based on the above statistics, the coaster will offer 10-30% annual average fuel savings with corresponding cut in emissions. The rotors can provide 100% of the propulsion power when wind conditions are favorable.
Prototypes of the Eco Flettner are under construction at Leer. Final dimensions will be 18 meters height and 3 meters width with a sail area of 108 square meters. The diameter of the end plates will be 6 meters.
Rotors generate thrust in accordance with the Magnus Effect. Their efficacy in creating this thrust depends on:
- the direction and speed of the wind
- course of the ship
On voyage from Rotterdam to Istanbul, the Wind Hybrid Coaster will arrive 28 hours earlier than a conventional freighter. For the same voyage, the vessel will also cut:
- fuel use by 7.94 tons
- carbon dioxide (CO2) emissions by 23.8 tons
- sulphur (SOX) discharges by 7.94 kg
- nitrogen (NOX) pollution by 15.9 kg
Being a totally automated sailing mechanism, the Wind Hybrid Coaster does not:
- need extra crew
- impact cargo operations
- create safety risks during storms
- require trimming, setting, or storing sails
E-Ship 1: by Enercon GmbH is a Ro-Lo cargo ship that uses four rotor sails 27 meters high and 4 meters wide. It made its maiden voyage in August 2010.
E-Ship 1 has a cargo capacity of 20,580 cubic meters and can sail at a maximum speed of 17.5 knots. Operations do not require additional crew. Neither do you require crew with special training. Plus the rotor sails absorb sea disturbances in bad weather.
Rotor Sail: by Norsepower offers Flettner Rotors to those tankers, ro-ro vessels, bulk carriers, and ferries where:
- the vessel spends a large part of its time at sea where wind conditions are favorable
- deck layout provides enough space for installation and its components such as cranes and other cargo handling mechanisms do not interfere with installation
Typical heights of these rotor sails are 18, 24, or 30 meters for vessels of diverse speed, size, and operating profile of each vessel. A rotor sail unit includes rotor sails, control panel, wind and GPS sensors, automation unit, and power supply.
MS Estraden: is a 9,700 DWT Ro-Ro carrier that saved 20% fuel using Flettner Rotors in 2015 when operating on routes with favorable wind conditions. Average savings are in the vicinity of 5%.
People eating meat don’t like to be reminded that they are eating dead animals. Similarly, fossil fuel dependent people – like us – don’t like to be reminded that the world is running out of oil.
This is not another one of those doomsday prophecies that fly around thick and fast these days – it is only a clarion call for us to step up our efforts to refine green technologies such as the Flettner Rotor.
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