6.2 Are cylinder ships a dead-end invention?
In 1924 Anton Flettner, a German physicist, tested a prototype of one of his inventions, a rotor ship. An expert in hydrodynamics and aerodynamics, Flettner had already experimented with metal sails, which he found increased sailing ship efficiency by 50 per cent. Next he moved on to an ingenious application of the Magnus effect – the idea that a sphere or cylinder spinning in an airstream develops a force at right angles to the moving air. This theory was developed to account for inaccuracies in the flight of cannonballs but the effect is more recognisable today as being responsible for the swerving of golf, tennis and soccer balls when hit with a slice.
Flettner discovered that when the wind blew at right-angles onto rotating cylinders a vacuum was created ahead of the cylinder and an area of high pressure behind, resulting in forward propulsion. Experiments showed that a rotating cylinder could extract up to 15 times as much energy from the wind as the same area of sail.
In an attempt to demonstrate the promise of such a technology Flettner converted a 680-tonne ship, the Buckau (Figure 21a), replacing her masts and rigging with two hollow cylindrical towers made of sheet iron. The cylinders were 20 metres high and 3 metres in diameter, and were rotated at 120 rpm by small electric motors at their base, driven in turn by a small diesel engine. In addition to being able to extract more energy from the wind the cylinders were lighter than rigging and sails and had a lower centre of gravity, making the ship more stable and manoeuvrable. A further advantage over conventional sailing ships was that the Buckau was less susceptible to sudden squalls and therefore could sail through strong winds where an ordinary sailing vessel would have to take down its sails. As a result it could be faster and fewer crew members were needed to control the rotors compared with sails.
Results of the trials with the Buckau suggested that substitution of rotors for sails in vessels up to 3000 tonnes was a practical possibility. The Hamburg-Amerika line ordered 10 rotor ships but only one, the Barbara, was built. In 1926 Flettner, having renamed his ship the Baden-Baden (Figure 21b), successfully crossed the Atlantic in an attempt to publicise and gain support for his invention.
Flettner had suggested that rotors could be added to all steam or marine diesel-powered vessels to reduce expenditure on coal and oil. However at the time there was an abundance of cheap fuel that reduced the strength of this advantage. In addition the ships developed serious mechanical problems because of the constant vibration of the rotors and they still relied on wind for this type of propulsion. So at the time rotors did not offer a significant advantage over existing technologies, they merely offered added complexity and unreliability. Less than 20 years after their invention the last of these potentially revolutionary new ships had been broken up for scrap.
During the oil crises of the 1970s the idea was briefly revised and more recently found expression in the Cousteau Society's expedition ship, Alcyone (Figure 22). Built in 1985 the Alcyone uses two fixed cylinders with movable shutters and fans to achieve the same ‘lift’ produced by Flettner's more cumbersome rotors. When combined with modern computer control that switches between the Turbosail and the ship's engines according to the state of the wind, this system allows for a 35 per cent saving in fossil-fuel use.
So this invention might not be the technological dead-end it appeared to be with the demise of Flettner's ships. Indeed as fossil fuels become more scarce and expensive there's every chance that cylinder technology might be taken up again on a larger scale.