"We turn the clean free energy of the ocean winds into marine propulsion. Blending naval architecture, aerospace dynamics and structures, and computer control, we provide silent speed at the touch of a fingertip, and we can save fuel and reduce pollution on commercial ships. It has to make sense".
The earliest use of aircraft type wings to propel ships, so far as we know, was proposed by Anton Flettner in Germany just after the First World War. As well as a successful and prolific inventor, he must have been a great salesman, because he persuaded the board of Krupps to re-rig an elderly barquentine, the Buckau, with a revolutionary set of metal vertical wings. Work had already started in the yard when Flettner, on holiday on a Baltic island, was taught how to swerve a tennis ball by applying side spin. Later that afternoon, relaxing on the beach, he was idly pouring sand grains down the slope of a dune while twisting a tennis ball in the flow. Suddenly, he realised that by spinning a tall vertical column in the breeze a propulsive force would be generated. This is called the Magnus effect, after Heinrich Magnus, a German discoverer of the effect in 1852. However the phenomenon had first been identified and described by the great British physicist Isaac Newton in 1672
Excited by this concept, completely new to him, Flettner rushed back to Berlin, and told the astonished board of Krupps that he no longer wanted aluminium wings, he wanted tall spinning rotors. As we have said, he must have been a super salesman, because Krupps agreed to scrap all work on the metal wings, and to replace them with tall metal rotors. The Buckau became the world’s first, but also almost the last, Flettner rotor ship
Flettner had in fact made a serious mistake, in our view. His rotors, spun at some 400 rpm by a steam donkey engine, certainly gave plenty of thrust, but the aerodynamic efficiency, the ratio of thrust across the wind to drag downwind, was not much better than the original cloth sail rig. So the Buckau could broad reach beautifully, but was pretty terrible at going towards the wind.
And to make matters worse, there was no way to reduce the aerodynamic forces in strong winds. The poor Buckau could become “embayed” on a lee shore, desperately reaching backwards and forwards, unable to sail upwind and out of trouble, the only solution being to use her low powered steam auxiliary engine. The third fundamental defect of the Rotor ship was that the minimum aerodynamic drag which could be achieved was relatively high, and so in extremely strong winds it was not possible for the captain to slow the vessel down at all.
If the rotation of the rotors was halted, matters became even more difficult, because the stationary rotors were subject to the pulsating effects of Von Karmann vortices, which have been responsible for the structural failure of many factory chimneys of similar proportions.
A second vessel, the Barbara, was also equipped with Flettner rotors, but she suffered all of the disadvantages mentioned above, and was comparatively quickly taken out of service.
In the 1970’s a pair of aerodynamically rather crude wing sails were fitted to the Shin Aitoku Maru, a small Japanese freighter owned by the Aitoku Shipping Company. Her name, rather unromantically, simply means New Aitoku Ship. Her wingsails were composed of two slightly curved and approximately square panels hinged on each side of a vertical mast.Hydraulic rams could fold the two panels together to make a sort of flag arrangement, but made of solid sheet metal, or they could be opened, to form a large and slightly curved barn door arrangement. The central mast was mounted on a vertical axis turntable bearing which could be motored around through approximately 320 degrees by a powerful hydraulic motor and gearbox.
The very low aspect ratio aerodynamic surface presented to the wind, and only slowly and approximately trimmed to the wind by the hydraulic system, meant that, like the ill-fated rotorship Buckau, the Shin Aitoku Maru could not sail well to windward.
A very fundamental problem arose from the fact that the shipbuilders had not realized that the centre of pressure of any aerofoil is approximately 25% of the width of the aerofoil back from the leading edge. This meant to that in stronger winds the torque which had to be supplied by the rotary hydraulic motor arrangement at the base of the mast became extremely high, and in due course, as the relative wind strength increased, the hydraulic motor became no longer capable of holding the wingsail at even an approximation to an appropriate angle of attack, and the wingsail panels had to be folded back into their metal flag configuration.
A further disadvantage of the arrangement chosen by the shipbuilders was that, in order to avoid the complexity and cost of a multi way hydraulics slip-ring set, the wingsail could not be motored round through 360 degrees or more. This meant that a combination of the desired course of the vessel and any change in the direction of the wind made the folded wingsail come up against one of the end stops of its rotational range of movement. In that case the ship could be in grave danger of overpowering or major structural damage, unless a rapid alteration of course was made by the crew. If this new course was likely to carry the ship into danger, matters became even more challenging.
John Walker was most interested to be invited in 1983 to go aboard the Shin Aitoku Maru by Mr Aitoku, and to have a full demonstration of the system. He was told by the chief engineer not only about the engineering which had been developed to make the system reasonably practical, but also about the fundamental disadvantages referred to above. Mr Aitoku never built another wing ship, and within a couple of years, even in the presence of $55 a barrel oil prices, all wingsail activity on the Aitoku system had ceased.
