Think you understand how rigs work?

Not an April fool, Stephen Thorpe's Rotorboat adds a new dimension to aerodynamic theory, reports Toby Heppell

It is rare in the sailing world that any of us come across an idea or concept that is entirely new and innovative, but this is what we appeared to have uncovered when we came across the Rotorboat sitting in the dinghy park at the recent Weymouth Speed Week. After further discussion and research it transpired that although some aspects of this unique craft were original it was a case of an older technology being rediscovered and redeveloped.

The Rotorboat was being talked about by a significant number of people in the clubhouse in Weymouth in equal measures of mirth and intrigue. It was clear straight the boat is like no other you have seen. A first glance and the boat looks like a normal small dinghy, around the size of a Finn, but on closer inspection it soon becomes clear the mast is much wider and tapers more significantly. But upon closer inspection you realise this is something radically different - the mast has no track on it for a sail and seems to be attached to the floor and able to rotate.

Rotorboat was designed by Stephen Thorpe of the Amateur Yacht Research Society - the collection of boffins who were the original movers and shakers behind Speed Week ( a quick flick through the AYRS journal Catalyst makes you wonder whether it is the same sport we're involved with...) Thorpe first came across the idea for the rotating rig not from sailing but from flying, believing at first it would be a useful solution to a glider's wing, but soon realised its potential application as a ‘sail’ on a boat.

“The theory goes that if you have a rotating cylinder the and air flowing past it, then the side that is rotating with the air will help to speed it up, making low pressure. Conversely the side going against the air will slow it down, creating high pressure. This pressure differential creates a force pushing the rotor in one direction,” explains Frank Ball, skipper of Rotorboat at the Speed Week in the absence of Thorpe. Evidently this is known as the 'Magnus effect' after the German-born scientist Heinrich Gustav Magnus who first theorised it.

So the theory behind the rotor boat is not new at al - the concept was first used on a boat by Anton Flettner (1885-1961) who built a ship in the late 1920s with two rotating cylinders to power it.

On the Rotorboat, the rotor is spun not by the wind but by a small electric motor which sits in a sleeve on the inside of the cylinder - in what appears to be the true tradition of the Amateur Yacht Research Society the whole project is built at home by hand.



There are those who will question the ‘wind powered’ credentials of a craft using an electric motor to effectively power it but Thorpe argues otherwise. He says all sail boats require some form of power to harness the wind: in the majority of cases this ‘power’ comes from people pulling on sheets to power up the sails. In this case the power used to harness the wind is the electric motor and though not by any means a traditional un-powered craft it does seem to sit in a kind of grey area between a boat with a motor and a sail boat.

Furthermore the motor drivinng the rotor has been limited to a low enough voltage to be powered by a single solar panel, blurring the line further still. ”On this small scale, the power input the rotor requires is within that which solar panels can provide, and a rotor boat so equipped could be considered self-sustaining," hypothesises Thorpe. "Solar panels are the ideal power source as they add the least drag penalty. At some as-yet unknown hull size, presumably the greater energy density of hydrocarbon fuels would need to be resorted to; but bearing in mind that 'the wind does nearly all the work', the fuel consumption would be a fraction of that required to propel the same vessel by water screw.”

Thorpe is keen to point out early on that the concept is not meant for high speed sailing, rather it is aimed towards “developing bigger boats for unskilled, lazy and impecunious people like me.” It is in this case a surprise to find the boat being displayed at Weymouth Speed Week. “We brought it to Weymouth Speed Week so people could have a look at it," explains Ball. "Realistically it is not going to break any records but it is a good place to show off innovation.”

The first boat to use this rotor technology was, as mentioned above, designed by Anton Flettner and was called the Backau. This, although rumoured to be superior to her sailing sister-ships, had, as Thorpe succinctly puts it, “one or two 45hp Diesel engines in her belly.”

What is new about the Rotorboat, is the design of the rotor. Previous designers, like Flettner, had utilised large cylinders which were very short relative to their diameter and spun at a very slow speed. “I envisioned a boat with a strong, cantilever, tapering, high aspect ratio rotor of less length than the overall hull length,” Thorpe states.

So while the theory behind Rotorboat's technology is proven the question is does it work on this new small scale dinghy version of Thorpe’s creation? The answer is, inexplicably, yes. “This week we have recorded speeds of up to four knots, and though this is not record breaking it does prove that the theory works well,” comments Ball. “In a boat with a ‘sail area’ this size - the rotor is 3m high, has a projected area of 1/3sqm and an actual surface area of 2sqm - that is a reasonably impressive speed,” he maintains. The boat is so efficient in fact that many believe on their first viewing that it is not powered by the wind at all; most seemed to think initially it was powered by a propeller somehow attached to the rotating cylinder.

Far from just being a radical experiment Thorpe believes a spinning rotor does have different pros and cons compared to a fixed wing. He says that although the rotor as he conceives it is effectively a wing, it differs crucially in that it is active not passive. “A fixed wing uses its shape to induce the necessary asymmetry in the fluid passing over it: my rotor is itself entirely symmetrical in cross-section, and instead uses spin to achieve the same effect. The immediately obvious shortcoming of this approach is that the rotor needs a power source to make it spin, making the system complex, and an absorber of energy over and above the absorbed energy (induced drag) of a conventional rig. That's why right from the start, I thought only of mechanical simplicity and aerodynamic refinement, believing that only thereby could I produce something both technically reasonable and valuably more efficient than what has gone before."

Success on a small dinghy is by no means the end of the project for Thorpe who believes the rotor could well have applications on significantly scaled-up versions, as he states in his findings on the boat. “From experience gained with my prototype I estimate that not only would the rotor for a large boat be much lighter than a conventional sailing rig producing the same thrust, but that the centre of gravity would be correspondingly lower. A common early objection to a rotor is windage (excess of it), but by observation of my prototype's behaviour I suspect the windage to be in the same realm as that of a conventional rig with sails furled - listen to the wind singing through a yacht's rigging. Further, there is the real prospect that by spinning the rotor at some low rate, the turbulent wake (synonymous with windage) can be much reduced,” he explains.

Thorpe, having proved the theory on a small scale, is now keen to increase the size of the concept. But as with all things that will require significant external funding and so is not a viable option for the time being. Thorpe is currently unsure what a scaling up process will actually entail. “It may seem that having proved the idea on one scale, one can simply apply some clever conversion factors to derive the effect of putting rotors on a ship. Indeed, a comment I frequently encountered when enumerating my doubts early in the project, was ‘oh, they'll know all about that - it'll all be in some book.’ They being academics, I suppose. This trusting view that by now all simple-seeming physical principles must be all buttoned up into irreducible equations would mean, if true, the world's wind tunnels may best be employed for drying clothes. In fact, although 'Flow around a circular cylinder (rotating circular cylinder, oscillating circular cylinder)' is a classic topic in fluid dynamics, most of the work done relates to non-turbulent flow and infinitely long cylinders. There is everything to learn about real world applications of the principle."

And there were we thinking we knew it all...

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