I have written this article to introduce my rotor boat, which has sailed a score of times over the last two summers off Seaford in Sussex. I hope you'll look at my very basic website, www.rotorboat.com
, which is designed for the uninitiated reader but which will provide a more graphic sense of what follows.
I should point out three things immediately: one is that I cannot yet provide the scientific data that many readers will crave–graphs of lift against wind speed, L/D ratio at different rotor speeds, etc. I hope these will follow, of course, and I'm working on the means of providing them. Another thing is that the rotor uses an electric motor to spin it. Readers for whom such a revelation is anathema should fling the paper down right now. A third point to make right away is that a rotor boat is built for comfort not for speed. In other words, this project is not aimed towards developing fast boats, but bigger boats for unskilled, lazy and impecunious people like me.
I suspect most readers of this article will already be familiar with the Flettner rotor in theory. A spinning cylinder entrains air to create a vortex. If that air has a flow relative to the rotor, the vortex will interact with that flow to speed it up on one side of the rotor and decelerate it on the other. The faster-moving side experiences a lower static pressure, the slower side a higher static pressure. In cross-section, the theoretical points at the cylinder surface that separate low and high static pressure regions—points at which the pressure remains at its freestream value, are called stagnation points. The rotor and whatever is attached to it will move from the higher pressure towards the lower. The phenomenon is generally referred to as the Robins effect or the Magnus effect. Simple to grasp in essence, the fluid dynamics involved are in fact extremely complex, such that this is still a large area of study with new research appearing continually. I am no academic, and the selection of papers I set myself to read in an attempt to bolster my instinctive confidence in the idea were so involved, equivocal, tentative, and for want of a better term, 'virtual' (they nearly all rely on pre-existing fluid dynamics software to create computer simulations) that I rather gave up looking at them. Not that their subject matter is not pertinent: the thickness of the vortical layer with different Reynold's numbers; the reduction in vortex shedding at different rotational ratios; re-circulation in the vortex detaching the stagnation point: these subjects define the problem, but for me it was quicker (and more fun) to do the real world experiment than to grapple with the theory.
Flettner gave his name to the rotor after a ship converted to his design was a success. Since then various patents relating to the idea have been filed—mostly elaborations without much stress on practicality, and since lapsed.
I became interested in the idea in blissful ignorance of this history. I expect readers will be able to point out rotor trials and projects I'm unaware of. I came to it from the perspective of flying rather than sailing. I felt that my idea merited investigation—even after learning that I was not the first but about the ten-millionth person to have that particular eureka moment—because I thought it would be more efficient than the Flettner rotor, which was short relative to its diameter and spun slowly. I saw a very strong, cantilever, tapering, high aspect ratio rotor, of a length less than the hull length, that spun fast.
Before moving on let me argue the advantages of my form. I always saw my rotor as a glider's wing, which is long and thin because high aspect ratio means less induced drag. The longer span you've got, the less tip loss can erode the lift which that span delivers. This is equally true of the rotor. Then, the longer the rotor the more it projects into faster moving air higher in the atmosphere's boundary layer. The smaller the diameter the less deck space is used. For any given rotor surface area, a smaller diameter means a smaller moment of inertia, and this is critical in minimising the power needed to run the system. The inertia and the induced drag arguments reinforced each other and made me set on a high aspect ratio. In practice, my doubts as to whether I could build the thing in the first place led to my initial effort being just 2.4 metres long, and the current rotor, that I consider the true prototype, being 3 metres long.
The hull is a homemade and regrettably crude glassfibre sandwich, made using a vacuum-bagging technique I'm still rather coy about, which I dreamed up to suit the purpose. I think the technique could be practised to turn out fine and economic results, but you wouldn't think so looking at my boat. It's 3.6 metres long and weighs about 40kg without the rig. Due to the core density of 80kg per cubic metre, the dinghy wouldn't sink even if full of holes. It also has a large sealed bow compartment and a stern seat of large volume to make the thing determined to float even if full of holes and people. It features a very strong ply bulkhead to provide a rigid mounting for the rotor drive assembly. That's all you need to know about the dinghy, which after all is neither here nor there in regards to the rotor concept. Oh, except that I built the centreboard trunk to allow the board to be slid back and forth to establish the best CLR in relation to CLE. This point is important to the rotor concept as a whole, and will be revisited.
The rotor itself is 3 metres long, with a root diameter of 300mm and a tip diameter of 150mm, giving an aspect ratio of 13.3. In essence it's just a sheath: a single-skinned carbon fibre sheath connected to a drive shaft by two carbon/foam sandwich discs, the lower of which incorporates an aluminium brake disc. A couple of extra carbon tow stiffeners and a thin plastic cap complete the rotor. The drive shaft is carbon. Excluding the rotating bearing elements and rotor element of the motor, the whole rotating mass is about 3kg, and the all important peripheral mass—the rotor sheath itself—is 2kg. The upper half of the rotor is watertight.
To turn this 2kg weight, on average 112.5mm from the axis of rotation, at over 500rpm, with the drag that entails on a 2 square metre surface area, I use a 12 volt electric motor 72mm long, diameter 63mm—smaller than a tin of beans or a coffee mug. This if nothing else should illustrate why this rotor is something to consider. By the way, the motor's not having to work hard to do this. The system as I've built it has the motor encased very tightly, with no assisted cooling, and it starts to struggle with high temperatures somewhere above a continuous 1,000 rpm. The motor is coupled directly to the drive shaft, and the whole sits in a rigid 2024T alloy tube with two hugely overspecified bearings keeping the lot where it should be. There is a sadly ineffective brake at the head of this tube. An aluminium frame clamps the tube and allows the whole rig to be mounted or demounted very quickly to the dinghy. The weight of this drive system, less the battery, is less than 10kg.
From the motor's controller a tough, watertight cable assembly is connected to the bulkhead, where two leads go to a 10kg battery in the watertight bow compartment, and other signal leads are passed via a conduit to the stern seat, to which the control box can be connected. Thus, I sit at the tiller with a small buoyant and watertight box that allows me to brake the rotor, control its speed and direction of rotation, and monitor battery voltage. It should allow me to read the rotor rpm too but I've never yet managed to make the gizmo work.
Launching from Seaford beach is a struggle. My sailing experience was nil before I made this boat, and often I haven't launched when the braver and very experienced folk of the sailing club have. The shingle is like ultra-coarse-grit sandpaper to the fully loaded hull; the wind most often close to directly onshore. Often help is required, and the people at the sailing club are very helpful at all times, even though I think some see the rotor as the devil's work. An advantage the rotor has over conventional rigs in this situation is that you have the rotor running before you leap aboard, so you should sail out of the danger zone instantly if your alacrity in getting the board down, and the beach break, allows.
I started sailing, in July 2004, with the board almost directly under the rotor. This put it very well forward, damping the steering. I have since sailed with it significantly further aft and the boat feels better. Unlike a conventional rig, one can determine the centre of effort, CE, of this rig with absolute accuracy at all times. On the face of it, sailing with the board where I had it this summer places the centre of lateral resistance, CLR, aft of the CE. This is traditionally bad news, but all I can say is that it certainly doesn't feel that way. I suspect that more things determine the CLR, including influences above water, than geometric considerations allow for.
As the reader knows, one reason that tradition requires a CLR forward of CE is so that a boat will round into wind if set free. If it broaches when sheeted hard it could blow over. At this angle of attack the sails are producing pretty much no lift and huge drag. The rotor boat conversely may well benefit from a fixed trim that brings it off the wind if the tiller is freed, because the rotor produces the same lift to drag ratio regardless of the track of the hull underneath it: a rotor boat tending out of wind will simply heel less and speed up, as the lift vector comes home. The greater danger for the rotor boat is inadvertently rounding into wind under full power, when the rotor will try to capsize the boat. However, the relationship of CLR to CE can be more accurately determined than that of a conventional sailing boat, so the designer can ensure that the boat will tend to do the docile thing. You would end up with a boat needing lee helm not weather helm in this case, with a consequent penalty in trim drag, but again I invoke the predictability of trim that can be designed into the vessel in comparison to the conventional sailboat whose helm changes with each point of sail. The trim could conveniently approach neutral, minimising trim drag. More than one rotor on a ship would allow accurate aerodynamic trimming.
What of performance? As I said, I would like to present polar curves and performance figures, but can't yet. Young people examining the boat on the beach have almost invariably labelled it "cool", but I always have to answer the inevitable young person's question, "is it fast?" in the negative. There is a clear and strong relationship between rotor speed and boat speed in any given wind, but the subtler question of which rotor speed is most efficient in terms of speed for power consumption in any given wind, is beyond me as yet.
I run the rotor at about twenty five watts most of the time because that's a realistic figure for a reasonably sized solar panel to provide, and it seems to be a sweet spot in terms of system vibration (the rather badly-balanced, homemade rotor hums at certain frequencies, and generates just a hiss from the bearings at others. In general terms it's very quiet). Twenty five watts equates to about 800rpm, and intriguingly, I've measured the power required for any given rpm decreasing in wind as opposed to still air. If this effect is felt over a large wind range it could be important. An unpleasant truth to face though is that the power requirement rises enormously relative to rotor speed. 2 amps provide about 800 rpm in still air, but 7amps are needed to deliver 1400 rpm. If this is a consequence of electric motor behaviour or bearing friction it may be mitigated, but if it's due to aerodynamic drag it could signal an inherent limit to the efficiency of the system on any scale.
If there is just one useful statistic, I suppose it would be, 'what wind is required at x watts to get hull speed' or a variation of the same equation. I cannot even answer this as yet. At about 25 watts the boat will achieve hull speed in a wind I believe to be in the upper half of force 4. Clearly this is spectacularly unimpressive compared to the Lasers, Darts, etc which whizz around my little prototype: I can only reiterate that we're not after the same things.
Furthermore, at this point vagueness possibly serves, because remember that the rotor I've made is a very rough prototype, which I was able to construct by gift of what was possible, not what was best. Given resource, I feel that a somewhat longer, much stiffer and better-balanced rotor could be made, of the same weight. A better-matched motor could be found to operate at its peak efficiency around the pertinent speed range. Importantly, more suitable bearings could make a big difference to power consumption: I've learned that bearing drag is the greatest power drain of the system, at least in the speed range I adhere to, and the bearings I'm using could carry a truck. I envisage an optimal performance significantly better than I am able to show currently. Even then however, what about scale effects? Greater Reynold's numbers, thinner vortical regions, more angular difference in flow direction from root to tip, different bearing losses and motor efficiencies—these and twenty other issues make predictions of a yacht's performance based on a dinghy's, less than clear to put it mildly.
A couple of final points to address are those of gyroscopic forces and windage. People often asked me before the boat sailed, 'won't the boat spin round the rotor?' Well it does, of course, but bearing in mind that the rotating mass is some 3kg, 0.113m from the axis, and this is opposed by a couple of hundred kilos maybe 2m from the axis, then you can see that the answer is "not much". Interestingly, what gyroscopic force there is on the hull (believe me, it's far too small to detect on the sea, even in no wind) acts towards the wind—in other words if it exists at any meaningful level it would reduce the lee helm required to keep the boat true if CLR is behind CE. Windage is a more serious concern., but my feeling is that the problem is not as bad as people tend to think. It sometimes seems to me that the rotor has hardly greater cross-sectional area than the aerofoil masts that modern racing yachts use: more importantly, it has nothing else. No boom, trees, stays etc that make up the average rig and must contribute plenty of windage. If the rotor is stationary a von Karman vortex street may develop in its lee. Although the device should at minimum be strong enough to laugh off any shaking this implies, it certainly wouldn't make for a comfortable motion at harbour. A couple of studies I've scanned seem to suggest that spinning the rotor at a speed too low to generate problematic lift would give enough energy to the boundary layer to reduce the wake significantly, and disrupt formation of organised vortex shedding. I look forward to being able to test this. If it's true, a band of flapping plates slung round the rotor to make it a kind of surrogate Savonius may suffice to create this low-rpm spin for free, and perhaps generate a trickle of charge current too—but then the windage increases and. . . . The initial design thoughts I've given to a yacht or workboat-sized project assume a rotor that can be lowered in extreme circumstances and for maintenance (although once you've solved the problem of reliable bearing lubrication the system's virtually maintenance free). This feature would be much harder to incorporate at the scale of commercial shipping though.
I hope this brief introduction does not lead too many readers with a long perspective on alternative sail types to cry "oh no, not that again." Having said this, any comment would please me, and I'll do my best to answer any enquiries, via firstname.lastname@example.org