SHADOTEC plc - HOW DO WINGSAILS WORK? A wingsail is directly derived from the vast store of knowledge built up from the development of aircraft wings over the last century. An aircraft wing is set horizontally to provide a vertical force, called lift, to hold the plane up in the air. A wingsail is set vertically to provide a horizontal force, to propel a ship or yacht. That's the easy part. However, a wingsail must work effectively when the wind is blowing on the vessel from the widest possible range of wind angles.  Basically, no wingsail can generate forward thrust from a wind blowing from directly ahead. However, with a device of really high aerodynamic efficiency such as a Walker wingsail you can get forward thrust from wind angles ranging from ± 25° off the ship's centre right round to 335° , leaving only a 50° dead zone. So altering course by as little as 30° can get the vessel sailing, and if she is using a combination of engine and wingsail, fuel consumption and pollution can both be reduced. If the vessel needs to maintain a course closer to the wind she can be tacked, and the wingsail will automatically tack too. It's nowhere near so obvious, but it can also be difficult to get benefit from wingsails when the wind is coming from astern.   Because the vessel is sailing away from the wind there is much less relative wind over the deck (or even, if motor sailing, the vessel can be experiencing a head wind). It's worse of course, if the wind is light. The best technique with the wind aft is very much like the procedure when the wind is on the bow. Then, the vessel bears away from the wind to get more benefit from it. Now the course is altered to bring the relative wind off the stern and more towards the bow. hardening up towards the wind. Altering course so that, instead of blowing on the stern the wind is now blowing at say 30° to the centre line from aft of the boat provides significantly greater wind thrust to propel the vessel along. This “downwind tacking” is just as useful a sailor's tool as upwind tacking, and just as useful to a big ship as a tiny yacht. And when the captain needs to restore his course by getting on to the opposite tack, (called gybing) he simply turns the helm to move on to the new course, the wind gently crosses the stern, and the wingsail automatically and quietly resets itself to adjust to the new wind angle. There is no “boom” to crash across on gybing as there is on traditional rigs, which can endanger rig and crew alike. The process of gybing, so far as the wingsail is concerned, is more or less the same as when the wind is on the bow, and the captain tacks. Equally quiet and automatic, the wingsail firstly “feathers”, aligning itself precisely with the wind angle, then when the wind angle has arrived in the right zone, it “unfeathers” to restore thrust. And software can provide a few other refinements. When the wind is coming hard on the bow at say 30° to the centre line of the boat, the wing will be set to maximum aerodynamic efficiency, or optimum lift/drag ratio. The thrust level is reduced, but the actual forward thrust on the vessel increases. Then, as the wind angle increases to say 60 ° to the centre line of the boat the thrust level is increased. (All of these angles, by the way, are “apparent” or “relative” wind angles, that is to say they take into account the speed of the ship. True wind angles, the angle of the wind to the ship's centre line when the vessel is stationary, are of no interest to the wingsail, although very useful to the navigator). Then, as the wind angle continues to increase, to and beyond around 120 ° to the centre line of the boat, the wingsail will automatically go into “stall” mode, since it will have detected the fact that it can produce more forward thrust on the vessel by increasing downwind drag than by optimising crosswind force. At 180°, as the wind passes across the stern the wingsail will automatically gybe, and the vessel continue to sail, but on the other tack. Wingsailing, as an American enthusiast once said, impatiently waiting to prise his wife away from the controls of a Walker Zefyr, is more fun than the law allows. Air braking is available.   Of course it's most effective when the wind is on or ahead of the beam. The thrust lever is pulled back into the astern quadrant, the wingsail computer understands that astern thrust is required rather than ahead, and it carries out an automatic tack so that instead of thrust say left of wind (if she was on starboard tack) you now get thrust right of wind. The boat slows, stops, and if the lever isn't‘t pushed into neutral, she'll gather way astern. If you allow that, and put the helm over, a wingsail yacht (or giant tanker) can do a perfect three point turn. All the captain has to do, when the vessel is heading on the desired new course, is to centralise the rudder. In all the above we are assuming the wingsail is just a box which does what you want with simple inputs. You can, and you may always, think of it like the engine of your car. You turn a key, it starts, you put it in gear and press the gas pedal:- it goes. Brake pedal, slow down. Some of us used to be able to lift the hood of an older auto and recognise all the parts. Fix them, even, sometimes. A modern car? Forget it. It looks to most of us like a nest of snakes, and if you don't have the right industry standard plug you can't even begin to interrogate its twenty computers. You can look at a wingsail like that. It can be a comfort that you can push the thrust lever forward, and off you go. However, our customers may wish to cross oceans. So wingsails are built to the best known standards from the highest possible materials, with back-up systems to ensure that all will continue to go well. The diagram shows a section through a typical airplane wing, where something like 80% of the volume is occupied by fuel tanks. So, the designers have only about 8% of the chord width at the nose and maybe 20% at the trailing edge to squash their high lift devices into.  At the leading edge these are typically leading edge flaps, slats or droop snoots. At the trailing edge, the designers squeeze in single or even double slotted flaps, which can extend out and down to keep the airplane aloft, also increasing the effective wing area. For take off, leading edge devices and half flap are usually used, because the plane has to pick up speed quickly and there is plenty of thrust available from the engines. On landing, the pilot is looking for a moderate landing speed, with the nose low for best visibility forward. So full trailing edge flap is used, with massive increments of lift to enable the plane to fly slower, heavy nose down pitching moment to give the pilot his view forward. Plenty of engine thrust is still available to overcome the massive flap and undercarriage drag on approach. The plane is now much less aerodynamic than when cruising at 38,000 feet, so instead of tiny movements of the inner ailerons to stay level the Captain will use heavy deflections of tail, rudder and elevators to stay in the glide path and wait for that reassuring bump as the main wheels touch down. Then lift can be dumped and reverse thrust powered in to take the strain off the brakes. A wingsail works in an entirely different environment. Its job, instead of holding a plane up in the air, propulsion being provided by the engines (or gravity in the case of a glider) is to be an engine itself, to provide propulsion in the most efficient and user-friendly way possible, from a resource, the free clean ocean wind, which is highly variable in strength and direction. And its direction relative to the boat is not only variable as a function of the weather, it is directly affected by the captain's choice or change of course, and it can be affected by the motion of the boat as it rolls, yaws and heaves in a seaway. A wingsail must provide as much forward thrust along the vessel's cent reline as it can from whatever wind there is, with the minimum side force to heel the ship. This calls for a high aerodynamic efficiency, or put more simply, plenty of thrust for not too much heeling effect. The traditional sailing rig, unchanged in principle since the gradual introduction of the fore-and-aft rig in the 16th and 17th centuries, still has to be looked after by its crew. Shadotec believes that a true modern wingsail should look after its crew, their craft and itself, whatever the weather. So it has a computer control system, backed up of course, receiving inputs from the captain's control lever and his wind speed and direction instruments, as well as feedback from strain gauges and from its own internal systems to define for example position and speed information about its parts. Most importantly, it must set a maximum level of force which can be reacted on to the vessel, whatever the thrust level called for the by the crew, to prevent overpowering and capsize. It can itself then be designed and built to withstand higher stress levels than this, therefore protecting itself from damage. Finally of course, it must be capable of being switched off. This is not true of the aircraft wing, which to survive bad weather on the ground must either be tied down securely or (better still) be safely locked away in a hangar. After the oil price hike of 1973 Japanese wingsail engineers, experienced in deck cranes and other rotating devices, decided to install a powerful hydraulic rotating mechanism at the base of their relatively crude folding metal barn door wingsails. This worked for the relatively short life of their largely experimental (and generously funded) devices, but had two fundamental flaws which, added to the woefully poor aerodynamics of their thrusters, accelerated the demise of their efforts. These were that by definition hydraulic rotation mechanisms are slow and steady whereas the apparent wind direction can change at something like 60 degrees a second, and back again half a second later. This meant that the efforts of the rotator to cancel out the effects of the highly variable wind direction on the device were never better than approximate, and could even, dangerously, get into antiphase. Even more dangerously, of course, the rotator could fail. If the barn door flaps were unfolded, the ship could well be overwhelmed, and even with the flaps folded the smallest change of wind direction would react massive and uncontrollable forces on the vessel. Because of these serious disadvantages the Japanese wingsail program was cancelled well before the 1986 oil price crash set Walker wingsail technology back for two long decades. The basic concept of a Walker type wingsail is quite simple. One or more wings are set up vertically, thrusting horizontally as the horizontal aircraft wing provides vertical lift. The wing unit is mounted on a freely rotatable bearing, and its angle to the wind is controlled by a vane, just as in the familiar windvane, perhaps a cockerel or a fish, on the local church tower. Because a thrustwing must work equally well with the wing coming from either side of the boat, everything must be symmetrical, or capable of making an asymmetrical set of mirror images, which can be brought back to symmetrical for the neutral case, where no thrust is needed to either side of the wind. If you imagine climbing up the tower and bending the golden cockerel's tail feathers a little one way, and let go, the bird will still weathercock to the wind, but the body of the bird will now be held at a small angle of attack to the wind. There will be higher pressure on one side, lower pressure on the other, and the cockerel will be trying to sail the church up the street. Bend his tail feathers a little the other way and he'll be trying to sail the church down the street. Bend them straight again, and there'll be just a little downwind drag. Unbolt him, take him to the marina, screw him on to your boat and you've got a thrustwing! All we need to do with this simple concept is to provide a more user-friendly control mechanism than having to bend tail feathers, separate out the wing and tail elements for higher aerodynamic efficiency, and aim to get as much thrust per unit area as possible. Conventional trailing edge flaps as used on aircraft are not much use in thrustwing technology, because the extra thrust comes along with a regrettable increase in drag, bad for windward performance; and because they introduce a lot of moment about the pivot axis, which makes control aerodynamics very difficult. The clever thing about the original Walker concept was that very high thrust was achieved with moderate drag and very low levels of moment. It has however disadvantages of complexity and cost, in that a separate powered system is needed to operate the flap. Shadotec has been working on a simple but powerful high thrust device which needs no separate powered operating system Leading edge devices such as slats and slots can boost thrust very satisfactorily with no particularly adverse effect on moment, but are conventionally rather difficult to make symmetrical for the neutral, case, and to obtain mirror images for either tack sailing. Until now that is. When thinking about the Shadow, a new product for Shadotec, we naturally took a long hard look at the work of WWS, and came to the conclusion that two new features would be desirable. The first is that the original Walker wingsail design, while powerful and effective, was complex. We therefore set about designing a new thrustwing which would be lighter, simpler and cheaper to build. Secondly, their fixed trimarans were rather wide, making berthing in a marina occasionally problematical. So the first Shadotec product, the Shadow, is designed to fit neatly into a standard marina berth, and can even to be towed behind a car. After a lot of hard thinking and trial and error, Shadotec has come up with a slat and slot device which answers all the requirements very well. This has been incorporated into the Shadow thrustwing. The blue illustrations are derived from the Shadow computer file. The prototype Shadow will be a 9m (30ft) variable beam catamaran folding into 2.5m (8ft) wide for berthing or towing behind a car or van. Her patented wingsail is articulated by the wind itself, so that the only powered surface is the control tail. This brand new development should dramatically cut costs and complexity by comparison with earlier Walker-type wingsails, at similar thrust/weight and thrust/area levels. Before folding the side hulls in the wingsail are "captured" by locking the usual free vertical axis bearing so that the wing can be folded down along the vessel's fore and aft axis. This action will be automatic, controlled by a single press of a key on the Shadow's computer.