Peter Worsley - Wingsail Experiments -Article
Since the invention of aircraft, a similarity has been noticed between the operation of sails on boats and the function of wings of aircraft. Sails on boats provide thrust in a horizontal direction derived from moving air, and wings on aircraft provide “lift” in a vertical direction to support a plane in the air, also from moving air (relative to the plane).
In order to fly, wings had to have a certain degree of efficiency, and some experimenters have realised now that aircraft-type wings could be used on a boat and would be more efficient than sails.
Having tested wings on boats in place of sails (“wingsails”) designers noticed another feature used on aircraft that would be useful to use in conjunction with wingsails, that you could control the angle of attack to the wind of the wingsail with another smaller wing mounted behind or in front of it (a “tail”).
There are many examples of tails used to control the direction of bodies
in a fluid, such as arrows, darts, bombs, torpedoes, submarines, and of course
the rudder on the boat serves the same function, in that case in water instead
Aircraft use them to adjust, to a precise degree, the lift or (angle of attack) of their wings, in that case the pivot point is the centre of gravity of the aircraft.
The Self-Trimming wingsail "sailset"
When fitted as a way of propelling a boat, the wingsail and tail combination (also known as a "sailset") can adjust to maintain maximum thrust during every small change of wind direction, in this way relieving the sailor of continually manually adjusting for every slight wind direction variation, using a tail can of course do a better and more precise job than any human can do. Such a wingsail/tail combination is referred to as a self-trimming wingsail.
As in the diagram below, the a pivot point for the whole wing and tail assembly is arranged at or near the wing's centre of pressure, and the tail, which is mounted some distance behind this point, exerts a stabilising force to keep the wing at the correct angle. The whole assembly, or "sailset" then follows the wind direction at all times in a similar way to a weathervane, and the angle the tail makes to the wing centreline determines the angle of attack of the wing, causing the wing to exert lift from either side, according to whether the wind comes from the right of the boat or the left, and whether it is required for the boat to go forwards or in reverse.
Diagram by J Walker
If the tail is aligned perfectly with the centreline of the wing, the sailset will provide no thrust to the left or the right and is in a "neutral" position. This can be used when the boat is moored or at anchor. Since the profile of the wing and the tail are made to an aerofoil low drag section, the neutral position the rig creates less drag even than a boat with bare poles (no sails raised). Therefore when moored, it is not necessary to lower the wingsail as you would have to do with a conventional flexible cloth sail. This is the most frequently asked question: how about reefing or lowering? It is not necessary to do either of these operations, and the rig will just weathercock about and take care of itself without any danger. When you want to move off in either direction, all that is required is to alter the angle of the tail, and the rig will find the wind and set the thrust of the wing pushing in the required direction.
Non self-trimming wingsails
Some wingsails have been made that are not self-trimming, that
is, the helmsman has to adjust the angle of attack of the rigid sail to the wind
manually. This is the most popular option lately, since the the America Cup
boats have brought wingsails more to attention. There are some disadvantages,
here because the system does not look after itself, has to be adjusted manually,
cannot be left up and is more difficult to adjust correctly, since a sail is
very forgiving in adjustment, but with a wing the angle of attack needs to be
more exact to maintain efficiency, quite apart from the fact that with such a
tailless system the sailor cannot hope to
keep pace with the changes in direction of the wind which occur all the time. It
seems strange that whilst accepting aircraft principles with respect to a wing,
many have not even considered that in order for a wing to work best it
requires governing by a tail, as in a plane. They have accepted half the
technology, but not noticed that wings on planes require tails to make them work
After all, you don't see the pilot of a plane pulling a rope to adjust the aircraft's wing!
A self-trimming wingsail sailing system gives the following advantages over a traditional cloth sailing setup.
No sail raising.
No sail lowering.
No reefing required.
No sheeting required.
Reverse is as easy as forward - and as fast.
Sailing in comfort from inside the cabin, there is no need to get cold and wet on deck.
In fact, the only ropes required on a self-trimming wingsail boat are those required for mooring.
The advantage of using a rigid wing is that it can be pivoted vertically on or near its centre of pressure and therefore requires very little force to change its angle of attack. (Such a system is very difficult to arrange on a conventional non rigid cloth sail).
Since the wingsail is balanced and needs only a small force to change its angle, it lends itself to control by a tail vane.
The tail vane follows the wind instantly and in this way takes away the need for manual adjustment of the wing and is in this way self-trimming.
In its simplest form, the wing is of symmetrical section which allows it to develop lift on either side according to whether the boat is on a left or right tack.
Manually controlled tail.
A wingsail/tail combination, as
described above, freely
pivoting through 360 degrees, automatically finds the wind direction and keeps the wing
working at the correct angle.
Although the operator does not have to adjust for wind direction all the time as you would with a normal sail, there still has to be control of the tailvane for forward neutral and reverse.
Typically, a lever is provided for control with a neutral position in the centre for no power, and left and right positions to drive the boat when the wind is from the left or the right.
There is nothing else needed for control, no ropes or anything else. Reversing is as easy as going forward.
However, you do have to change the setting for changing tack, so need some awareness of the direction of the wind.
Automatically controlled tail.
A more recent development is an
automatic self-tacking device, which makes self-trimming wing sail control
completely automatic and controlled with only one
This is the most simple to use system yet devised.
This has been achieved before in a complicated way with electrical sensors, computer control and servos, but my own patented system is entirely mechanical and much more simple to build and operate. To see a video of how it works go here.
The operating lever is the same as a throttle on a power boat. Go, neutral, reverse. That's all! You don't need any knowledge of the wind direction.
The only difference between a boat with this system and a power boat is that the wind is your engine and your speed will depend upon its strength.
If the wind threatens to be too much you can reduce power with your lever towards the neutral position accordingly, this is equivalent to reefing a normal sail.
Answers to some objections made to wingsails often made by conventional sailors.
Q: Why use a tail or trimming surface to control a wingsail?
A: A tail relieves the helmsman of the duty of adjusting the sails to get the best angle to drive the boat. The wind is invisible, and with conventional sails the best adjustment can only be judged by indirect means such as the speed of the boat and the “feel” of the boat etc.
Luckily, conventional cloth sails can tolerate inaccuracies in adjustment quite well, and still drive the boat. At best, the adjustment of the angle of attack of conventional sails is only approximate, due not only to the adjusting system, but also from the small strength and direction changes which occur in the wind all the time.
To get the best from a wing, adjustment has to be more precise, and this is best provided by a secondary trimming surface, - the tail.
You only have to observe a self-trimming wingsail in action to see how it automatically adjusts all the time to the wind. These adjustments could never be achieved by a manual sheeting system.
Q: When I pull in my sails on my conventional yacht it takes a lot of strength, how can a tailvane develop similar strength?
A: The answer is that your conventional sails are not “balanced” and you are pulling against the full power of the sail. Conventional cloth sails pivot on their leading edge (luff) and it is very difficult to arrange for them to pivot at any other point.
With a self-trimming wingsail the pivot point is arranged to be about a third of the way back from the leading edge, and therefore most of the lift developed behind the pivot is counterbalanced by the lift produced in front of the pivot - therefore allowing adjustment with minimum power.
It’s like the difference between trying to lift a person on a seesaw with no-one on the other end, compared with lifting them when the opposite seat is occupied.
(The lack of “balanced” adjustment on conventional sails leads to many complications such as winches and block and tackle purchases, resulting in yards of rope on the cockpit floor, and dangerous accidental gybes where the boom comes crashing over from one side of the boat to the other, (sometimes resulting in head injury). These kind of inconveniences have always been regarded as part and parcel of sailing and “part of the adventure”.
However, they are really unnecessary, and they are all avoided on a self-trimming wingsail system.)
Q: In their simplest form, these wingsails have symmetrical aerofoil sections, aren’t these inferior to asymmetric sections?
A: They are only inferior in the respect that they stall at a lower angle. Most aircraft-type wings will lose lift if presented at an angle of more than about 15 degrees to the wind. With a symmetrical section the stalling angle is lower, usually about 10 degrees.
Evidence that symmetrical sections produce nearly as much lift as asymmetric sections can be seen in the fact that many aerobatic aircraft use symmetrical sections and still perform spectacularly! Various schemes have been used to make a boat’s wingsail adjustable so that the wing can present an asymmetric section on either tack.
Such ideas are trailing-edge flaps or even pivoting the entire wing horizontally about its centreline. Although these ideas can be useful they are by no means essential.
Q: Can a wingsail be lowered in a gale? Does it need to be lowered? Surely, the drag is more than bare poles?
A: A self-trimming wingsail in “neutral” – that is with its tail at zero degrees – (not angling the main wing to lift in either direction), presents a perfect streamlined section to the airflow and has much lower drag than a circular section of the same thickness, and since it is constantly aligning itself to have the least-resistance to the wind is therefore better than “bare poles”.
All experimenters so far have found that in “neutral” there is no problem in any strength of wind. The Walker Wingsail, which is the most prominent design of wingsail so far, has survived many Atlantic hurricanes without damage.
So the consensus seems to be that there are no conditions where they would have to be lowered.
Self-trimming wingsails and electric in combination
Recently, for the first time, a solar powered boat crossed the Atlantic. This boat used photovoltaic cells on its roof to charge batteries, which in turn, drove electric motors with propellers underwater to push it along. The achievement was hailed as a triumph of low-carbon sustainable transport. Whilst this was an interesting technical first, it was by no means the first time craft had crossed the Atlantic without the use of fossil fuels. Atlantic crossings have been going on for hundreds of years using windpower, (which also originates from the sun).
The problem with sailing, is that it has always been there, and so has been taken for granted, and largely ignored in recent times as a inexhaustible source of marine power.
Consider this scenario. Our solar powered boat is struggling to get into port, batteries low, overcast conditions and getting dark, a strong wind is blowing, but the boat cannot use the powerful wind because it has no sails or means to convert the wind into forward motion. Eventually, the crew have to be rescued as the craft is in danger of being blown onto the rocky shore. Surely, if sustainable non-fossil fuel motion is the aim it is clearly a serious omission to fail to equip a boat with a means of using the wind!
The probability is that designers feel that the traditional methods of using sails, which still predominate, and the inherent awkwardness of handling them and adjusting them make their use to impractical for normal transport use.
Of course, this works in the other direction also. A sailing boat is becalmed on a beautiful sunny day and is going nowhere in the doldrums. The sun is beating down, but our boat cannot make any progress because it has no means of converting the abundant sun’s energy into power to drive itself along. Supplies are getting low, eventually the crew have to be rescued.
Clearly, an effective eco-boat needs to use all available sources of natural power to be a success.
We could call such a boat a “hybrid” craft, since it uses both solar and windpower.
At the present time, there are hardly any examples of boats that use both windpower to sail and solar power to motor. Windpower and electric power do not integrate very well together, their methods of operation are very different.
The following example compares using electric power with using conventional sails for windpower.
To power up your boat with electric power, you need only to move a lever a few inches and the power comes in and pushes you along.
To drive your boat using the wind, it is a little more complicated, an example would be as follows.:
1. Assess wind strength, decide what sails to use, or how much to reef.
2. Connect up necessary sails.
3. Hoist sails (usually more than 1) secure them up.
4. Adjust sails for best result.
5. Monitor course and wind at all time to get best sailing result.
6. If your intended direction is more less than 45 degrees towards the wind, tack as necessary.
That is at least 6 separate and quite often awkward steps.
Compare this with the simplicity of only moving your power lever on your sail/electric system!
To integrate electric power with sailpower, clearly a more user-friendly sailing system would be an advantage!
A self-trimming wingsail would be much more suitable for use with electric solar power than the traditional cloth sailing system.
Perhaps an ideal system would be one where a speed-sensitive switch was employed which, when the speed of the boat drops below a certain value, the electric power was automatically switched on.
With this system, if there was sufficient wind from a convenient direction the boat would sail happily along using the self-trimming wingsail only.
If however, due to headwind, or the need to tack, the speed falls, the electric power would cut in and provide the needed impetus.
In this way you could achieve the best of both worlds. Better than a pure electric boat, with undreamt of range (since it uses the wind when available). Better also than a pure sailing boat since it can overcome any loss of speed by providing power during tacks, calms, and harbour manoeuvring.
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