I have been working on the transition rig idea for some years. This is a rig that changes shape in use, and folds away when not required, rather like the wings of bats and birds. Because of the variable geometry created by the mast, the sail cloth must be stretchable to accommodate the shape changes. But not too stretchable, or the sail will be in danger of blowing out of shape in stronger winds.

The idea came in the late 1980s. For a number of years after that, I worked in the traditional way of the inventor, on the living room table in the evenings and in the garage at weekends. In 2000 and 2001, though, I have been able to work on the transition rig project full-time, thanks to funding by NESTA, the National Endowment for Science, Technology, and the Arts.

I call my design the transition rig because it marks an evolutionary transition in sailing rig design. I believe this approach to sailing rigs will have applications across the sailing spectrum, from small to large, whether on water, land or ice.

My story begins, however, with windsurfing. My first experience of this exhilarating sport was in 1983 when I began working as a lecturer in the medical school in Papua New Guinea. The conditions there were ideal for learning - warm azure water that was a pleasure to fall in to, and reliable SE trades that began each day as balmy breezes and built steadily to a cheerful force 4 to 6 by mid-afternoon.

Back in the early 1980s, windsurfing sails were rather like the yacht sails of the time – triangular in shape, supported by the mast at the leading edge, and held out at the back by a long wishbone boom. The sails were made from Dacron and cut into a rather baggy shape. They worked fine in lighter winds, but as the wind rose beyond about a force 4 they would become increasingly difficult to control. The mast would flex, the sail would deform, and the centre of effort - the place where all the forces of lift and drag generated by the sail seem to be focused - would move back. You would then be straining every muscle to hang on in a most undignified position.

Rig design has come a long way since then, particularly in windsurfing. The thing about windsurfing is that you are literally holding the wind in your hands, and you can feel the shifting aerodynamic forces as conditions change. I believe that this direct contact with sail forces, together with the relatively small size which encourages experimentation, is why the pace of rig development in windsurfing has been so rapid compared with sail development in other branches of sailing.

As my windsurfing skills improved, I began to think of ways of improving on the sails then available. I began to make sails with a more oval, wing-like shape, with a shorter boom and full length battens. I took an increasing interest in making sails that would go fast – speed sails. However, I found that while it was possible to make a sail that performed well in a steady wind on flat water, generally conditions were much more variable – the wind would be gusty and the water surface choppy. The smaller the craft, the greater these perturbations become, and the more unsteady the airflow over the sail.

I began to think about how to make sailing rigs that were more adaptable, rigs that could change intelligently as the conditions changed and still give a good performance. Inevitably, sails experience unsteady airflow, and this makes it quite a challenge to design a sail that works well across a range of conditions. As the saying goes:

"We cannot direct the wind,
But we can adjust our sails"

Let’s now fast-forward to the present time. We can see that modern windsurfing sails have tackled this problem of changing wind-strengths by the use of twist – on the sail alongside I have drawn a dotted line from the mast tip to the boom end, and shaded the part of the sail that extends behind that line. This pronounced roach helps the upper part of the sail to twist away in gusts, depowering the sail and allowing the sailor to remain in control over a wider wind range.

An alternative method called reefing is widely used in other sailing applications. Here a proportion of the sail is folded or rolled down to reduce the area presented to the wind. This is carried out when the wind becomes stronger.

The transition rig takes a rather different approach. Instead of relying only on deformation under aerodynamic loading, or reefing to reduce area, it introduces the possibility of changing shape.

I can’t remember a eureka moment when the idea for the transition rig suddenly popped up, but I am pretty sure that my lifelong interest in wings and flight had something to do with it, together with my background as a biologist. I have for example clear memories of many happy hours spent on hills and cliff tops watching seabirds effortlessly gliding past at eye-level with their wings outstretched, deriving lift from the air currents rising over the hill. From such a vantage point, you can see that with every gust, their wings flex closer to their bodies and take on a more streamlined planform, while in a lull their wings extend fully, to derive the best lift. Thus, they change the geometry of their wings to suit the conditions.

In contrast with the changing geometry of bird’s wings, modern, advanced sailing rigs remind me more of insect wings set vertically instead of horizontally. The mast at the leading edge of the sail corresponds to the strengthened veins at the leading edge of insects’ wings. In both cases, the changes in shape when loaded up aerodynamically are the result of passive twistings and bendings. Some distortions can be useful, as in the case of twist in the upper region of the sail, while others can reduce the effectiveness of the foil. Much research has gone into reducing unwanted distortions in modern sailing rigs by the use of high-tech materials and design.

You will have noticed by now that I tend to talk about wings and sails as if they share common features. Not everyone would be comfortable with this, but I believe it is justifiable.

Actually, we didn’t invent sailing – like so many of our inventions the biological realm beat us to it by quite a margin. Recorded human history goes back only a few thousand years, and charts the development of sails and rigs to the present day, but let us look back many millions of years to the time when small insects that were destined to be the ancestors of today’s stoneflies were learning to make the most of moving air currents to propel them across the water, raising wing-like rudiments in response to the wind. This is the finding of Marden and Kramer published in 1995. Thus, there is evidence that sailing movements by insects across water are linked with the evolutionary emergence of flying about 330 million years ago. Of course, other animals took to the air too - flying dinosaurs called pterosaurs, other smaller dinosaurs that are now thought to have evolved into birds, and then the mammalian bats appeared. Each developed different solutions to the problem of unsteady airflow to achieve flight, but there are common denominators that link these solutions. It is those underlying principles that I am trying to incorporate into the transition rig.

The wings of today’s birds and bats differ from insect wings in being able to change shape actively. Their wings can take on a variety of shapes to optimize their aerodynamics under different conditions. Flapping wing flight is a prime example of unsteady aerodynamics in action, and gradually we are learning how it works.

When I began making prototype rigs with a variable geometry I thought that the most difficult element of the design would be the mast - working out the geometry and making joints strong enough to bear the sailing loads. Actually, it has turned out to be the sail that is more difficult to optimize ...

When modelling a complex system, which most biological systems are, first you try to capture the essence stripped of all distracting detail. Then you try to understand the variables at work by trial and error experimentation with prototypes, and you can begin to add some of the finer details if they are justified. I have drawn on the biological realm quite widely in terms of structure and function in this rig. Although the transition rig resembles the bat’s wing most closely, it incorporates features from a variety of other sources, both living and extinct. Modelling the feathers of a bird’s wing would be a very difficult task, and I chose instead to stick with the more familiar membrane approach for my sail surface, so this has inevitably given the transition rig a bat-like appearance. This has delighted copy writers in papers and magazines and I am now getting used to seeing headlines such as ‘batty professor’, ‘batty sailing project’, and ‘To the bat boat Robin’!

The transition mast has three segments: upper, middle, and lower. The middle segment is distinctive in having two parallel struts. This arrangement co-ordinates movements at the two sets of joints – if the lower joints become more extended, then so do the upper joints. There is also a rotation that occurs in the middle segment that resembles the pronation/supination rotation of our own forearm. This swings the lower joints to leeward when going from tack to tack and enhances the aerofoil section in the lower part of the sail.

As for the joints themselves, I have tended to model them on biological joints, making use of concavo-convex surfaces with a large surface area. I have found this approach results in joints that can cope well with the high twisting forces experienced by the rig without having to be too heavy. They are made from carbon composite materials and stainless steel.

The mast is extended by means of a tensioning device, usually made of webbing, that passes down the mast skirting around the joints. The greater the tension in this webbing, the more the mast extends. Thus the tensioner controls the configuration of the mast. The degree of tension is controlled in different ways according to the application. The windsurfing version is the hardest to achieve, since ideally it should occur automatically in response to the changes in downforce on the boom exerted by the sailor. When sailing in light winds, the downforce is minimal because the sailor is standing almost upright on the board alongside the sail, but in stronger winds the sailor leans back to counterbalance the increased power generated by the sail, and the downforce on the boom increases. This becomes the controlling force that causes the mast to flex into a more zig-zag shape. In a dinghy, it is possible to arrange a separate flexion/extension control that can be adjusted by the crew. In all the applications, if the tensioning is released completely the mast can be folded into a compact bundle, rather like a bird’s wing or a bat’s wing. The boom is pivoted too, so that it can also take part in the changes.

To summarise, the mast is jointed and can change shape. It can be straightened or flexed in use: the rig is more extended in light winds and more flexed in strong winds, and then can be folded away when not required.

This ability to fold is actually quite an important feature - if you go down to a marina and look at the boats, you will see a forest of masts and rigging, even though the boats are moored. The wind resistance, or ‘windage’ produced by the mast and standing rigging can be significant, especially when out at sea in storm conditions, and can become hazardous even when carrying no sails. I believe that the marinas of the future will look different - all the sailing rigs will be folded neatly down to deck level or even retracted away below decks. This is sailing with the convenience of an umbrella – you raise the rig when you need it, and fold it away when you don’t. I believe the day will come when fixed, stayed masts will look quaint. By way of illustration, wouldn’t you be surprised to see a bird walking around on the ground with its wings stiffly outstretched and braced with wire?

Now, because the mast changes shape, the sail membrane also has to change shape, and therefore an elastic sailcloth is required to provide a taut foil surface. (By using a stretchy sail material, I am single-handedly going against all those years of research and development that have produced modern sailcloths that do not give!) Initially, I tried stretch fabrics such as Lycra and Spandex. These materials have excellent stretch properties and are tough and light, but they have two shortcomings in this application: they are porous meshes that allow the relatively easy movement of air from one side of the cloth to the other. This dramatically reduces the power of the sail, allowing equalization of the pressure differences between windward and leeward sides of the sail the fabric becomes saturated when immersed in water, making it heavier.

One way to overcome these problems is to coat the cloth with a stretchy film that is waterproof and windproof. I found a source of stretch material coated on one side with a polyurethane film. Polyurethane is a versatile material that can be made UV resistant and can be brightly coloured. In combination with a knitted stretch fabric it provides a reasonable sail cloth for the transition application. For several years I could only find a single-coated version. This meant that the knitted fabric was exposed on the uncoated side of the cloth and therefore could still absorb water. It is only in the last 12 months that I have been able to source a double-coated cloth that largely solves the water-uptake problem – although there is still a slight capillary uptake of water along exposed cut edges of the material.

For windsurfing, there is an additional need – at least part of the sail has to be transparent so that you can see where you are going. This is especially important when you are sailing at speed in crowded waters. I had no success in finding anything both stretchy and transparent so I mentioned this to a reporter who was preparing an article for New Scientist. She included this observation in her article, and soon after publication of the article I was contacted by the designer of Femidom, the female condom. She recommended a transparent stretchy film that they had used and I was able to obtain a thicker version of the same material to try. Early tests have revealed that it is not quite ideal for use in sails in the sense that it tends to sag permanently when overloaded and is rather prone to punctures caused by contact with sharp objects such as stones on the beach, but nonetheless marks a significant step in the right direction.

Changing shape and the use of stretchy sail cloth are central to the transition rig idea, but they are also the hardest parts to get right. It is a bit like the sailing equivalent of squaring the circle. Let me explain. If we flex the rig as the wind increases, in the same way that a bird or bat may flex its wing to obtain a better high-wind shape, the obvious happens - the leech of the sail, that is the trailing edge, has a tendency to become slacker. In a strong wind, this is probably the last thing you want. So what can be done to minimise this effect? I have found that there are one or two ways of keeping the leech tight, and one of course is automatically achieved by the increased load on the clew end of the boom. The battens also play a key role in balancing tension across and along the sail. Ideally, though, we need an active membrane, one that will tense up when required to reduce any areas of slackness.

Let’s go back to the bat for further help. The bat achieves this by having muscle fibres arranged within its elastic wing membrane, the fibres orientated along the lines of best action. Thus, when part of the membrane becomes slack, the muscle fibres contract to reduce the slackness. Quite how we would emulate this muscular action in a sail membrane I don’t yet know.

What are the possible applications of the transition idea? The sailing spectrum is broad: from small sails for canoes and kayaks to sail assistance for giant tankers and bulk carriers, a range of sail sizes from 2 sq metres to 20,000 sq metres. Windsurfers, dinghies, yachts, cruise ships, and research vessels might all benefit from having variable geometry, foldable rigs. Then, of course, sailing is not limited to water craft – from the e-mails I have received it is clear that there are enthusiastic sailors on land, sand, and ice. Awnings and sunshades for buildings also come to mind. Then we have aircraft such as hang gliders and ornithopters, so there is no shortage of potential uses for the transition approach.

"To see a World in a Grain of Sand
And a Heaven in a Wild Flower,
Hold Infinity in the palm of your hand
And Eternity in an hour."

(from Auguries of Innocence, William Blake, ca 1803)