The concept

The concept
Other ideas

The Transition Rig is a mast and sail that takes inspiration from Nature, particularly the wings of bats and birds. The mast is jointed, and this gives the rig a capacity for variable geometry.

There are two main reasons for developing this idea:

  • a variable geometry sailing rig has the potential to adapt to changing wind conditions, in the same way that a bird can adjust its wing shape when gliding in gusts and lulls and during flapping flight
  • the jointed mast allows the whole rig to be folded away when not required, which is convenient and also in stormy conditions may enhance survival.

Although the concept is a relatively obvious one in the sense that we see successful biological solutions to variable geometry wings around us every day, the Transition Rig is still at an early stage in its development. I have numerous prototypes that demonstrate different aspects of the idea and these are described in these pages, but there is not yet a fully-functional version that makes the most of the variable geometry approach. I hope that this website will give an insight into the potential of the concept and offer a few solutions that I have found to problems that arise when making variable geometry sailing rigs. I shall be delighted if you can build on these ideas.

Here is a short film made in the year 2000 giving an overview of the Transition Rig idea:



The following is a broad overview of the Transition Rig concept. This was first published in Catalyst (the Journal of the Amateur Yacht Research Society), volume 18, pages 14-20 (October 2004).


The transition sailing rig takes its inspiration from the wings of bats and birds. It can change shape in use according to changes in wind strength, and can be folded into a convenient bundle when not in use. The idea arose in 1986, and I made a series of windsurfing prototypes in the years that followed. From mid-2000 I was funded for one year by NESTA (the National Endowment for Science, Technology and the Arts), and I now have working versions of the rig for canoes, kayaks, windsurfers, and dinghies.

variable geometry wings

The concept

I have always enjoyed watching birds such as gulls as they slope-soar along hill sides and cliff faces, fascinated by the way they can alter the geometry of their wings to cope with gusty conditions. In light airs, their wings are fully extended, while in gusts their wings are drawn in closer to their bodies and the outer segments of the wings become more sweptback.

In the 1980s I developed a passion for windsurfing in the warm conditions of Papua New Guinea. As a hobby I began to make specialised sails and boards for speed sailing. I soon discovered that each sail worked most effectively over a rather narrow range of wind speeds. At that time we did not appreciate the value of additional roach and controlled twist in the upper part of the sail which now gives contemporary windsurfing sails a wider wind range. The combination of an unforgiving sail and unsteady wind greatly reduced the chances of sustaining a good speed over worthwhile distances. It was then that I began to wonder whether it would be possible to make a sailing rig that would be more adaptable to the changing conditions encountered on the water.

the main components and their names

The idea of making a variable geometry rig came in 1986. Although the original inspiration came from the shape changes I had observed in birds’ wings, I felt it would be more practical to follow the structural example of the bat’s wing, where the flight surface is an elastic membrane rather than overlapping flight feathers. This approach would simplify construction and experimentation, and had the added benefit of making it easier to reverse the aerofoil when changing from tack to tack.

The jointed mast would have three segments: the lower one attaching to the sailboard would be a single strut, the middle segment would consist of two parallel struts, and the upper one would be a single strut. This mimics the arrangement of the skeleton in bats and birds, as well as in our own upper limbs. As a biologist, I quickly fell into the habit of naming the parts of the rig in the way that the biological equivalents are named. Thus, the lower segment became ‘humerus’, the middle struts became ‘radius’ and ‘ulna’, and the upper segment became the ‘carpus’. (Carpus means wrist, so the use of the term here is not accurate - other bones such as the metacarpals and phalanges contribute to the skeleton of the tip segment of a bat’s or bird’s wing.) The two middle struts (radius and ulna) co-ordinate the movements at the upper and lower sets of joints - as the ‘elbow’ flexes and extends, so must the ‘wrist’.

downforce on the boom flexes the mast

The main aim of having a variable-geometry rig was to use it fully extended in lighter winds, and then flex the rig in stronger winds so that the centre of effort of the sail was brought lower and the upper segment of the mast made more sweptback. This I believed would make the rig more effective and more controllable over a wider wind range than a conventional rig, adapting better to gusts and lulls. In the context of windsurfing, I envisaged that these adaptive changes would occur in response to changing loads placed on the boom by the sailor. In light winds, the sailor stands more upright on the board, putting very little downforce on the boom, while in strong winds, the sailor hangs most of their bodyweight from the boom to counterbalance the lift generated by the rig. Part of that increased load would be experienced by the boom as a down force, and this could be used as a controlling force to bring about the shape-change.

(There is a video about shape-changing here.)

An additional advantage of a jointed rig is that it can fold into a compact bundle for transport and storage, without the need to dismantle any of the components. Thus, rigging and de-rigging can be achieved rapidly and conveniently.

biology-inspired joints

the tensioner passes in front of the upper joints
and behind the lower joints

The joints and tensioner

At first, my approach to joint-making was also influenced by biological structures. I made the hinge joints from glass- and carbon-reinforced epoxy resin, using large bearing surfaces with a saddle (concavo-convex) shape. These components were time-consuming to design and make, requiring the preparation of wooden blanks and the intermediate step of mould-making, but when completed had the advantage of being very resistant to the twisting forces they would experience in use without being unduly heavy. From start to finish, each generation of mast development would take about a year. More recently, I have started to use much simpler metal joints which can be made and modified comparatively quickly, and this has speeded up the development process.

(There is more about joints here.)

The first prototype was crude and was never tried on the water, but it did enable some of the practical problems to be worked out. For example, for the mast to change shape in the required way, it has to be elastically tensioned so that it is fully extended when rigged with the sail, and then begins to flex when downforce is applied to the boom. I found a way to direct tensioning webbing around the front of the upper joints and around the back of the lower joints, making it adjustable at the foot of the mast. This made it possible to balance the various forces acting on the rig and also make allowances for sailors of different weights. Releasing the tensioner completely then allowed the rig to be fully folded.


the pronation/supination rotation occurs between the upper
and lower parts of the mast

the joints at the upper end of the ulna and lower end of
the radius allow the pronation/supination movement


The basic geometry of the mast, the control of flexion and extension, and how to achieve folding were worked out through trial and error, but I found that many design problems still remained. For example, the lower joints of the mast forming the elbow are set back from the leading edge of the lower part of the sail at about 1/3rd of the chord. This is a part of the sail that benefits from having a good aerofoil section to produce power lower down. To achieve a good section, the lower joints need to be displaced to leeward - away from the sailor - on each tack. If the mast can only flex and extend, this is not possible and the shape of the lower part of the sail is compromised.

It was not too difficult to achieve the necessary rotation - I followed the solution provided by the arrangement of our own forearm and the forearms of birds and bats. If, in addition to flexion and extension, the radius is able to rotate around its long axis at the lower end in relation to the humerus, and if the ulna is able to rotate its long axis at the upper end in relation to the tip segment (carpus), then the lower joints can swing from side to side in relation to the boom when tacking and gybing. The axis of rotation for this movement passes between the universal joint at the lower end of the radius and the universal joint at the upper end of the ulna. The interesting thing about this arrangement is that the boom, radius, and carpus work together as one unit, while the ulna and humerus work as another unit during these pronation/ supination movements. The rotation produces an interesting ‘cupping’ effect on the overall form of the rig, where the carpus leans slightly to windward in relation to the humerus. Before trying this system of joints in practice, my belief was that the correct rotation would occur automatically when the lower part of the sail ‘powered up’. This turned out not to be the case.

(There is more about pronation and supination here.)

shape changes - rig extended in light winds (left)
and more flexed in strong winds (right)

a sail made entirely from stretch materials

The sail

The next challenge was to make a sail that could accommodate the shape-changes – flexion and extension, pronation and supination when going from tack to tack – and at the same time keep a good aerodynamic shape. Here I was faced by a dilemma: conventional reasoning and experience say that if you want a sail to remain stable, particularly in higher winds, you need to use a sail material with minimal ‘give’. Indeed, one of the main thrusts in the development of contemporary sailcloth has been to reduce stretch under load.

As with many design problems, the trick is to find the correct balance between apparently opposing requirements, in this case shape change and aerodynamic stability. One of the main concerns with this type of rig is that as the mast flexes, the tension in the trailing edge of the sail (leech) becomes less. Within limits this is acceptable, since it allows the upper part of the sail to ‘twist off’ in stronger winds and reduce the power being produced. However, if the leech becomes too loose, the rig becomes difficult to control.

Early prototype sails made from stretch materials such as Lycra and Spandex worked reasonably well in the sense that they allowed shape-changes over a useful range while remaining reasonably taut, but they were unsuitable for a sailing application. The porous nature of the cloth allowed air to flow at least partially through the sail from the windward to the leeward side rather than flowing around it, greatly reducing its power, and if the cloth came into contact with water it became saturated, baggy and heavy.

The next step was to experiment with stretch fabrics coated on one side with a thin elastic film of polyurethane. These fabrics fulfilled many of the requirements that I had for an elastic sail cloth. They are lightweight, stretchable, tear-resistant, UV-resistant, available in a wide range of colours, and airproof. Their big disadvantage was that the fabric exposed on one side still absorbed appreciable quantities of water. However, the single-coated fabrics were good enough for the prototype sails to be tested on the water. In some sails, I laminated two layers of the single-coated cloth together so that the coated surfaces faced outwards, but this proved to be a time-consuming process and even then water was able to infiltrate between the laminations over time.

For several years I searched for a double-coated stretch fabric that would overcome the water logging problem. Technically, double-coated cloth is harder to manufacture than single-coated, and coating specialists were not prepared to experiment on my behalf without a substantial guaranteed order, which I was unable to provide. Eventually a double-coated material became available - it had been developed for use by the health service on operating tables and trolleys.

For windsurfing it is helpful - and safer - to have a see-through sail so that you can see where you are going and avoid collisions. I had searched in vain from the beginning of the project for a transparent and stretchy material, and had to make do by inserting windows made of non-stretch clear plastic into the prototype sails. Then, by a stroke of good luck, an article about the transition rig was published in a science magazine and I received a useful tip from one of the readers - a clear elastic film had been developed for use in the female condom, and might have the required properties. Of course, for sails a much thicker gauge of film is required, and fortunately this was also available. The material has proved to be very useful, fulfilling most of the criteria for a clear, stretchy sailcloth. It does however have two disadvantages - it is very difficult and frustrating to sew because it clings tenaciously to the sewing machine’s flat surfaces, impeding its passage through the machine, and it is also quite vulnerable to puncturing. In time I hope to replace sewing with heat welding, and it may be possible to incorporate a puncture-resistant mesh in the film, but for now the film provides a step towards finding the ‘ideal’ material.

(There is more about sails here.)

The testing

By now I was regularly testing the prototype windsurfing rigs on the water, and repeatedly being reminded of the gulf between theory and practice. One by one my assumptions and pet ideas were being severely challenged. The pronation/supination movement gave the biggest headache. As soon as the sail powered up, the lower joints would flip across, but always in the wrong direction. Instead of moving away from the sailor on each tack, they would do the opposite, moving forcibly towards the sailor and giving the rig a very un-aerodynamic shape. I tried many different systems to produce the correct movement and lock the rig in the required shape, and although some worked reasonably well, none of them gave a simple automatic rotation when changing tack. I had hoped all along that from the sailor’s point of view the transition rig would be used just like a conventional rig, with all its variable-geometry features looking after themselves automatically, and yet now I was having to fit additional controls. A compromise solution did eventually present itself, and now ensures that the correct rotation occurs without the need for levers and locks. (For more on this click here.)

raising the rig on a Mirror dinghy

Free-standing version

Up until 1999, my attention was focused on developing a variable geometry windsurfing rig. However, my involvement as a volunteer on a nearby project to build an ocean-going catamaran caused me to think about how the transition rig idea might be adapted for use as a free-standing rig and applied to other types of craft. With the help of friend and colleague Alex McCall, a Mirror dinghy was modified to accept a cable-operated folding mast and sail. The most complex engineering occurs in the region of the mast foot, which has to be strong enough to support the unstayed rig, be free to rotate, and also allow the passage of multiple control cables close to the axis of rotation. The rig incorporates all the movements described above and has an additional control to allow tuning of leech (trailing edge) tension when the rig takes on different shapes. The steel cables for raising, lowering, and rotation are attached to horizontal levers beneath a false floor in the dinghy and operated by blocks and tackle. This version of the rig has come the closest yet to fulfilling the different aspects of the original concept. (For a video click here)

Simplified versions of the free-standing rig have also been developed for smaller craft such as canoes and kayaks. These offer umbrella-like convenience in that they can be raised quickly when required, and folded away into a small bag when not required and stored out of the way. However, these smaller rigs do not have the capacity to change shape according to conditions - they are either fully extended or folded.

Having worked out the arrangement of a free-standing version of the transition rig and the control systems required, it then became possible to propose folding rig modules for larger ships. Tankers and some bulk carriers have significant areas of relatively free deck space, and it may be possible to provide sails as a way of reducing the amount of fuel they use. Even a 15% reduction would be worthwhile, given that oil is a finite, diminishing resource and that the burning of heavy fuel oil has a harmful effect on the environment. Folding, removable sail modules bring several advantages over fixed masts and rigging, and one of my ambitions now is to generate enough interest in the concept to be able to build a working prototype for testing. (For more on this, click here.)


In conclusion

The transition rig changes shape as it adapts to different conditions, being fully extended in light conditions and more flexed and streamlined in heavy conditions. Added to this is the convenience and safety of a rig that can be folded away when not required. These advantages come at a price – the rig is more complex and will require good design and materials if it is to match the strength and durability of conventional rigs without a significant weight penalty. And with complexity comes the potential for increased cost, at least in the early stages of development.

The variable geometry approach to sailing rigs offers an alternative pathway for development compared with more traditional rig designs. The name ‘transition’ was chosen for this concept because it implies both the evolutionary transition and the shape changes in use.


Richard Dryden


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