 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?
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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.
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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.
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