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“I WAS always fascinated by flight,” says Richard Dryden. “Wings, how things
fly, aerodynamics and aircraft.” So why are we perched precariously in a small
dinghy on the Teign estuary in Devon?

The answer is the sail that’s propelling us over the water. Bruce Wayne would
be proud of Dryden, because there’s no mistaking its shape: it’s a bat wing.
Dryden has taken one of nature’s best flying machines, turned it on its side,
and launched it onto the water.

“It takes a bit of a leap of faith, I suppose,” he says, “but the more I work
with these things, the more I feel that a sail is simply a vertical wing,
experiencing all the usual forces a wing does, plus a few others.” He calls his
sail and mast the “transition rig” because not only is it shaped like a wing, it
folds up like a wing too. That makes it uniquely responsive to changing
conditions, and certainly far easier to adjust than a conventional sail.

Designing sails began as a hobby for Dryden, a University of Plymouth
biologist turned inventor. But now he is working on the rig full-time, courtesy
of a year’s grant from the National Endowment for Science, Technology and the
Arts, a lottery-funded organisation that backs “adventurous innovators”. He has
made sails for windsurfers and dinghies and even canoes and kayaks. Perhaps most
ambitious, his drawing board is covered in plans to put bat-wing sails on
gigantic ocean-going ships.

Dryden’s interest in sails began when he became hooked on windsurfing in the
rather warmer waters of Papua New Guinea in the early 1980s. In those early days
of the windsurfing craze, sails were triangular, a bit baggy and floppy, and
difficult to control in a strong wind. Dryden reckoned he could do better, so he
started making his own, cutting them more oval in shape, and gradually moving
towards designs that were more and more wing-like.

“From aircraft I knew that some shapes were better than others,” he says. “On
aircraft, you don’t make triangular wings with the leading edge sticking out,
like you see in a sail with the mast as its leading edge. It doesn’t work. So I
began to take shapes that I was more familiar with in aircraft and make them
into sails.”

As his windsurfing skills improved, so did his designs. He could soon make a
sail that would work efficiently at a given wind strength, learning how each
modification affected the sail’s performance. But then, of course, the wind
would change. Dryden began to ask himself the question that would eventually
lead to the transition rig. How could he make a sail that adapted spontaneously
to changing conditions?

In the late 1980s, he shifted his thoughts from aircraft wings to biological
ones. He remembers standing on a cliff top near his parents’ home in Devon
watching birds gliding on the rising air currents, noticing how they would keep
their wings close to their bodies during stronger gusts and stretch them out in
lighter winds. “I think that’s what I had in my mind when I thought about how I
could make a mast and a sail that changed shape,” says Dryden.

The mast of the transition rig is arranged like the bones in a wing
(see Diagram),
with two joints that allow it to fold into a Z shape. The middle
section has two parallel “bones”, just like a forearm, allowing the whole
structure to flex. The upper section is like a bat’s outstretched hand, with
“fingers” forming the leading edge and a series of three battens to support the
membranous fabric of the sail.

Joints might have been Dryden’s first preoccupation, but sailcloth has turned
out to be a bigger headache. To be compatible with the mast it had to be
stretchy—not a property that’s usually recommended for sails. “When I
mention that I’m making a sail from stretchy material, most sailmakers look at
me with a mixture of horror and sadness,” says Dryden.

Early on, he used Lycra and spandex but they soaked up too much water and let
too much of the wind through their loose weave. So he tried a knitted nylon
fabric coated on one side with polyurethane. It was better, but still a bit too
absorbent. He’s just tracked down a material that has a polyurethane coating on
both sides. That’s what we’re testing out on the Teign estuary. It seems to work
pretty well, stretchy enough to stay taut and waterproof as well. And it goes
like the proverbial bat.

The only downside is its colour—a sickly shade of blue. “I think it was
made for coating hospital inspection tables, which is why it’s this rather
unpleasant shade,” says Dryden. The colour isn’t just a matter of aesthetics:
it’s less than ideal for a windsurfer because sails need to be
see-through—or at least to have a clear panel—so you can see where
you’re going. “I just sail where there are not too many people at the moment,”
Dryden confesses. If anyone knows of a transparent, stretchy and waterproof
material, he would love to hear from them.

To set the sail up, you haul it upright with an elastic tensioner running
from the top of the mast, around the outer edge of the joints and down to the
base. This tensioner keeps the mast fully extended, but its elasticity also
allows the rig to compress down into a more compact Z shape. It even folds up
altogether, so when you’ve finished for the day you don’t have to remove the
sails or cart a full-length mast home.

But the main point of the transition rig is that it changes shape while
you’re on the water. You don’t have to swap your sail or reef it in when the
wind changes. When windsurfing, this shape change happens almost
automatically.

As the wind gets stronger, you intuitively lean back more to counteract the
pull on the sail, so you don’t fall in. Doing this creates a downward force
which compresses the sail slightly, reducing its height by 15 to 20 per cent and
giving it a more swept-back shape—a better shape for stronger winds. The
“centre of effort” of the sail—the point where its driving force seems to
be focused—is lowered, making the board easier to control. If the wind
slackens you lean back less and the sail opens out again.

On the dinghy, you have to control the extension with a rope attached to the
elastic tensioner. But it’s still a lot quicker than reefing a traditional sail,
and a good deal easier than swapping sails.

Dryden’s ideas put him at odds with conventional sail designers, at least of
boat sails. Their aim is to make rock-solid sails with as little give in them as
possible, and masts that are held rigid by stays and shrouds. That’s an
unbeatable strategy—if you don’t mind swapping sails every time conditions
change.

On a windsurfer, where the sailor plays the part of stays and rigging, a bit
of flexibility is normal. But exactly how flexible the sail should be is an open
question. “Anything too stiff and inflexible, while perhaps being more
aerodynamic on paper, will actually be considerably slower and more difficult to
use than something ‘softer’,” says Bill Dawes, editor of Boards
magazine. “Having said that, it can’t be too soft—a good sail needs to
transfer gusts into extra power and drive.”

So maybe the transition rig isn’t the best thing if you’re a competitive
windsurfer trying to be faster than everyone else. Dawes doesn’t think it will
ever replace the modern sails developed for stronger winds and more competent
sailors. These already spill a little of the power of a gust by twisting, so you
get a smooth increase in speed. But he thinks there might be a place for the
transition rig in lighter winds, or for beginners. “It’s an interesting idea,”
says Dawes. “If nothing else, the idea of a rig that is so portable and so
easily erected is wonderful.”

Dryden agrees he’s aiming at less experienced, recreational sailors who, he
says, have different needs from skilled windsurfers. He hopes novices will be
attracted by a forgiving rig that can be used comfortably in winds up to force
4. “You’re not going to be overpowered so quickly by a sudden gust,” says
Dryden. “The sail absorbs some of that energy.”

The wing shape is still evolving. Dryden wants to improve its speed, and for
this he’s planning a sail based on a swallow’s wing. It’s narrower and sleeker,
and he hopes it will make a faster rig.

As for designs for other vessels, Dryden has no shortage of ideas. “My
feeling is that this approach could be used on any sailing vessel,” he says.
He’s already looking at yachts, including sand yachts, ice yachts and land
yachts. Perhaps the first to go to market will be a small, simplified version of
the transition rig for canoes and kayaks. Dryden has already had a lot of
interest from canoeists who tour with a small foldaway sail.

At the other end of the scale, he’s even thought about oil tankers. A naval
architect colleague did a few calculations for him and concluded that five or
six transition rigs, each about 40 metres high, would deliver significant fuel
savings on big vessels. When not needed, or in stormy weather, they would just
fold away. And, just for a change, he’s designed a set of sails for a windmill
that are completely self-regulating and can automatically adapt to different
wind conditions.

But an interest in wings still occupies his thoughts. He often wonders
whether the behaviour of variable-geometry sails in unsteady wind conditions
could reveal more of the mystery of flight—especially of a flapping wing.
But he doesn’t think he’s ready to move over from sail to wing design quite yet.
“With sails, especially when you’re windsurfing, you can hold them, be part of
the structure and feel the shifting forces,” he says. “More importantly, it’s a
lot safer to fall off a windsurfing board than it is to jump off a church tower,
like people used to do to test out wings.”