Maximum thrust

Solid wing rigs can develop three times the power of a conventional spar/softsail. Steve Clark describes the example on Cogito

This article follows on from part one of our interview with Steve Clark about his Little America's Cup winning C-Class catamaran Cogito . To see a lot more images of Cogito 's rig see Christian Fevrier photo gallery published in conjunction with the BlueGreen Picture Agency .


C-Class catamarans have narrow, lightweight hulls, but the single greatest development in their technology has been in their rigs. There is no question that for the Cogito brains trust the complex technical challenge of conceiving, building and developing their catamaran's triple element solid sail rig was the most challenging part of their project.

"From a technical standpoint, the wing has no sail-making voodoo," he says. "You're actually designing what you want. With the wing I know exactly what shape it is, because it is the shape I design. I know exactly how much it twists because I have got a string that I can ease that allows it to twist. I have a string that allows me to play the variable flap angle and I can put in the camber that I want. So I am sitting there working with the shapes that I actually designed to have happen - it is real aerodynmics rather than voodoo aerodynamics. And the plan form isn’t pathetic... Would you fly across to the US in an aeroplane that had a America's Cup Class rig sticking out either side of it with a bunch of AC guys cranking winches in the middle of it?"

But the main issue is simply that compared to conventional masts and sails they are a legion more efficient. Clark compares lift co-efficients between a conventional softsail rig and a solid sail one: "It is about twice as good as a sail and you can get better than that depending upon what you play with for high lift devices. A soft sail never really gets up to a lift co-efficient of 1 and these are all operating at more than 2 or 3. Lindsay’s [Cunningham] double slotted wing with the flaps all over it was allegedly delivering a lift co-efficient of 3.1 - so lots of growl. The problem with that is that as the lift goes up with increased elements, so does the drag. So the lift to drag curve ends up flatter and you have to work out how to manage that.

"The second place where a wing defeats the sail, is where the angle of attack gets small on a sail it starts losing its shape and the lift to drag ratio falls apart there because it gains section inefficiency. Basically when the sail starts backwinding and flagging it isn’t an aerofoil anymore. Whereas the wing will continue to develop lift as it approaches 0.5 deg. As long as it has some positive angle of attack on it, it will continue to develop lift, so upwind it is closer winded and more efficient and will remain more efficient until the parasitic drag of just the air rushing by starts to be enough to actually slow you down."

Ironically the original reasons for using wingmasts in the C-Class were more structural than aerodynamic. "People went to wings in C-Class catamarans because they couldn’t pull hard enough on the mainsheet to control the leech of the mainsail," says Clark. "In the late 1960s and early 1970s, the Lady Helmsman-style boats had huge main beams made with 1/4in wall of aluminium that they were breaking with compression loads! Part of it was because the mast weighed a lot and because it had a semi-circular traveller on the back which was an aluminium I-beam, tagged on to a big back beam and a 64:1 wire and rope strop on it. And the cars were made out out of steel were all custom-made maxi boat stuff.

"There was a picture of my father sailing where you could look at the boat on a broad reach with the main sheet loaded up and the traveller is bending its height in between the two supports! And I remember sailing on those boats which were fairly heavy so they didn’t accelerate away, thinking 'if something breaks here I am just going to die!' I suspect the ORMA 60ft trimarans and Formula 40s which are relatively heavy catamarans you still have this thing that when a puff hits, the thing is just winding up and it is like bone and tendons stretching..."

In contrast Clark says current C-Class and A-Class catamarans are not like that as they are substantially lighter, made from modern materials and the rig loads are resolved. "You don’t get that sense of loading up and I’m about to die here. They are very different animals."

Designing and engineering a state of the art C-Class rig is no mean feat. Adjusting its overall angle of attack and camber between its three elements is just the first problem. Being able to adjust the camber in such a way that the wing twists is much harder and then there is the issue of how the crew can control all of this.

Clark describes the wing rig controls you have to play with on Cogito: "There is a mainsheet which is run to both sides. There are two mainsheets, but basically it is like a traveller control. There is a camber control, which controls the flap angle and then there are essentially two twist controls. There is a rotator control that looks like the standard A-frame on the back of a catamaran mast and the distance from the boom to that rotator is the amount of camber you get, like the over-rotation control."

"The sheet is on the back and as you tack the aerodynamic load makes it want to fold in half. Coming off that boom is a spreader that matches a spreader at the top. Control lines come into the wing criss-cross and come out the back, so that little control arm does what the boom does - it pulls the top of the flap so that as the bottom of the wing folds in half it pulls the top of the wing. So tacking it is like tacking a Laser. You just put the helm down and the wing goes across and the settings are repeatable from one side to the next."

"If you want to have less flap angle aloft than you have down low because you are overpowered and you want to twist and essentially have it flatter up high, there’s the ability to have the upper control arm lag behind the bottom control arm by having a quadrant on top of the boom that you can ease a little bit. So that allows the flap to wash out because it will be eased relative to the bottom of the sail." This control system was devised by Duncan MacLane and Dave Hubbard almost 20 years prior to Cogito on Patient Lady III.

However this is only half the story when it comes to the twist that can be induced in Cogito's wing. "The beauty part is when you want to twist the leading edge," continues Clark. "And the reason you want to do that is the boundary layer - the air at the water level is going a lot slower than it is up high which means your apparent wind down low is much further forward than it is up high. So that happens most pronounced in the first 25ft."

The control technique for this is similar to the trailing edge flap. In the front element of the wing there is a carbon fibre tube (the mast section of the Arab racing dhow...) on to which the cowling for the element is tapped. There are also two control arms inside the element. If the lower control arm is centred then the top of the element does what the bottom does, but if it is eased the top of the element will twist relative to the bottom.

Thus it is possible to induce 15deg of twist between the top and bottom of the trailing edge flap AND a similar amount in the leading element.

Check out that twist!



Clark says the Cogito wing offers a marked improvement over its predecessors. "You used to say twist was just washing out the flap angle, so you’d exchange lift co-efficient for flap angle to maintain attached flow. What are able to do now is to keep flap angle and rotate the top of the wing out so that it is all working at the high lift co-efficient, high flap angle."

The leading edge twist mechanism, he admits, is a reinterpretation of an idea he saw on an International Canoe rig developed by Sven Alenius. "Sven had a double luff canoe sail with a cowling around a symmetric mast and was saying the nice thing about this is that the leading edge twists. So when Duncan was talking about making the leading edge twist I said ‘I know how to do that’.

The sections of Cogito's rig were direct copies of the sections developed by John Ronson and Bert Rutan for Dennis Conner's solid sail Stars & Stripes catamaran that won the America's Cup in 1988.

Over previous incarnations of this wing Clark says they spent a lot of time improving the rear flap. "The most important part of any wing is the first third of it. On the previous Patient Lady wings the nose of the number three element was kind of nasty and we went to some effort to make that better on this one."

During the construction of the wing, engineering and build went hand in hand. They built the main spar inside the forward element and placed loads on it. "We bent it with a bunch of weights and recorded that, and that told us what the main wing was going to bend like. Then in order to have it remain constant we had to predict what the aerodynamic loads on the rest of it were and basically to model what the sag of the trailing edge was going to be because that wanted to be the same and then model the sag of what the number 3 I beam was going to be. And you do that by building a bunch of samples and sticking weights on them – they’re 3ft long samples and you put weights on them and you measure them and you calculate and you come up with what the number has to be. So it takes a little bit of time." All along the various parts were weighed to ensure that Clark's demand of losing 25% of the weight of the boat compared to Patient Lady VI was met.

Hanging on the wall - the number two and three elements of Cogito 's wing. The transparent material is a lightweight polythene



Clark reckons that the design, engineering and build time took two and a half people the best part of a year.

While there are many benefits of solid wing rigs, there are also a number of disadvantages. A fundamental problem is what happens in gusts or if you're caught out in strong winds? "You can basically go upwind in almost anything," says Clark. "The view was that if we saw something big and black and we couldn’t get back to the beach and get the wing down in time, we wanted to be as far downwind as we could possibly be when it started so that we could sail upwind through it.

"Basically we felt we could pull the camber out of the wing and stick the thing on the wind and drive it upwind. We could basically sail upwind in anything. The bad news was going to be if you ever had to go back down at which point you do start to trip over the bow. We probably saw at least one terror field delivery to Newport from Bristol, when we saw some 30 knot puffs on a beam reach and we thought we were dead. We had the wing completely flat against the shrouds and we couldn’t bear away without stuffing the bows."

Of relevance directly to their use on C-Class catamarans is that Clark feels the lightweight catamaran platform is is possibly the least idea for such a rig. "A trimaran would be better because you could free stand it," he says. "The shrouds are just a disaster from a wing stand point, because you can't get the thing to go 360degrees. So there are no-go zones where you are in trouble.

"For example pulling the thing up a ramp is like having the vang on real hard on a dinghy and the ramp is downwind, and you bear away down the ramp and sail loads up and boat flops off the dolly. In this case, you’ve got something which weighs 375lb and is 40ft tall so that when that loads up it wants to fly away - right now! It doesn’t wait around for you! Whereas if you could let the wing go past 90degrees or past the shrouds you could let the wing feather and the wing feathered has got less drag than a bare spar...

"The second thing which is bad about it is that it is a multihull which can be tipped over and wings don't like to go in the water at all."

Interesting Clark says that an ideal boat to fix a solid wing sail would be Bruce Schwab's narrow Around Alone Open 60 Ocean Planet. "A narrow monohull would be perfect: Really clean going upwind, reduce drag and the ability to dump a lot of lift co-efficeint. From a performance stand point they are the best sailing rig you’ll ever see. And they are so much better than spars and sails that it is a joke!"

Just how many years ahead of its time this technology is remains to be seen.