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It’s all about wings (again…)

by John Curnow, Editor, Sail-World AUS 13 Jan 21:30 GMT
Inside the double luff Semi Rigid Wing © Advanced Wing Systems

In what may seem like back in the Jurassic age, well certainly in a sailing sense anyway, a certain, special, and never-before-seen keel was unleashed on the world. It had some wings on either side of a much reduced wetted surface area fin, and it could allow the craft above it to manoeuvre like no other 12m had ever done before. This of course, was super-important in match racing.

This time, the particular wings in question are not on the in water appendage(s), but rather, they're airborne, as the main propulsion unit. It all came about from the announcement by American Magic that they had chosen the award winning Advanced Wing Systems as a supplier to work with them on the design of their wing sail for their AC75. And yes, it is also the same system that is powering 'The Mule' so well right now.

In talking with brothers Patrick and Greg Johnston (the latter is the CEO), so much more came out about it all, like how to be an overnight sensation after 34 years at it. "The experience we have gained in soft wing design has flowed directly into the design of the wing sail on The Mule. The rig on the Mule is not the same as the as our Semi Rigid Wing, due to rule restrictions (OD rigs being just one of them). Whilst the designs for American Magic are covered by confidentiality agreements, the SRW is not, and it is available right now."

"The Semi Rigid Wing (SRW) system is a double surface (some refer to it as double luff) wing-sail that can offer great performance, and keep costs to reasonable levels. Its simplicity means that sail handling an operation are straight forward, as the system hoists, reefs, and stows like a normal sail."

So the main aspect that the SRW reduces is that the luff does not collapse once the boat is powered up, as these foiling apparent wind machines are so fond of doing. This in turn causes a resultant loss in power and the boat falls of its foils, much like when you over run your own kite in a planing monohull.

"The mast (which is sort of like a double D section) forms the leading edge of the sail, and parasitic drag from the mast is almost entirely eliminated. The unique, counter rotating design induces section shape and increases the rigidity of the wing. The mast is typically larger in section than a conventional rig, but it also means the SRW is a stiffer, lighter, and lower drag rig", said Johnston.

The parallels between Australia II's keel and this wing are meant, for without the huge gains and interest generated by the former, the latter may not be here now, and about to storm onto the highest stage in the game, the America's Cup.

Early on, the performance was certainly there to be used, but cost, difficulty of rigging, weight and other constraints outweighed them, at the time. Yet that blistering pace, and the ability to point so high as to get there on the one board, when others would take two digs to do the same, had not been lost on the brothers. In the intervening decades, costs of materials and advancements to them, along with better computing power, meant it was all on again.

A foiling International Moth was one early prototype for SRW, and the results proved that SRW could be built to a weight that is comparable to conventional sails. "We could not get a lot of attention with the Moth rig, but at the 2012 Australian Championships we did convince one sailor from the lower half of the fleet to try the rig in one race. His result from that race was 20 places better than any of his other results."

It also showed that the real potential would be with boats that had hoisting sails. A Classe Mini, and an 8m Sportboat have further ratified the point. It was happening, and the results spoke for themselves.

So there are five main operating controls and settings/adjustments:

  • Mainsheet
  • Outhaul
  • Traveller
  • Leech slip
  • Rotation

    The operation of an SRW is performed by doing thus: - The SRW consists of two sails (stiffened with battens) connected to either side of a rotating mast at the leading edge of the section, and to each other at the trailing edge. They are hoisted via two separate tracks with special slides.

    Thickness and asymmetry are induced by rotation of the mast relative to the boom. The mast is rotated away from the wind, so that the leeward side of the mast moves aft and the windward side of the mast moves forward, tightening the battens and therefore membrane. This places the battens on the leeward side of the foil in compression and the battens on the windward side in tension, thereby creating the classic, flat one side, curved on the other, aerofoil section that we are so used to seeing on planes.

    The compressed side of the foil cambers, with the amount of camber determined by the angle of mast rotation. The direction of asymmetry is determined by the direction of mast rotation. Figure 'a' shows the wing-sail with no mast rotation set. Figure 'b' shows the effect of 10 degrees of rotation.

    Additional camber can be introduced to the foil by reducing outhaul. This allows the windward batten to camber under the wind load, thus increasing the camber of both the windward and leeward battens. By control of the mast rotation and the outhaul tension a wide variety of section shapes can be produced. Figure c shows the effect of releasing the outhaul.

    To further control shape, the joint between the battens at the leech can be controlled to allow the leeward batten to push (or slip) past the windward batten. This aspect of the design was introduced to compensate for sail twist, which has the same effect as increasing mast rotation, since the batten pairs rotate around the mast. This control allows the upper sections to be substantially depowered by both twisting and flattening. Figure d shows the effect of 10 degrees of twist. Figure e shows the effect of allowing the leeches to slip past one another reducing the thickness and camber of the section.

    Through these mechanisms a wide variety of section shapes can be achieved. The section shape can be varied through the span of the wing to compensate for twist and to depower the top of the wing. This image shows an example of how the shape can be varied through the span.

    We'll talk about how to use the controls to achieve the results in another piece, but for now, if you're one of the souls that has been captivated by The Mule (and we are not talking about Clint Eastwood's latest film of the same name that does look truly brilliant), then the above may go some way to explaining just how it has all come to pass....

    Before leaving and speaking of foiling, and as SailGP's inaugural event is now imminent, Australia Team's Skipper, Tom Slingsby, said to me during the week, "The very first SailGP event is coming up quickly, and we at Australia SailGP Team can't wait. Hopefully we will have some big crowds out there to support us on Sydney Harbour. Grab your tickets now at sailgp.com"

    Right oh here today there are some gems for you to review. We have information about Youth Sailing, Finns, Tornados, Five-Ohs, IMOCA/The Ocean Race (yes, training myself to say the new name), Moths, skiffs, Lasers, Dragons, Knut Frostad joins Navico, Bella Mente 72 Maxi, Golden Globe Race, the Fastnet gets inundated, the Clipper, and certainly there is much, much more.

    Remember, if your class or association is generating material, make sure we help you spread your word, and you can do that by emailing us. Should you have been forwarded this email by a friend, and want to get your very own copy in your inbox moving forward, then simply follow the instructions on our newsletter page, where you can also register for different editions.

    Finally, keep a weather eye on Sail-World. We are here to bring you the whole story from all over the world...

    John Curnow
    Editor, Sail-World AUS

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