Steve Clark gives an expert view on BOR90's America's Cup wingmast

by Richard Gladwell on 10 Nov 2009
Steve Clark is widely acknowledged as one of the most astute and most experienced inshore multihull technologists and sailors on the planet. His is a family pedigree with his father Van Alan Clark being heavily involved C-class catamaran development in the early days of the Little America's Cup.

From that upbringing, Steve has continued and led the team that developed the C-Class challenger Cogito, which broke the Australian domination of the Little America's Cup in 1996 at Macrae, in Australia, breaking the dominance of the Lindsay Cunningham designed succession of C-Class Yellow Pages.

For reasons best known to others, Steve has not been involved in the multihull America's Cup. His co-designer and helmsman in 1996, Duncan Maclane works as part of the Alinghi design team.

With the release of the first images of the wing mast for BOR90, Sail-World asked Steve Clark for his analysis of the largest wing mast ever built.

(As we noted yesterday, the BOR90 is a two element wing mast which essentially just has a tail section rotating off a front section, a little like an aeroplane rudder. The more sophisticated three element wings, used on Cogito, comprise a smaller third section which sits between the other two for greater efficiency and control. Wing sails are two to three times more efficient than a conventional spar and soft sail rig).

Steve Clark:' The rig on BOR90 is a two element single slotted wing. This is a 'cleaner' configuration instead of a 'more powerful' configuration. You will not get the high lift coefficients without the #2 element ( the flap in the slot that directs the flow through the slot.) This is probably not a big deal because they have the ability to set head sails for down wind work.

'It may or may not have twist capabilities. For one thing, the use of head sails makes this less necessary. For another, the majority of the wind gradient is in the first 30' off the deck, so given the scale of this wing, it may not be as big a deal as it is for the smaller C Class wings.

(Sail-World: One of the features of the Cogito three element wing mast was that it gave the crew the ability to adjust the twist in the wing, meaning that leech tension was a thing of the past, as the sail 'leech' was completely controllable.)

'The control system seems to be the tried and true Patient Lady (the predecessor to Cogito) system, which is self tacking and very well worked out.

'This system scales well and can accommodate as many control arms as needed.

'Stretch in the control cables is a big deal, but given the super low stretch lines available it is less of a problem than it was even 10 years ago. The exact function of the short control arms is not entirely clear. It may be necessary to reduce the stress on the longer ones and thus reduce the stretch on the control cables.

'Or it may be that given the need to set head sails, the lower arms have to be shorter than the upper ones to facilitate sheeting head sails to the proper angles. These are both pretty good reasons, and I doubt there is much more in it than that.

'The ability of the wing to operate at low angles of attack will be a big step forward for these boats which operate with so much apparent wind. I think it's going to help. If it blows hard enough for them to fly the main hull without the a jib being set, they will be faster upwind without the jib. This was true of Stars and Stripes 88 and should be true here given that this is a very clean wing. Stated another way, this rig will be able to sail longer without being backwinded by the jib. If that becomes a problem, the boat will be faster without the jib.

'If this were a C Class wing, I would have believe it was part of a super light program in order to be frightened by it, but that is completely driven by the sail area limitations, which, once again, don't apply here.

'I expect the raising and lowering procedure will be very much like what we do with the C Cats except it will be rotated 90 degrees. The wing will approach the vessel lying flat on its side, with the leading edge 90 degrees to centerline. The leeward ( down side ) shroud will be connected, the fore stay will be connected and the running back stays will be connected. I expect the leeward shroud will be positioned such that the hydraulic cylinder is completely drawn.

'There will be a pivot at the base of the mast such that as the mast is raised, the ball will mate with the socket.

'There is a gin pole to which the weather shroud will be attached. They will then winch the wing up. The vessel may need to be allowed to swing such that it is close to head the wind, but not allowed to tack. As the wing becomes upright, one may expect the leeward hydraulics to be eased and the running backs to be ease such that the wing can feather as soon as possible. A temporary shroud, or halyard will be used to enable the lifting shroud to be disconnected from the gin pole and attached to the hydraulic ram. The rig will then be stable and can be tensioned as per normal.

'At this point it is probably imperative that the vessel get underway as soon as possible. So I would expect the entire sailing crew to be on board and ready to go before the wing starts to rise. If flow attaches to the wing when they aren't ready for it they will have a large problem on their hands.

[Sorry, this content could not be displayed]'Once again this is just speculation based on what little I have seen on the Internet. There are people who know much more about the specifics of this particular rig than I do.'

The new wingsail on BOR90 is expected to be sailing off San Diego in two or three days, and will obviously be watched with great interest.

For some some other thoughts on the C-Class and wingsail development http://www.stanschreyer.com/?p=62!click_here

For an earlier analysis fro Steve Clark of the respective America's cup multihull campaigns http://www.sail-world.com/NZ/Americas-Cup:-Steve-Clark-on-Bows,-Stress-and-Weight/58754!click_here