The America's Cup from the armchair...
by John Curnow, Editor, Sail-World AUS 14 Feb 2021 21:00 UTC
PRADA Cup Final day 2 - Airtime for Britannia in the race 4 pre-start © COR36 / Studio Borlenghi
Which is not necessarily a favourite place of mine from which to review things. I have kind of always been a 'if you want to know what the weather's doing, stick your head out the companionway hatch' kind of guy. So like many, early on I thought the 36th America's Cup may have been a bit of a fizzer, but the deeper we got into the event, the better it all seemed, and the more you got into it.
So the current talk is of the AC75s closing in on 55 knots, which in foil tech terms is a bit like hitting the speed of sound (Mach 1). Paul Larsen on Sail Rocket 2 showed us that you need to be on super cavitating foils to break through this barrier caused by the vaporisation, and it would seem the current set up is way too fat for that.
Now it seemed like a really good time to investigate the engines of the AC75 - their sails. Many had thought that single element soft wings would not be able to perform to the same level as their double element hard wing cousins. Alas, this has not proven to be the case at all. We could finish there, but it is exactly at that point at which the really good, if somewhat technical story of this particular need for speed gets really juicy. A lot of reading eschewed, and I am so deeply indebted to the friends who helped me understand the material in question. This one is dedicated to you.
OK. Strap in. Get set.
It is true that the single element, soft wings do not have the same maximum lift as the double element hard wings, and this makes getting onto the foils more challenging. However, once there, and as soon as the apparent wind moves forward, the single element wing is capable of better lift to drag ratios than the higher drag double element configuration.
The soft wing sails are also performing at a level that has not been achieved by conventional sails, and whilst they might look similar, the differences could not be more distinct and marked. A conventional mainsail has a region of flow behind the mast that is disturbed by the very mast to which it is attached. To get the best performance from the mainsail, the headsail is used to slow the air through the slot to encourage the air to re-attach to the leeward side of the sail.
Avel Gentry's paper from 1981, A Review of Modern Sail Theory, explains this interaction very well. He explains how the slot between the jib and the main assists the mainsail, and why we see the setup below in modern sailing boats, where the front of the mainsail is inverted.
Whilst all of the same principles apply to a wing sail, there are some important differences in the characteristics thereafter. Notably, the smoother and more aerodynamic leading edge of the wing sail is much less likely to have separated flow. The flow around this part of the mast is very fast, but because of the wing shape it will conform smoothly.
Consequently, there is much less need to slow the flow down in the slot to encourage re-attachment. This means that the headsail flow can exit into much faster air and, as Gentry explains, this improves the performance of the jib.
Flow. Speed and VMG
Next, Gentry also explains the roll of upwash and downwash in the headsail / mainsail interaction. Gentry states: "The upwash flow ahead of the mainsail causes the stagnation point on the jib to be shifted around toward the windward side of the sail, and the boat can be pointed closer to the wind without the jib stalling or luffing."
The implications for this on an AC75 wing are profound. By opening the slot between the wing and the jib, the air is faster over the entire system. This creates an upwash effect into which the jib can be sheeted, adding to height. So, counter-intuitively, a wider slot ends up giving better height and more speed. Accordingly, this is translated into much better VMG.
If you're a petrol head like me, think of it as extractors on a four-stroke motor, or an expansion pipe on a two-stroke. Remember, a wing is not a sail, and the mast is no longer integral to the situation, where it creates a lot of suction as it curves around. There is a reason the leading edge of a keel is fatter than the trailing, after all...
So there has been a tendency to fit flatter headsails in an attempt to gain height, which can lead to a loop that results in a sub-optimal set up with cataclysmic results. The headsail has to be sheeted in closer, slowing the air onto the wing and creating adverse pressure. Consequently, the wing has to be set flatter to prevent backwinding. So now the flow over both sails is reduced, and there is less upwash. The outcome is less power, and less height.
Is the quickest way between two points a straight line?
This image of Luna Rossa Prada Pirelli and American Magic Patriot shows a clear difference in jib cambers at the foot. Luna Rossa consistently had better upwind VMG than Patriot in the Prada Cup Semi Finals.
Here is the catch. If the cambers in the lower part of the wing are reduced, then to get the same power overall, the cambers in the upper part of the rig have to be increased. This results in the centre of effort being significantly further up the rig, and the one word to be very mindful of here is leverage.
Also, if the bottom of the wing inverts, as shown in the image of American Magic, the COE goes even higher, and worse still, it is moving around. That creates tremendous control issues, as you have significant changes to the moments acting on the boat, which must be countered by changes in the underwater foil set-up. Big corrections to the set-up of the underwater foils means drag. Importantly, any reduction in drag created by having a thinner and flatter section in the bottom of the wing are very quickly negated, especially in shifty conditions.
What we're really saying is that it is symbiotic and holistic. INEOS improved out of sight early after some changes, so who knows where it's all going. What we can say from the latest racing is that both INEOS and Prada had flatter set-ups in smooth conditions. Generally, Prada have got things set up better for both VMG and uphill. Downhill things are pretty much on a level playing field.
Ultimately, it is all about control. More control = More stability. If the COE does not change, then it also helps with the entire hydro package. A massive drag saving below the water is a huge pay off for a slightly higher drag upstairs.
Sharing the love...
The Etchells had firstly their unanimously praised Summer Championship (Eaton Cup), and then more recently the intergalactic Milson Silver Goblets. Both of these were staged out of the awesomely picturesque Royal Sydney Yacht Squadron. Incredibly close racing was the cornerstone for each event, and no doubt the major contributor to the amount of smiles, and the vigour with which they were displayed.
Two-time Etchells World Champion (along with nine Australian titles), John Bertrand, was one of those that took part in 'The Goblets' as they are affectionately known, finishing in second place by just the one point. The Great Man said, "There were 30 boats out there, with just six coming from Mould 11, and I cannot see any difference."
"Most of the fleet have updated to the new rig, sails, and inboard sheeting. You could tie a string around all of us, no matter where you are in the fleet." Bertrand reprised his crew from last year's nationals, with Olympian Jake Lilley on the bow, and living legend Noel Drennan on mainsheet. The Great Man's enthusiasm for racing is totally infectious; "This is the best racing in the country right now, with members of the Australian Sailing Team dotted throughout the fleet. It's a really high standard." Indeed the Medal Maker, Victor Kovalenko, concurs, commenting about how good it is for training as it is so tactical, and the quality of racing is exactly what they need in the lead up to Tokyo, with travel heavily restricted right now.
Youth and smiles were a focus on the 8m Defiance for the Australia Day Regatta, as is evidenced by Jed Cruickshank here. Aiden Mansley is right there with him, and together with Charlie Alexander, they also competed in that Etchells Summer Regatta and Goblets. Other youth members of the Defiance crew were Kyra Early, Emily Keg, Zac Quinlan, and Zac West. Notable 'oldies' included Nev Wittey and Nicholas Ingate. The metre boats also recently had a rally on Sydney Harbour, which is where this image comes from.
So whilst some may not be able to sail at present, everyone here in Australia wants you to know that it will all come back, and hopefully some of the joy felt at events like the ones mentioned here can climb over the equator and every other boundary to give a little ray of light to all.
Right oh - there is plenty of information on the group's sites for you to review when you can. Please avail yourself of it.
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John Curnow
Editor, Sail-World AUS