America's Cup: AC75 - Race Mechanics and the Foiling Dance
by Richard Gladwell, Sail-World NZ 27 Feb 2019 02:56 PST
Emirates Team New Zealand testing a soft wing concept sail in Auckland, New Zealand, February 2018 © Hamish Hooper
Part 2 of the interview with Burns Fallow, a North Sails designer and long time member of Emirates Team New Zealand, on the development of the AC75 class rule and how the various components of the AC75 are expected to interact. For part 1 of the interview click here
Forward of the wing spar, the situation is more familiar and closely resembles the situation that prevailed with the AC72, in that open design jibs are permitted (within some measurement restrictions). The open design Code Zero also makes a return.
“As with the mainsail, the jibs are open design – there’s a maximum LP dimension (forward face of the wingspar to the point where the forestay intersects the foredeck at the bow) with a series of girths which are related to the leech length of the sail.
“So if you have a full hoist jib it can have a little bit of roach on it. But if the head comes down the forestay and the leech gets shorter, then the girths including the head are allowed to get wider,” says Fallow - explaining how the rule works to allow teams to develop different size jibs within the allowed foretriangle.
“We can have square top jibs that are not full hoist”, he continues. “Steve Collie [Aero Coordinator for the team] came up with an elegant little formula in the rule which figures out a nice envelope of sails that you can have.”
“You are only allowed one jib on board the boat at a time - there's no sail changing during a race. The nature of the race is now that if you commit to a sail, it is there for the race. Jibs can have up to eight battens (including flutter battens), and they can be full length. I thought the AC72's had nice restrictions on the jibs, so the rule we have written, while it is different, encourages that type of headsail.”
The return of Code Zero gennakers set off a bowsprit will evoke a mixed reaction from Kiwi America’s Cup fans in particular.
Memories are still fresh of the 2013 America’s Cup where Oracle Team USA had very accurately worked out the crossover point at 10-11kts of windspeed when the Code Zeroes were ineffective – another heart-breaker costing the Kiwis a vital race and the Cup.
“The rule has a girth restriction and LP restriction. The largest that you can put on is around 220sq metres which is quite a flat gennaker compared to a TP52.”
“The Code Zeroes will be used in a similar envelop to the AC72's - so if you are not foiling they will be the downwind sail. If you are foiling, then you are probably going to be better off with a jib.” [As with the AC72.]
“There is no specification on handling - you can roller furl or free hoist. That decision comes down to the weight you spend and what we call racing mechanics - which is how you manage getting around the course with these sails. And there are no battens permitted in a Code Zero. Only one Code Zero is permitted on the boat at any one time.”
Ballasting for the Code Zero
That raises the question as to whether Code Zeroes would be on the raceboat at all in wind speeds in 15kts.
"I would say it was highly unlikely," is Fallow’s response.
"The rule makes you put the equivalent ballast weight onboard - any difference between the actual and assumed weight of a sail carried as ballast in a prescribed position. The prescribed weight for a Code Zero is 90kg, and if you don't have one on board, then you have to ballast up the boat.”
As with the AC72, the vexed issue with the Code Zero is that in displacement mode the Extra is required in marginal conditions to get the AC75 up to foiling speed, but once the AC75 is up and foiling the sail is an impediment as the apparent wind comes forward very quickly – and even when furled the drag is significant.
“If you happen to be in puffy conditions it will be interesting viewing to see how boats manage racing around the course as versus keeping on the foils."
“It's all part of the foiling dance. A different way of racing. They will be very tricky sailing in more wind - all boats are, of course, and they will be tricky in the cross-over between foiling and non-foiling.”
Burns Fallow raises an important issue inherent in the AC75 design concept with its reduced stability at low speeds - when there is no or little lift being generated by the leeward foil. In this regard, they are very like a 75ft skiff than a 75ft keelboat.
“When you are in displacement mode because the boats don't have a fixed keel, they are incredibly tender,” he explains. “We don't really need a lot of heeling moment, and therefore you can get away with a short rig. The rig height for the length of the AC75 is very short. “
“Then when you pop out on the foils, the apparent wind speed rockets up, and therefore you don't need a very big rig. It is a happy coincidence that in both modes - displacement and foiling - that we only require a short rig.”
Balancing the power and weight
Fallow says the key difference between the catamarans and the AC75, lies in the height and weight of the rigs.
“To be honest in the cats, the rigs were far too big for when boats are foiling,” he explains. “The AC72 needed a big wing and a lot of power just for the part of the performance envelope to go from displacement with both hulls in the water to flying a hull and then foiling.
“Once the AC72 was foiling the top quarter of the wingsail was unnecessary, and they would probably have been faster with it chopped off.”
"The catamarans do have a lot of power in displacement mode, the AC75 does not," he adds, underlining the instability issue with the AC75 when at slow wind speeds.
The transition from displacement sailing to foiling can only be modelled at this stage. However it is likely to be rapid – taking place within a few seconds, and requiring a careful response from her crew, during the rapid speed build and as the apparent wind increases and shifts forward.
“Upwind the AC75 will need to build speed by coming off its optimal VMG upwind course for some time, and may have to sail a reaching course to some degree.”
“Eventually speed will build, and from a rig point of view while reaching sheets will be eased a little, and come back on as the boat pops up. At that point, the boat will have a lot of righting moment as well.”
“It’s not just apparent windspeed that builds - but the righting moment as well. So you'll see a big trim on as the boat pops out of the water and the boat will come back up to proper VMG sailing angles up and downwind.”
Big decisions downwind
That’s the scenario upwind.
Downwind with the Code Zero set, the physics and race mechanics become rather more complicated.
“You would have a bit on,” Fallow chuckles. “The Code Zero would be sub-optimal, as you have suddenly doubled or tripled your apparent wind speed.
“You've got a big sail on when you really want a small one, and you have to deal with it. As soon as you start foiling you will not be as fast with a Code Zero as you would with a jib.” [As occurred in the AC72’s.]
“The counter to that is if you don't have a Code Zero on and you're in sub-foiling mode, then how long is it going to take you to pop out onto foils when you just under jib alone?”
The obvious answer is that it is going to be longer, and at 30kts of boatspeed that could translate into quite a jump on your jib-only rival AC75.
“It's a function of what you think is going to happen in a race as to what sails you are going to pick,” he explains. “It's going to be tough - and with some hard decisions to be made. That's what these sailors get paid to do.”
“If you're trying to sail upwind with a Code Zero hoisted and furled, ready to go - that is probably going to be a very draggy item.
“As soon as you start foiling and your apparent wind gets higher, you become much more aware of drag than you were ten years ago sailing IACC keelboats.
“Drag becomes a very important consideration when you have 45-50kts of apparent wind coming across the deck,” he notes.
He expects the AC75 to be a lot better proposition than the AC50 sailing downwind in lighter airs in displacement mode.
“It shouldn't be quite as bad as a couple of races in Bermuda when the AC50's were gybing through 220degrees -but you will see some of that trying to build speed out of a gybe, for sure.”
“But hopefully the America's Cup won't come to trying to race downwind in 5kts of breeze,” he adds.
“I don't think these boats will be as stuck in the water as the AC50's because of the one hull of the AC75 versus two hulls of the AC50. But there will be a stark contrast, as has happened in the past between being stuck in the water in displacement mode and being on the foils.”
When does a skin become a sail?
In the AC75 Class Rule, there is a new differentiation between what is a sail and a skin – which is a terminology new to most sailing fans.
In the AC75 Class Rule, “Sails” are not a specifically defined term, and so the common English usage of a “sail” is used. A “Sail Skin” is “a thin and predominantly flexible membrane of a headsail [including Code Zero] or mainsail.”
In other words, the rule intends that a mainsail consists of two “skins” or sailcloth. A jib consists of a single “skin”.
The skins are mandated to be flexible and foldable without being damaged, which is an old measurement term to ensure that they are a sail. There allowances for certain distances from the corners to be stiff, to allow for reinforcing. Fallow expects North Sails 3Di to be used for the mains and jibs and probably the Code Zeroes.
“The skins are still sails,” he explains. “We are using the same structural modelling program we use for sails, but it has had to be highly adapted to allow some of the interactions between the skins and the mast that we didn't previously have.”
He says that one of the issues which had to be worked through very early with the AC75 concepts – was not just the concept itself, but how it was going to be modelled and verified.
One of those concepts was whether the skins in the mainsail would share the total load between them, or if the combination of the skins would take the load – and if so in what proportions.
In other words "does the sail have to be the same weight as if it was a single skin? Meaning that with two skins – had we doubled the sail weight, or would the load be shared between the two skins?”
“The answer could have been either,” Fallow explains, “and it was not until we had the North Sails Membrain program working for the AC75, that we had the answer in terms of adopting this idea. That only happened in December 2017, and by then we had already committed to doing a practical test in the Farrier trimaran.”
“We found that, yes indeed the load is shared between the two skins – but it is not quite 50/50. The skin on the leeward side will see more load than the one on the windward side. It means that each sail can be around 65% of the weight of a single skinned mainsail.”
“In total, the two mainsail skins will weight more, but it is on a shorter rig, giving potentially 20% more lift than a conventional rig, and definitely with lower drag. Overall the combined Mast and sail weight is similar to a conventional rig generating the same heel moments.”
“Those were the sort of parameters we needed to know before answering if we could say "this is the system we want to adopt” We went ahead after building two little test sails in the shed and then tested a double skinned mainsail on the Farrier 22 ft trimaran.”
“It was quite a bold move, but we had confidence in the modelling we had at the time. Thankfully it has proven correct with all the additional stuff we have learned in the last year which still holds true.”