America's Cup: AC75 - there's more to the rig than meets the eye

British Challenger for the America's Cup, INEOS Team UK was the first team to launch a first test boat in the build up to AC36. Known as T5 it is a 28-foot foiling monohull - and was first proof that the foiling monohull was a viable concept. © Harry KH / INEOS Team UK British Challenger for the America's Cup, INEOS Team UK was the first team to launch a first test boat in the build up to AC36. Known as T5 it is a 28-foot foiling monohull - and was first proof that the foiling monohull was a viable concept. © Harry KH / INEOS Team UK

by Richard Gladwell 21 Feb 2019 22:19 EST

Emirates Team New Zealand sail designer, Burns Fallow says adapting the two skinned mainsail concept to the new class rule for the AC75 foiling monohull came during a 20-minute brainstorming session held a month after the America's Cup win in Bermuda.

“The general concept of double skinned mainsails in the AC-75 style was invented there,” he claims. “Hopefully it will prove to be practical and fit for purpose. I would love to see it trickle down, into uses outside the America’s Cup, because it was an exciting thing to be part of after being in the industry for 30 years. But I don't think the take up outside of the AC75 will be immediate.”

A long time designer with North Sails, Fallow is back with the America’s Cup champions, after it was decided that the AC50’s used in the 2017 America’s Cup would have one design rigs, meaning sail designers were excused.

Stepping back into the team after a four-year absence has been something of a revelation.

“One of the things that was very apparent when I came back into the team, having not been in the last one [2017 America's Cup] was how much trust there is in the modelling. That was a huge advance from 2013 to 2017.

“That trust was right from the time the first concept for the AC75 was floated.

“There is a huge confidence in the level of the modelling. I can only assume that when they were in Bermuda, the performance parameters that the modelling was suggesting must have been very close to reality.”

"Also it's been impressive to see how these guys minds work. "It is never good enough, and let's just think harder about how we could solve this problem," says a seriously impressed Fallow who is now on his sixth America’s Cup campaign which includes two victories in the period 20 year period to 2013.

He has seen the America’s Cup Class progress from a conventional monohull rig last used in the 2007 America's Cup in Valencia. He headed the Wing Aero development team for the AC72 foiling catamarans in the 2013 America’s Cup in San Francisco and watched with interest the performance of the one-design hard wingsail and supplied one design jibs used for the AC50’s in Bermuda.

For 2021 there is the two-pronged challenge of developing the AC75 monohull foiling concept along with the two-skinned soft mainsail, writing a Class Rule – and then designing a race boat for the Defence.

“The last few America’s Cup cycles have used a hard wing sail which has been very impressive in terms of its performance. But it had too many drawbacks - the most obvious being the handling side. Every day - particularly in 2013 with the AC72 at least 30 people were required to get the wing into the boat.”.

“In Bermuda with the AC50 they needed a few less, but it was still a personnel intensive job, and not practical on any size of race boat outside of the America's Cup.”

Rigs half the weight of AC75 wingsail

“The other big drawback with the hard wings was the weight. They were heavy. If you tried to build a light displacement boat, then every kilogram that you add anywhere to the boat make it harder to get foiling.”

“So the soft sail rig was an endeavour to try and save some weight as well.”

“The two starting points were that we wanted something more practical, and lighter than a wing would be, but trying to pick up as many of the aerodynamic features of the hard wing sail as we could.”

One of the surprising aspects of the AC75 compared to its multihull sister, the AC72, is that the rig above the deck on the foiling monohull is about half the weight of that on the 72ft foiling catamaran.

“The mast on the AC75 is a lot shorter than the AC72, but generally we are around half the weight of what the AC72 wingsail was - including mast sail and rigging,” he says.

Of course, the double skinned mainsail doesn’t generate the power of the hard wingsail of the AC72.

Mid-way between hard and soft sail rigs

Fallow points out that the two element wing with the slot between helps to re-energise the boundary layer on the second element, and means that you get a much higher lift coefficient of 2.5 or higher.

“I think we can approach a lift coefficient of around 2.0 with a soft wing and headsail configuration where a conventional sail would be 20% lower than that”, he adds.

But that is not the whole story.

“Bear in mind most of the time that when you are sailing at high speed and particularly with foiling applications, you are not looking to maximise lift - and part of the equation is to minimise drag. Because we are incorporating the mainsail seamlessly by hanging a sail/skin off the D-shaped mast, the parasitic drag is significantly lower than a conventional sail and rig.”

“The AC75 rig definitely sits between the hard wingsail and the soft conventional rigs.”

“The key part of the AC75’s double-skinned mainsail and the rotating D-shaped wingspar is not seeing the wing and sail as different elements, but to integrate both elements into one aerodynamic surface – not dissimilar to a bird’s wing in cross-section.”

To join or not to join at the leach

A key to achieving this homogenous surface is the way in which the mainsail skins attach to the D-shaped wingspar – and similarly at the leech where the options are to have the two skins attached or connected in some way to each other, or for the two skins to be completely independent of each other.

“In the rule, there are two specific zones - near the mast and near the leech - where we have left it up to the teams to have a lot of design control as to how they attach the sail (to the mast) or connect the skins (at the leech).”

“We don't know what the best solution is at the moment.”

“We are not saying that you have to use cars, or you have to use a bolt-rope to attach the sails to the mast. We have left that open for design development.”

“Near the leech, there is an area where you can connect the skins together if you wish.

“One thing that has to be remembered is that the mainsail has two skins and when you are going from one tack to the other, the leeches won't align with each other. For the sail to assume its correct sectional shape, you need to have more camber on one side than the other.”

“You could pin them together - there are systems which do that - which may or may not be the most efficient means”, he adds.

The designers are allowed to connect battens together physically. But Fallow points out that if you do this from a design perspective, you have to cover off a lot of different sailing situations.

“Any connection you have will affect how the two sails interact with each other.’

“There is nothing mandated that they have to be connected. The rule doesn't say that the skins have to be the same size. So there is a lot of flexibility. When we wrote the rule, we wanted each team to figure out what is needed and what is not.”

A wing like a bird

The AC75 mainsail will have a different flying shape from the IACC monohulls sailed from 1992-2007 America's Cups class, and the hard wingsails used on the AC72 and AC50 in 2013 and 2017.

“The leeward skin is more curved, and therefore the leech is further forward. We don't say how the skins have to come off the back of the D-Shaped mast. But if the designers do it smoothly, the leeward skin is more curved, and therefore the leach sits further forward than the weather skin which is straighter and the two skins combine to form a really nice wing-shaped section - very similar to the cross-section of a bird’s wing.”

“We saw that in the first mock-up we did a few days after coming up with the double skinned mainsail concept."

“While there is no fixed weight limit on the mast and mainsail, it does affect the total weight of the yacht - so if you spend weight in the rig, you have to take it from somewhere else or vice versa.”

Sail controls are another issue with two skins, which while the sails may set independently of each other there will probably be some global systems – like a mainsheet which controls both sails, and secondary controls such as two different outhauls which control one skin only.

“You have to go through and look at what you have - adjustable halyards, cunninghams on each one, two outhauls, two mainsheets. It gets very complex very quickly. As we saw in the last two AC cycles once the boats start foiling they get very dynamic, and you need to be able to respond quickly to changing conditions and circumstances - and that is all part of the equation.

“In some areas, you might combine controls and others may be less more independent.”

Welcome to the Passive Zone

The AC75 class rule splits the mainsail into three zones. The top four metres is known as the Upper Zone, and the bottom 1.5metres is the Lower Zone.

In between is the largest area of the sail which Fallow refers to as the Passive Zone. The intention with this zone is that it performs and is controlled in a similar manner to a single skinned mainsail on a conventional rig.

“In the top part of the sail, there is a four-metre section where extra controls are allowed for twist, halyard systems or whatever,” Fallow explains.

“And then in the bottom 1.5 metres of the sail, it is the same. There is nothing to say you must have a boom - it is optional, and nothing is mandated. It is over to the teams to control the sails as they wish.”

Six full-length battens (luff to leech) are permitted in the Passive Zone, along with six flutter battens in the leech – which are allowed to be just one metre long.

There is also a restriction on how wide a batten can be - so a four-metre wide batten is prohibited - which would effectively make a hard top section of the mainsail.

In the Upper and Lower Zones, there is a lot of design and control freedom, which begs the question as to whether Team New Zealand will adopt the boom-less deck sweeper mainsail style that skipper Glenn Ashby uses on his world championship winning Int A-Class catamaran.

"I’ve seen a couple of photos. but I do not know the detail of what his set up is like down there”, says Fallow. "I see Glenn every day - but I have not spoken to him about that yet!" he adds.

How do you adjust the foot camber in a boom-less sail?

"There are lots of ways – for instance, you could do it the same way in which you adjust the foot camber in a jib,” explains Fallow. “We are not restricting any of that by saying you must have a boom and an outhaul.”

“You could come up with a mainsheet system that has a downhaul control like some Jib Control systems do.. Each will have its own issues, like the ease of use, functionality, effectiveness and those all have to be weighed up. But we want teams to figure that out for themselves, and design their own systems rather than have these specified in the rule.”

In Part 2, Burns explains how the "front sails" work