america's cup yacht design

Yachting World

• Digital Edition

America’s Cup boats: 8 facts about the AC75 and why they’re unique

• Toby Heppell
• August 20, 2024

The America's Cup boats to be used on the 2024 edition of the event are immensely complicated high tech bits of kit. They might be officially sailing craft but they behave in some remarkable ways

The AC75 is the class of boat that takes part in the America’s Cup and are arguably the most radical boats the compeition has ever seen.  This type of America’s Cup boat was first used in the 2021 America’s Cup so this is the second event in which these boats have been used.  

The America’s Cup is, fundamentally, a design competition, and successive America’s Cups have featured the most extreme yachts yet – for their time – ever since the first race in 1851.

However, the foiling boats we have seen in the last four editions of America’s Cup racing (the AC72 and AC50 catamarans, and now the AC75 monohulls) do represent a new direction for the highest level of sailing.

There are plenty who argue that this technology is so far beyond the bounds of what most people consider sailing as to be an entirely different sport. Equally, there are those who believe this is simply a continuation of the development that the America’s Cup has always pushed to the fore, from Bermudan rigs, to composite materials, winged keels, and everything in between.

Good arguments can be made either way and foiling in the world’s oldest sporting trophy will always be a subjective and controversial topic. But one thing is certain: the current America’s Cup boats, the AC75s, are unlike anything seen before and are showcasing to the world just what is possible under sail power alone.

Photo: Ian Roman / America’s Cup

1 Unimaginable speed

Topping the 50-knot barrier used to be the preserve of extreme speed record craft and kiteboarders. A World Speed Sailing Record was set in 2009 of 51.36 knots by Alain Thebault in his early foiling trimaran, Hydroptere , and was bested in 2010 by kite boarder, Alexandre Caizergues who managed 54.10 knots.

Only one craft has ever topped 60-knots, the asymmetric Vestas Sail Rocket 2 , which was designed for straight line speed only and could no more get around an America’s Cup course than cross an ocean. Such records are set by sailing an average speed over the course of 500m, usually over a perfectly straight, flat course in optimum conditions.

America’s Cup class yachts, designed to sail windward/leeward courses around marks, are now hitting speeds that just over a decade ago were the preserve of specialist record attempts, while mid-race. American Magic has been recorded doing 53.31 knots on their first version of the AC75 class, Patriot.

Perhaps even more impressive, in the right conditions when racing we have seen some boats managing 40 knots of boatspeed upwind in around 17 knots of wind. That is simply unheard of in performance terms and almost unimaginable just three or so years ago.

Article continues below…

How to watch the America’s Cup live streams

The 37th America’s Cup is due to start on Saturday 12 October and will see America’s Cup Defender Emirates Team…

WATCH: Will the USA’s Radical America’s Cup Design Choices Pay Off?

American Magic’s new AC75, Patriot, has garnered significant attention due to its unique approach. While all the teams are bound…

2 A storm onboard the AC75

Related to the speeds the boats are sailing through the water, particularly upwind, is the wind speeds the sailors will feel on deck.

When sailing, the forward motion affects the wind we experience onboard, known as apparent wind. The oft’ trotted out explanation of how apparent wind works is to imagine driving your car at 50mph. Roll down the window and stick your hand out of it and there will be 50mph of wind hitting your hand from the direction your car is travelling.

So when an AC75 is sailing upwind in 18 knots of breeze at a boatspeed of 40 knots, the crew on deck will be experiencing 40 knots of wind over the decks plus a percentage of the true wind speed – depending on their angle to the wind.

The AC75 crews might be sailing in only 18 knots of breeze – what would feel like a decent summer breeze on any other boat – but they experience winds of around 50 knots.

To put that into context, that is a storm force 10 on the Beaufort scale!

3 Righting moment changes

The single most radical development of the AC75 is to take a 75ft ‘keelboat’, but put no keel on it whatsoever.

When the then America’s Cup Defender and the Challenger of Record, Emirates Team New Zealand and Luna Rossa Prada Pirelli respectively, announced that the 36th America’s Cup (to be held in 2021) would be sailed in 75ft monohulls, conventional wisdom had it that the boats would look something like a TP52 or a Maxi72 – both impressively high performance keelboats.

By doing away with the keel entirely, the design is now like nothing we have ever seen, particularly when it comes to how dynamic the power transition is between foiling and not foiling.

The boats are designed to foil on the leeward foil, with the windward one raised to help increase righting moment: to help balance the boat. This means that when the AC75 is not foiling they are extremely tippy – much more so than most other boats of the same size.

Essentially, when the wind catches the sails, the boat wants to fall over as there is too much sail area for the amount of weight underneath the boat – something a lead keel usually counters on a yacht or keelboat.

Once the boat is up and on the foils, however, that all changes, as everything to windward of the single foil in the water balances the sails. That means, the hull, the crew weight, the sail and rig weight, and the windward foil, all work to counter the sails.

What all this means is that the boats go from being extremely tippy, to hugely powerful in just the few seconds it takes to get up on the foil. “The [AC75s] are really very tippy pre-foiling and then they go through the transition where they will need to build significant power. Then immediately [once they lift off] you have more stability than, well, take your pick, but certainly more righting moment than something like a Volvo 70 with a big canting keel.

“That change all happens in a very short space of time,” explained Burns Fallow of North Sails, who was one of the team who developed the soft wing concept back when the concept was revealed.

Photo: Ricardo Pinto / America’s Cup

4 ‘Cyclors’ return to power America’s Cup boats

Bak in 2017 Emirates Team New Zealand stormed to America’s Cup victory in an AC50 foiling catamaran which was, by some margin, quicker than any of the other teams.

The most glaring difference was their use of pedal grinders to produce power rather than traditional pedestal arm grinders. ETNZ’s sci-fi style term for their grinders was ‘cyclors’, cyclist sailors.

The idea had actually been tried before in the America’s Cup; Pelle Petterson used pedal grinders on the 12-metre Sverige in 1977. But ETNZ’s set-up now was very different: here it was part of a linked chain of innovations, the most obvious emblem of a radical approach.

One obvious benefit was the greater power output from using legs to pedal, but beyond this it left cyclists’ hands free and allowed the team to use a highly sophisticated system of fingertip control systems, and thus to use faster, less stable foils, and then to divide up crew roles so ETNZ could be sailed in a different way.

When the AC75 was first introduced in 2021, Cyclors were specifically banned by the class rule. However, with a reduction of crew numbers from 11 to 8 in the second AC75 class rule – in use for the 2024 America’s Cup – cyclors are now allowed once again and all teams look set to be using pedal power onboard.

5 America’s Cup boats may not be heading where they point

With the AC75 sailing on its foil, drag is dramatically reduced, vast amounts of power can be generated and so speeds rapidly increase. But the foils can serve another purpose too.

In order to be able to lift each foil out of the water, the foil arms must be able to be raised and lowered. Hence the foil wings, which sit at the bottom of the foil arms (and are usually a T or Y shape), do not always sit perpendicular to the water surface and the AC75s often sail with them canted over to something nearer 45º to the surface.

The further out the leeward foil arm is canted – essentially more raised – the closer the AC75 flies to surface and, crucially, the more righting moment is generated as the hull and rest of the boat gets further from the lifting surface of the foil.

There is another positive to this: as the lifting foil is angled, it produces lift to windward, which can force the boat more towards the wind than the angle it is sailing.

Due to this negative leeway (as it is known when a foil creates lift to windward) the boat can be pointing at a compass heading of say 180º but in fact will be sailing at eg 177º as the foil pushes the boat sideways and to weather, essentially sailing to windward somewhat diagonally.

6 The foils are heavy. Very heavy.

As the foils work to provide stability to the boat (when it is stationary both foils are dropped all the way down to stop it tipping over) and to provide massive amounts of righting moment, they are incredibly heavy.

A pair of foil wings and flaps (excluding the one-design foil arm which attaches them to the boat and lifts them up and down) weigh 1842kg. To put that into perspective, the entire boat itself with all equipment (but without the crew) weighs between 6200kg and 6160kg. So the foil wings at the base of the foil arms are nearly ⅓ of the total weight of the boat.

It is partly due to this that you will see some teams with bulbs on their foils. If you decide to go for a skinny foil wing (which would be low drag and so faster) then there will not be enough volume to cram sufficient material in to make the foil weigh enough. So some teams have decided to add a bulb in order to make it weigh enough but to also keep a less draggy, slimmer foil shape.

7 Sails can invert at the head

As with everything on the AC75, the mainsail was a relatively new concept when the boat was first announced. It consists of two mainsails which are attached to both corners of a D-shaped mast tube. This has the effect of creating a profile similar to a wing.

It is well established that solid wing sails are more efficient at generating power than a soft sail and for this reason solid wings were used in both the America’s Cup in 2013 and 2017. But there are drawbacks with a wing: they cannot be lowered if something goes wrong and require a significant amount of manpower and a crane to put it on or take it off a boat.

One reason a wing makes for such a powerful sail is that the shape can be manipulated from top to bottom fairly easily with the right controls. With the AC75 the designers wanted a sail that could have some of this manipulation, produce similar power but could also be dropped while out on the water. The twin skin, ‘soft wing’ is what they came up with for this class of America’s Cup boat.

In addition to the usual sail controls, within the rules, the teams are allowed to develop systems for controlling the top few metres of the mainsail and the bottom few metres.

What this means is that the teams are able to manipulate their mainsail in a number of different ways to develop power and control where that power is produced in the sail. But it also means that they have the ability to invert the head of the sail.

Doing this effectively means ‘tacking’ the top of the sail while the rest of the sail is in its usual shape. The advantage here is that instead of trying to tip the boat to leeward, the very top of the sail will be trying to push the boat upright and so creating even more righting moment. The disadvantage is that it would come at the cost of increased aerodynamic drag.

We know that a number of America’s Cup teams are able to do this, though whether it is effective is another question and it is very hard to spot this technique being used while the boats are racing at lightning speeds.

8 America’s Cup meets F1

A new America’s Cup boat is a vastly complex bit of kit. Each team has incredibly powerful Computational Fluid Dynamics (CFD) software packages and simulators in order to try to understand the various gains and losses.

To make these simulators and computer projections as accurate as possible each team has been getting as much data as they can over their three year development cycle.

In the case of this America’s Cup it does seem the development process is genuinely getting closer to Formula 1 (albeit with smaller budgets than a modern F1 team has behind them).

INEOS Britannia have been work alongside the all powerful Mercedes F1 team (both of who are backed by INEOS) and have been open about how much this has helped their development process and after a relatively small amount of collaboration in 2021 the British team and Mercedes have created a much tighter relationship for the 2024 America’s Cup .

But the British team is not alone. When two-time America’s Cup winner, Alinghi announced they would be coming back to the event after some years on the sidelines, they also announced their own tie-in with current F1 World Champions, Red Bull Racing, to for Alinghi Red Bull Racing .

“It’s really similar to F1,” explains Mercedes Applied Science Principal Engineer Thomas Batch who has 11 F1 titles to his name and is was with INEOS in Auckland 2021. “Certainly in this campaign the technology is close to what we have in F1.

“In terms of raw sensors on the boat you are probably talking in the 100s but then we take that and we make that into mass channels and additional analysis with computational versions of those channels that we then analyse and get into in more detail. So you are looking at 1000s of plots that we can delve into [per race or training session].

“That level of data analysis and then feedback with the sailors is very similar to working with an [F1] driver.”

If you enjoyed this….

Yachting World is the world’s leading magazine for bluewater cruisers and offshore sailors. Every month we have inspirational adventures and practical features to help you realise your sailing dreams. Build your knowledge with a subscription delivered to your door. See our latest offers and save at least 30% off the cover price.

THE BOATS: AC75, AC40 & LEQ12

Although foiling feels like a recent revolution to take the world of watersports by storm, it has been at the heart of America's Cup racing for over 10 years.

It was August 2012 when the sailing world was turned upside down by a 72-foot catamaran flying in the Hauraki Gulf. Emirates Team New Zealand had brought foils to the America's Cup, changing the face of top-level yacht racing forever.

Six years later, in 2018, the publication of the AC75 Class Rule marked the beginning of a new sailing era. The engineering and sailing techniques needed to get the AC75 to fly completely differed from anything seen before.

During the 36th America's Cup in 2021, the AC75 proved themselves to be unique and kept millions of fans worldwide glued to their screens. It was then no surprise that the organisers were keen to continue with the AC75 rule for the Louis Vuitton 37th America’s Cup in Barcelona, albeit with several improvements to promote faster flight and incorporate upgraded technology. But the organisers went further. With a focus on youth and women being paramount to growing the sport into the future, a new class of boats was introduced–the AC40–specifically for this purpose. As the smaller sister-ship of the AC75, the AC40 launched in Auckland to much acclaim with speeds hitting the "high 40s" almost immediately straight out of the box under the skillful command of Emirates Team New Zealand in September 2022.

AC75: AMERICA'S CUP BOAT

On the 15th of November 2021, eight months after the Kiwis successfully defended the America's Cup, an updated 'Version 2' of the AC75 Class Rule was released.

The boats will be a step on from what we saw in Auckland in 2021. The foils will be larger to promote quicker lift and faster flight. The boats will be lighter and, onboard, the electronics and software systems will be vastly upgraded. To save weight, the number of crew on the AC75 has been reduced from 11 to 8. The move to reduce the crew means cycle power is again legalised, and the cyclors, introduced by Emirates Team New Zealand in 2017, might return to the game.

Several of the elements are strict one-design, such as the mast, rigging, foil cant-arms and cant hydraulics but the scope for design elsewhere is broad. To keep costs under control, it was also determined that teams would only be allowed to build one AC75 so the demand on getting the design right is at a premium. The boats will be expected to fly at speeds of 50 knots (with a record speed reported of 55.6 knots during racing) in winds that average 12 knots in late summers in Barcelona. 

RELIVE THE RACING HERE

AC40: YOUTH, WOMEN & TRAINING

New for the Louis Vuitton 37th America's Cup, the AC40 is a multi-use foiling monohull that all the confirmed teams will use for sailing and testing purposes. The AC40 is also the nominated boat for both the UniCredit Youth & Puig Women’s America’s Cup events where they will be sailed in strict one-design. The confirmed teams will also use the AC40 in competitive events leading up to the America’s Cup and several of the teams have two boats on order. The AC40 is being built by McConaghy’s in China. The foils and systems are being delivered by the Emirates Team New Zealand North Shore Facility and delivered in strict order of entry to the America’s Cup. Teams may modify and test out componentry on the AC40, and for this purpose, they will be deemed as a LEQ12 (see below), but for competition purposes in the UniCredit Youth & Puig Women’s America’s Cup, they will be returned to one-design according to strict rules around those specific events. Early trials of the AC40 by Emirates Team New Zealand have been sensational with speeds recorded in the ‘high-40-knot’ bracket and it is anticipated that this new class will form a global circuit both before and after the America’s Cup. For the UniCredit Youth & Puig Women’s America’s Cup, aside from the confirmed entries from the America's Cup syndicates, yacht clubs from around the world are being invited to form teams on a strict one-nation, one-entry basis and the finals of the youth and women’s events are scheduled on key dates of both the Louis Vuitton Cup (Challenger Selection Series) and Louis Vuitton 37th America's Cup Match dates.

LEARN MORE ABOUT THE UNICREDIT YOUTH AND THE PUIG WOMEN'S AMERICA'S CUP

LEQ12: TESTING ONLY

Entrants for the Louis Vuitton 37th America’s Cup are required to build one AC75 and purchase at least one AC40 but within the rules there is also the opportunity for teams to build one, new, test boat with the stipulation that it must be ‘Less than or Equal to 12 metres in overall length.’ The term LEQ12 was thus coined and it’s a nod to a well used term in computing ‘LEQ’ and was the most accurate description of the vessel. These boats are loaded with sensors, cameras and measuring devices in order that the design team can collate data and calibrate their design software packages in a real-world environment. For the sailors too, it is an opportunity to hone monohull foiling techniques and have the ability to try new sailing styles as well as test new componentry in a relatively controlled environment–with far fewer consequences than trialling something new on a powerful AC75 at full scale.

How many new AC75s can a team build

When agreeing on the new format for the 37th America’s Cup, the Defender and the Challenger of Record were determined to keep a lid on costs and deliver a sustainable event that could attract additional entries without a very high financial barrier to entry. During the 36th America's Cup, teams were allowed to build two AC75s but in the final Protocol for the 37th America's Cup, it was agreed that teams could only build one AC75 to compete in the America’s Cup in Barcelona. The introduction of the AC40 class allowed for teams to purchase these for either two-boat training or as test boats and a further concession was permitted in that teams could build one specific LEQ12 for testing and training purposes. By implementing the one-boat build rule for the 37th America's Cup, a high premium is placed on getting the final design right as there is little opportunity to change the fundamental design of the boats after launch. The world’s greatest naval architects, designers and engineers have been employed by all the teams with several partnering with complementary sports engineering functions such as Red Bull Advanced Technologies and Mercedes F1 Applied Science. The design race for the Cup is therefore, as intense as ever.

Can older AC75s be used and/or modified?

The first-generation AC75s that were used at the 36th America's Cup in 2021 may be used for training purposes by all of the confirmed teams in the run-up to the Louis Vuitton 37th America's Cup in Barcelona. To date, two teams–American Magic and Alinghi Red Bull Racing–have opted to re-launch these first-generation boats and use them as test boats and in the case of Alinghi Red Bull Racing, the Swiss who were not present during the 36th America's Cup, purchased Emirates Team New Zealand’s first boat ‘Te Aihe’ to train in Barcelona. There are, however, strict rules on developing these first-generation boats with the number of foil wings, flaps, rudders, foil arms, masts, hull surfaces and sails being strictly limited. This could well be the reason why the other teams have elected to develop LEQ12s or to adapt their AC40s into LEQ12s, as is the case with Emirates Team New Zealand, where the limits on componentry and testing are far less stringent, affording more scope for design development and analysis.

When does an AC40 become an LEQ12?

As mentioned above, in relation to the AC40, these boats are delivered in one-design format, but some teams may elect to take them out of one-design to test out componentry, and in this case, those AC40s will be deemed as ‘LEQ12s’ until returned to one-design configuration again. Change anything on your AC40 outside of the very strict class rules on componentry, and it becomes a LEQ12. In order for it to be used in the planned pre-regattas or the UniCredit Youth & Puig Women’s America’s Cup events it must be returned to the agreed one-design. Immediately after the necessary commissioning process of the very first AC40 that was delivered to Emirates Team New Zealand in September 2022, the team made changes to the cockpit configuration and specific controls that the crew use; This immediately converted the Emirates Team New Zealand AC40 to a LEQ12.