The AC75s, the America’s Cup boats currently racing in the Prada Cup and that will be used for next month’s Cup match showdown, are arguably the most radical boat the America’s Cup has ever seen. 

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 three 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.   

1 Unimaginable speed

In their final race before being knocked out of the competition, American Magic’s Patriot registered a top speed of 53.3 knots during a bear away. 

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, 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.

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. 

2 A storm onboard America’s Cup boats

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.

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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 America’s Cup Defender and the Challenger of Record, Emirates Team New Zealand and Luna Rossa Prada Pirelli respectively, announced the 36th America’s Cup 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. 

4 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. 

5 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 6508kg and 6538kg. 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. 

6 Sails can invert at the head

As with everything on the AC75, the mainsail is a relatively new concept. 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 2m of the mainsail and the bottom 1.5m. 

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.

7 An America’s Cup boat generates lots of data

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 Team UK have been able to 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. They even have some Mercedes staff out with the team in Auckland assessing their data.  

“It’s really similar to F1,” explains Mercedes Applied Science Principal Engineer Thomas Batch who has 11 F1 titles to his name and is with INEOS in Auckland. “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.” 

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