Stream Liners

The America’s Cup moves from catamarans to exotic foiling monohulls.

Author:
Rebecca Hayter
Publish date:
Apr 1, 2020

Cameron Gregory

The America’s Cup moves from catamarans to exotic foiling monohulls.

The past two America’s Cup regattas were sailed in frighteningly fast, wing-sailed foiling catamarans. The 2013 edition in San Francisco saw the dawn and dusk of the AC72, while the 2017 event in Bermuda featured a scaled-down version called the AC50.

Now comes the next America’s Cup regatta, to be contested in 2021 in New Zealand. It will be sailed in a yacht carrying the most outrageous superlative yet: the world’s largest foiling monohull. As the new AC75 frees its hull from the sea to foil above it, it is predicted to travel at four times the speed of the true wind, more than 50 knots, with the promise of a new brand of thrills and chills for spectators and TV audiences.

The foiling monohulls demand a high standard of athleticism: Bleddyn Môn training hard on the grinding machine at INEOS Team UK’s Fortitude gym.
Dan Wilko

The America’s Cup has always been a test bed for sailing innovation. On many levels, the AC75 monohull opens a new exploration into yacht design technology. The best way to explore potential in a new concept is to gather some of the world’s experts in that discipline and then give them yottabytes of computer power and few restraints, which is why the AC75 America’s Cup Class Rule, released in March 2018, specified the boats would be built to a box rule. The hull can be any shape its designers conceive that fits into a theoretical box of the length, beam and draft within the rule.

Unlike nearly every monohull on the planet, the AC75 has no keel or centerboard beneath the hull because that would hinder its ability to foil. There is not even a canard. The only appendage attached underwater is the rudder, which has a T-bar wing that can be just less than 10 feet (3 meters).

For the designers in the America’s Cup syndicates, the box rule was Christmastime come early. Most of them had never designed a monohull that would spend more time in flight than in the water, and that would be more about aerodynamics than hydrodynamics.

The American Magic team, sailing for the New York Yacht Club, partnered with Airbus to access that company’s aeronautical expertise. The result is Defiant, with a bow that looks like the nose cone of a fighter jet and a hull that looks like a fuselage.

INEOS Team UK has also gone boldly into an aerodynamic design fest with its Britannia, in a complex combination of curves and slab topsides aft that would suit Batman. Bow-on, she looks like a sensible shoe.

Emirates Team New Zealand, the defender, and Luna Rossa Prada Pirelli Team, the challenger of record, co-wrote the design rule, so it’s not surprising that their boats, Te Aihe, meaning dolphin, and Luna Rossa, respectively, are quite similar in looks. More surprising, given ETNZ’s reputation for innovation, is that Te Aihe is the most conventional of the boats.

The star appeal of the AC75 is its foiling arms and their wings, extending from either side of the hull. They are immensely strong and delicately graceful, as though the progeny of an engineer and a dancer. Luna Rossa Prada Pirelli Team built all the syndicates’ foiling arms and their mechanisms as one-design components, but every syndicate designed and built the wings on the ends of the foil arms, and will probably continue to redesign and rebuild them right up to just before the America’s Cup.

The foil-arm wings are not just pretty; they are lead-ballasted. The leeward foil arm generates lift, and the windward foil arm generates righting moment to balance heeling forces on the sails. More important, the wings have computer-adjustable flaps like those on an airplane wing. The flaps move constantly to keep the boat trimmed in smooth flight.

American Magic’s first AC75 clearly demonstrates its aeronautical DNA, in its nose cone bow and hull shape reminiscent of a fuselage. It is foiling on its rudder wing and leeward foil arm; the starboard foil arm is raised to provide righting moment.
Amory Ross / NYYC American Magic

As with many aspects of the AC75, at this stage, it’s impossible to say who’s gotten it right and who hasn’t, but Brett Bakewell-White of Bakewell-White Yacht Design in Auckland, New Zealand, says the size of the foil arm wings will be critical.

“They need to be big enough for early liftoff at low speeds,” he says, “but not so big as to be too draggy at high speed.”

Soon after the boats were launched, INEOS had the largest foil-arm wings; the Italians had the smallest. The Americans’ foil-arm wings were straight and daggerlike; the New Zealanders’ wings curled up at the tips, like the tips of water skis. Some had the ballast in a single, torpedo-shaped bulb. Some had bulblets. They are all works in progress.

How does the AC75 work in real life? In dock configuration, the AC75 has its twin canting foil arms tucked beneath the hull to present a compact profile. When it’s towed out to open water, both foil arms are extended on either side like stabilizers. The sail trimmers trim on, the boat gains speed, the wings generate lift, and the 75-foot, 6.5-ton boat rises out of the water, executing its dramatic transition from displacement hull to flying hull.

In pre-starts, maneuvers and rough sea conditions, the boat will ride on both foils, but it flies faster when riding on one. As the AC75 gains speed, it gains stability, enabling the boat to sail on just the leeward foiling arm wing and the rudder wing. The windward foil arm is raised to full height, levering the equivalent weight of six cars to counter the full power of the sails. The boat’s top speed is just nanoseconds away.

That may decide the winner in this America’s Cup: Will the winning boat be the fastest through the water and onto its foils, or will it be fastest in flight?

The obvious answer is that the winning boat will be the fastest onto its foils and the fastest in flight, but the laws of hydrodynamics and aerodynamics say that it may not be both. Bakewell-White says that in simple terms, a skiff hull will be the fastest boat to get onto the foils, while a scow hull will be fastest in flight.

“The majority of drag in a displacement boat occurs where the waves are formed in the boundary where air and water meet,” Bakewell-White says. “That’s why it’s going to be critical in this America’s Cup to get onto the foils as quickly as you can.”

Sir Ben Ainslie on the INEOS Team UK simulator.
HARRY KH

The Italians and New Zealanders agree. They have produced skiff-style hulls with flattish sections and a defined, center V-skeg to reduce the transition time between a displacement hull in the water and a foiling hull above the water.

Theoretically, a boat will need to make this transition only once, at the start of the race, but the Italians and the Kiwis are realists. In a world where stability is totally dependent on speed, all boats will have unplanned splashdowns, be they from coming slow out of a tack, a sudden maneuver to avoid the other boat, or a fluke in the breeze.

If a boat comes off its foils and goes down into the water, it’s going to be an expensive pit stop. It will need to lower its windward foil arm and build speed to regain liftoff. That might take 10 seconds or more while the opposition boat is doing more than 50 knots.

By carrying part of their displacement in a bustle under the hull, Bakewell-White says, the New Zealanders and Italians hope to recover quickly from splashdowns. Long-range lenses have captured this theory at work with Emirates Team New Zealand: It has lost speed out of a tack, just kissed the water and continued its flight. In the fluky winds of the Hauraki Gulf, Auckland, where the 36th America’s Cup will be raced in March 2021, quick recovery from splashdown will be nice to have. However, the bustle incurs some aerodrag, which will shave off a little speed in flight.

INEOS Team UK and American Magic have built scow-style hulls with smooth underneath sections to gain a speed advantage in flight. “Any drag effect, in air or water, increases with speed, so the AC75 whose shape develops the least aerodrag once it’s up on foils will be the fastest,” Bakewell-White says.

Aerodrag is a buzzword around the AC75. In simple terms, it’s the effect that an object’s size, shape and surface texture has on the air flowing over it. Usually, yacht designers are more concerned with hydrodynamic drag, but when the boats are mostly foiling above the water, aerodrag is getting air time.

INEOS Team UK and American Magic have chosen the streamlined underbelly of their scow hulls to reduce aerodrag, but, like the skiff option, there’s a trade-off. “That wetted surface area creates a lot of drag that you’ve got to overcome in order to get onto the foils,” Bakewell-White says. So, the scow will take longer to foil and, if it does splash down, its undersections will stick to the water and slow down, fast.

Every syndicate began building its second boat to race the America’s Cup soon after launching the first, so if they want to swap from scow to skiff or vice versa, they had to make that decision months ago.

So, in this intimate gang of just four syndicates, what will it take to win the most innovative America’s Cup? As always, success in every detail. Keeping the boat balanced on its foils at top speed will require precision helming and sail trimming. How well the crew do their job will depend on thousands of hours of data collection and design: The foil arm wings, rudder wings and double-skinned mainsails, jibs and code zero sails are all designed by each syndicate. Then there are the guys behind the screens, the mechatronic engineers.

Luna Rossa training on its first-generation boat. Its skifflike hull features a bustle in the center of its underneath sections, to help it lift quickly onto its foils.
HARRY KH

Unlike the catamarans, the AC75s have computer-adjustable flaps on the foils, run by a complex system of electronics and hydraulics, custom designed by each syndicate. Depending on wind and wave conditions, these systems will be tuned on the day to deliver various increments of adjustment and response speeds. If the mechatronics engineers do their jobs well, no one will notice. If they get it wrong or their systems fail, or if the weather team gets it wrong, then the boat will be slow, fail to foil smoothly or splash down.

A major drawback with the AC50 catamarans of 2017 was that two people, the helmsman and the wingsail trimmer, did most of the work while the grinders human-powered the boat’s systems, including the foils. It was boring for the crew and less-than-engaging for the viewers.

On the AC75 monohulls, batteries will power the foil arms, so that the grinders need only power the winches while other crew trim the sails.

Every America’s Cup leaves a legacy for its successors. As a foiling monohull, the AC75 would never have happened without the AC72 and AC50 catamarans showing the way. But it’s good to know that on the world’s largest foiling monohull, human sailing skills remain at the fore, along with the mechatronic teams behind the screens.