The design rule for the 36th America’s Cup is the most audacious yet: a 75ft foiling monohull that will race mostly in flight at four times true wind speed, up to 50 knots. It’s more an airplane than a boat; more a story of aerodynamics than hydrodynamics. That is the defining feature of the AC75.
Since September, three challengers—Luna Rossa Prada Pirelli Team, INEOS Team UK and the New York Yacht Club’s American Magic—and the defender, Emirates Team New Zealand, have all launched their first-generation boats and are foiling successfully, thanks to using simulators during the design phase. (A fourth challenger, Stars & Stripes Team USA, has yet to launch a 75-footer.) Simulators were also a major contribution to New Zealand’s success in Bermuda.
The boats are designed to the 36th America’s Cup box rule: in simple terms, the hull can be any shape its designers conceive that fits into a theoretical box of the length, beam and draft set by the rule. It’s much more interesting than one-design, but it’s expensive—American Magic reported more than 100,000 design hours to produce its AC75, Defiant.
It’s not surprising that INEOS Team UK and American Magic, which started designing their boats before the design rule was published, built highly unusual boats that looked nothing like each other. It also follows that the boats from Italy and New Zealand, the Challenger of Record and the Defender respectively, are quite similar because they wrote the design rule (although it still hasn’t finalized the wind range in which the boats will sail and foil).
So, those pairings reflect two defined concepts: the scow for the UK and United States, the skiff for Italy and New Zealand.
Brett Bakewell-White of Bakewell-White Yacht Design in Auckland, New Zealand, says that in simple terms the skiff is about being the fastest boat to get onto the foils; the scow is about being fastest when it gets there.
“The majority of drag in a displacement boat occurs where the waves are formed in the boundary where air and water meet,” he says. “That’s why it’s going to be critical in this America’s Cup to get on to the foils as quickly as you can. If I was designing it, I’d be thinking: ‘It’s going to be unstable when it’s not up on the foils anyway, so just accept that and don’t try and make it stable, just aim for the minimum drag possible.’”
So the skiff-like hull is best, right? Not so fast. Bakewell-White continues: “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.”
Oh, so a scow-like hull is best, right?
Again, that depends. In a world where stability is totally dependent on speed, all boats will have unplanned splashdowns—being slow out of a tack, a sudden maneuver to avoid the other boat, a fluke in the breeze: Boosh!
In Bermuda in 2017, the AC50 catamarans fell off their foils, too, but the transition between foiling and non-foiling was usually smooth. If a scow-type hull splashes down, it is more likely to stick to the water, and if it has to lower its windward foil arm to regain stability and speed while its opposition is doing 50 knots, that’s gonna hurt.
NYYC American Magic
The Americans partnered with Airbus designers and have committed to aerodynamics over hydrodynamics. The extreme, fighter plane-style nosecone bow protrudes clear of the water: Defiant by name and by nature.
This boat is all curves. Viewed bow-on, the hull develops into a gently squashed oval which continues to the open transom in highly refined aero detailing. The paint job is stunning and slippery—think Mercedes black, just whispering for air flow.
Underneath, Defiant has a super-smooth, consistent surface. The pivot points for the foiling arms, housed in soft bulges, are at mid-height in the topsides, which may explain why the American boat is a low-flyer. “They may be getting some benefit from a phenomenon known as ground effect,” says Bakewell-White. “The pressure of the air between the hull—or perhaps I should say fuselage—and the water holds them up so they can have smaller foils and still fly because they are getting increased pressure between the hull and the water.”
INEOS Team UK
Whereas Defiant has a nosecone bow, Luna Rossa’s is plumb, ETNZ has a dreadnought bow and UK has a sensible shoe—a blunt dreadnought bow drawn with a thick pen, not a fine nib. Britannia is also the widest boat, which will aid stability on the water and, bow-on, resembles a wave form as the gunwale swoops up to slablike topsides. From aft angles, it’s a Batcar.
Bakewell-White believes the design aims to channel air flow over the low bow and between the high topsides. “This reinforces the idea of the hull as a flying wing sail looking for that cushion of air over the water, especially as the undersides are a simple, straight section from bow to stern. I suspect a longitudinal section of the hull may look like a wing section,” he says.
The foil arms pivot on the waterline, taking the foil arms to the maximum permitted draft of 16ft 5in.
Luna Rossa Prada Pirelli Team
Luna Rossa’s design has aggressive, albeit elegant, aesthetics with a matt black finish and a bold red Prada stripe. The boat’s low profile will minimise aerodrag in flight. The sheerline flows from the bow to the waterline transom in a dramatic, torpedo-like taper—a clean exit.
She’s all about curves, except for the plumb bow, which curves into the forefoot. Underneath, the skiff-like hull has flattish sections and a defined, center V-skeg for most of its length, marking the Italians’ determination to reduce the transition time between displacement and foiling.
“Like New Zealand, Italy has produced a canoe-type concept where they are carrying part of their displacement in a bustle under the hull,” Bakewell-White says. “It appears those two boats are looking at a reduced wetted surface in that marginal foiling situation where they’ve got a partial displacement, where they might touch down in a tack or a gybe.”
Luna Rossa’s foil arms pivot in a recessed torpedo shape set quite high in the topsides, perhaps to lift the foil arm high for maximum righting moment.
Emirates Team New Zealand
Beam-on, Te Aihe is slender, low-profile and dedicated to minimal aerodynamic drag and turbulence, however, it’s also the most conventional of the boats. Like Luna Rossa, the reverse sheerline flows down to the waterline transom. The narrow stern will incur minimal drag to get airborne quickly.
The dreadnought bow features a short plumb stem from which a chine runs aft for about 15 percent of the hull, presenting a fine entry and reserve buoyancy in the forward sections. Underneath is ETNZ’s unusual bustle: a bulge like a keelson that deepens amidships. In the fickle winds of the Hauraki Gulf, Team NZ is regularly kissing the water out of the tacks, but is soon back in flight. The foil arms pivot low, just above the waterline in a soft dimple.
ETNZ has a decksweeper mainsail to stop air leaking under the boom. “I think that’s a pretty important aerodynamic feature,” says Bakewell-White, “and I suspect they’ll all be doing that by the time they get to actual racing.”
Foils are a drag
The T-join of the foil arms and their wings is a source of interference drag and there are two ways to deal with it, Bakewell-White says. “One is to make the junction as small as possible, but structurally that’s more difficult.” Only TNZ has opted for this, in a detailed join with a fin to minimise the low pressure zones at the T-joint.
The other way to reduce interference drag is to separate the flows over the foils. The Americans and British have done this with a torpedo bulb at the T-junction. The Italians have a much smaller torpedo bulb and smaller bulbs, which one observer described as a bulblet, at the wings’ mid-points.
The size and shape of the foil arm wings and rudder wings will continue to develop. INEOS currently has the largest foil arm wings; the Italians have the smallest. The Americans’ foil arm wings are straight and dagger-like; the New Zealanders’ curl up at the inner tips, like the tips of water skis. “The size of the foils will be critical,” Bakewell-White says. “Big enough for early lift-off at low speeds but not so big as to be too draggy at high speed.”
Skiff… scow… it’s a hull of a choice
Discussions in the yacht club bar will loudly debate scow versus skiff, but in these complex boats, the hull is almost a bystander. Syndicates are already designing their second-generation hulls and will be unable to make significant changes. Far more critical, but less easy to gauge, are the foils and sails. Development on these, finding maximum lift for minimum drag, will continue right to the America’s Cup. Equally critical are the skills of the helmsman and the trimmers who will make constant, sensitive adjustments to keep the boat in flight at high speeds.
But the real heroes may be the guys behind the screens, the mechatronic engineers. Unlike the catamarans, the AC75s have computer-adjustable flaps on the foils, run by a complex city of electronics and hydraulics, custom-designed by each syndicate. Depending on wind and wave conditions, these systems will be tuned to deliver various response speeds and increments, set up by software, monitoring systems and hydraulics.
WHERE THE BOATS ARE THE SAME
The foil arms and foil control systems, and the D-section masts are supplied as one-design components. Every team designs its double-skinned mainsail and sailing systems, rudder wings and foil arm wings.
Unlike nearly every monohull on the planet, the AC75 does not have a keel, centerboard or even a canard. Instead, the lead-ballasted foil arm wings provide stability and righting moment. The rudder has a T-shaped wing of up to 9ft 10in.
All the boats have rolled gunwales to keep the airflow attached and flowing smoothly across the lower portion of the jib.
Also with turbulence and aerodrag in mind, all boats have a central deck that slopes down from the foredeck to the transom, forming an endplate to the sail plan. The grinding and trimming crews are in trenches either side to avoid disturbing this flow. However, crew positions, including forward-facing or transverse grinder positions, and hydraulic and electronic controls will vary significantly among the boats.
SPECIFICATIONS
Hull length 68ft (20.7m) + bowsprit 6ft 6in (2m)
Maximum beam 16ft 5in (5m)
Weight 14,330lb (6.5t)
Mast height 87ft (26.5m) from deck
Mainsail 1,453-1,560ft (135 – 145m)
Jib 968ft (90m)
Code zero 2,152ft (200m)
T-foil rudder max draft 11ft 6in (3.5m), max span 9ft 10in (3m)
Canting foils max draft 16ft 5in (5m), foil wing span 13ft (4m)
Crew number 11
Max crew weight 2,116-2,182lb (960-990kg)
Photos courtesy of The America’s Cup
January 2019
