The boats being raced in this year’s America’s Cup are pushing the boundaries of both science and design.
The 75-foot foiling monohulls are unlike anything seen before in our waters and have been attracting huge interest and fanfare as they fly across the Waitematā Harbour daily.
But how exactly do these boats fly? What differences are there between Team NZ and Luna Rossa? And what are the responsibilities of each crew member?
Cheree Kinnear and AUT’s Sailing Professor Mark Orams break down every inch of Team NZ and Luna Rossa’s 75-foot, 7.5 tonne America’s Cup boats.
The twin cantilevering foil arms, which extend out the side of the boat, provide lift and stability and are controlled by battery-powered hydraulics.
When the boats are foiling, the force of the wind on the sail is essentially trying to push the boat over on its side, so the foil in the water on the opposite side of the wind helps keeps it stable.
The other foil, if kept in the water would drag, therefore, is lifted. But because it’s heavy it also helps with stability. The only time both foils are kept in the water is during a manoeuvre.
“The foils basically support the boat,” Orams says. “They’re around the size of an ironing board so you’ve got this massive yacht and all the force is supported by this very narrow foil.”
The foil arms also have adjustable flaps attached to the bottom of each wing, and the flight controller’s job is to control the flap’s angle when foiling.
“If you think of an airplane’s wings, the flaps go down to give you lift to take off and then as you take off, you’ll see the pilot retract the flap as you’ve got enough speed that you’ve got to reduce the drag. It’s the same principle with the main foils,” says Orams.
Team New Zealand have fairly straight foils while Luna Rossa’s resemble more of an inverted ‘Y’ shape and slant downwards to the tips.
“In aerodynamic terms that gives you more stability,” Orams said. “So somehow Team NZ have figured out how to stay stable but have a really low drag set up on their foils. That tells me they must have a very sophisticated way of controlling their flaps.”
Controlled by the helmsman, the rudder acts like the tail wing on a plane and is essentially a steering device.
The ‘elevators’ at the bottom of the rudder control the pitch of the yacht and enable stable flight -basically how much the boat flies nose up or nose down when foiling.
The rudder must work in unison with the foil arms, as Orams explains.
“If you’ve got them both going the same way, so both angles up, they’ll both go to the sky and the boat will breach and fall to the surface. In contrast, if they’re both down it’ll nose dive so they have to work together.”
Just like the foil arms, the larger the rudder, the more control the boat has, but once flying, the more drag it can create. The challenge is in striking the right balance.
While both teams have similar rudders, Team NZ’s is the smallest.
The hull is the main body of the boat.
The unique difficulty with designing the hull of these foiling AC75’s is, because it spends time both in and above the water, it needs to be built hydro-dynamically and aero-dynamically.
To achieve this balancing act, the hull needs to have enough buoyancy to take off yet be thin enough that when flying, has minimal drag.
Team NZ and Luna Rossa have relatively similar shaped hulls, in that they’re arrow-shaped in design but Team NZ is much wider at the back.
“What they’re trying to achieve is buoyancy in the hull by making it wide but as they present the hull to the wind direction, that it has as little profile as possible,” Orams says.
“A little bit like a fighter jet.
“Luna Rossa’s shape is less radical with no hard corners or bumps whereas Team NZ have a bit more of a narrow bow and boxy stern.”
The other important aspect of the hull design is the shape of the keel. When the boat is at full flight, wind travelling underneath from one side of the hull to the other can create turbulence.
Extended keels, as seen on both boats, force the wind to go front to back and tunnel either side of the hull.
“It’s a lot less drag and allows the boat to go faster,” Orams says. “So both teams have these keels that drop from the middle of the boat that closes the gap between the water surface and the hull.”
Mast & sails
Each boat must have an identical mast, but how it’s installed, how much it leans and where it’s positioned is up to the teams.
The sails are constructed out of carbon fibre. Team NZ and Luna Rossa both use North Sails, which are built in a mould.
The jib – being the triangular sail at the front of the boat – is single sided so when the boat goes from one tack to the other, the sail inverts in its shape onto the other side.
The mainsail, however, has two skins, and so creates a “soft wing”.
“This is the engine of the yacht so how teams adjust it is huge,” says Orams. “Mainsails are adjusted using hydraulics, powered by the grinders, how much power the grinders can produce determines how much adjustment the mainsail trimmer can make. And they have to be adjusted a lot for manoeuvres, puffs and lulls and they’re always trying to manage the compromise between power and drag.”
When it comes to race day, each team selects which sails to use based on recommendations from their meteorologist. The mainsails are selected fairly early but they leave their jib choice until much closer to the start time.
“You’ll have your plan A, plan B and plan C before you even leave the dock,” Orams said.
At the top of mast and front of the bowsprit, there are instruments that measure the wind strength and angle.
This provides the afterguard (helmsman and tactician) with valuable information when racing.
It also provides critical information about angle of attack, foil arm cant, pitch of the elevator and the rudder.
“They’ll look at some key things such as the target of their boat speed, turn rate in manoeuvres, the speed going into a turn, through the turn and coming out and how they accelerate to get up to speed,” said Orams.
“It determines where the mainsail is, the foils are adjusted to and the angle the helmsman is steering the yacht.”
In basic terms, grinders use hand winches to create hydraulic power for the trimmers to adjust the sails.
To create hydraulic power, pressure needs to be created to pump hydraulic oil through pipes so the grinders ensure pressure is kept in the accumulator tanks.
“Think about it like a hot water cylinder. When filled up, you’ve got plenty of hot water and pressure but when you have two showers going, the pressure drops and so goes the temperature, the grinders are basically keeping the pressure up in those tanks,” Orams explains.
“Without pressure, a person will essentially push a button and nothing will happen.”
Both Team NZ and Luna Rossa have eight grinders, four on each side of the boat.
The flight controller is in charge of the foils and plays an essential part in ensuring the boat pulls off manoeuvres while foiling.
With the more traditional approach, Team NZ have thrust the role onto Blair Tuke.
Luna Rossa, on the other hand, has opted for the configuration with two helmsmen in Jimmy Spithill and Francesco Bruni, meaning both alternate in the role of flight controllers as they manage the foil control system.
The mainsail trimmer controls the ‘engine’.
They’re key to spotting changes in wind and can change the top and bottom sections of the mainsail.
But they must coordinate with the helmsman, as Orams explains.
“It’s like if you were in a car and you’ve got one person on the steering wheel and the other with a foot on the accelerator. If they’re not in sync with one another, it’s really difficult,” said Orams.
“They have to be like fighter pilots, very cool and calm.”
The helmsman’s primary responsibility is steering the boat and making decisions in manoeuvres.
He talks the team through their routines when entering a tack or gybe and the helmsman has the final say.
Team NZ has one helmsman – Peter Burling. But as mentioned above, Luna Rossa has opted to have two – Spithill and Bruni.
“The tricky thing for teams is when they change from one tack to another, how do they control the boat? Team NZ has one primary helmsman, and you want to steer the boat from the windward side. So when they go into a turn Burling ends up on the wrong side so Glenn Ashby has to steer while he crosses the back of the sail,” Orams explains.
“In a new innovation, Luna Rossa have one helmsman on each side and swap between roles of steering the boat and controlling the pitch.”
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