Road to the Cup Ben Ainslie on developments in AC75 hull design
BEN AINSLIE ON AC75 HULL DESIGN
Four AC75S have now been launched, and each of these brand new America’s Cup class boats reveals the thinking of its team’s designers and sailors. There are more differences than similarities in the four boats, particularly in the hull design. So we’re going to take a look at what’s driving the design choices.
There is one point of consensus and that’s the divided cockpit. Each one has a deck layout with a trench down each side of the boat, separated by what’s effectively an extension of the foredeck that runs all the way to the stern.
“The thinking is to create a pod from the bow of the boat through to the stern of the boat, and to endplate the mainsail to that pod,” explained Ben Ainslie. “There’s some really, really strong aerodynamic gains to doing this.
“That effectively means you end up with two trench-style cockpits on either side of the yacht. And once you’ve got that pod in the middle and the trenches down the side, your crew is ideally hidden from the wind as much as possible to reduce the [aerodynamic] drag.”
The end-plate effect is well understood; the lift generated by an aero- or hydrofoil comes from the pressure difference created between the high and low pressures sides. If the air can flow from the high to low pressure sides over the ends or tips of the foils then it reduces the pressure difference and the efficiency. So an endplate is just something that stops this flow over the end. In this case, the deck of the AC75 is an endplate for the bottom of the soft wingsail.
BIG DIFFERENCES
So much for the similarities, what about the differences in the deck layouts? “There are definitely different approaches here as to whether all of your crew move from side to side – as is more traditional in yacht racing – or split, so are fixed on one side of the yacht, come what may, and whatever tack you’re on or manoeuvres you’re doing.”
In the first approach, where everyone moves as they normally would, then the big advantage is righting moment, because all the crew weight is always on the windward side. The disadvantage is the time lost getting them from one side to the other when they are no longer winding handles or sailing the boat. It remains to be seen where the fleet settle on this issue.
When it comes to the design of the hulls themselves, the most striking thing is that the boats can be split into two pairs. The INEOS Team UK and American Magic boats both have a flat bottom, while Luna Rossa and the Kiwis have a ‘bustle’. On the first pair it’s a V-shape, and in the second a more rounded longitudinal protuberance; both run fore and aft down the centreline of the hull.
“It’s a really fascinating dilemma with this style of boat, and we’ve seen the Italians and Kiwis come up with a bustle or blister which has two effects, really: a hydrodynamic effect in terms of the boat going through its acceleration phase; and lifting up out of the water.
“And then it also has quite a strong aerodynamic impact. We talked about endplating of the main to the hull of the boat. Can you effectively do the same thing with the hull of the boat to the surface of the water? Again, if you can achieve that, it has really big aerodynamic performance gains,” said Ainslie.
So the bustle is designed to both ease the separation of the hull from the water as it tries to lift off, and at the same time make it easier to recover a touch-down. These hydrodynamic effects should make it possible to fly the boat very close to the water and collect the aerodynamic gain from endplating the whole of the boat against the water’s surface.
COPY CAT TECHNOLOGY
Will it work? Technology now allows the teams to take images of each other’s boats, build a CAD model from those pictures using a process called photogrammetry and then analyse the design with their own tools.
“From that, you’ve got to try and work out why has that team come up with that design concept,” explained Ainslie. “It’s no good just copying because you don’t really understand what the emphasis is or what the priorities are of that particular design; why it is that particular shape? And then once you try and understand that, you can then perhaps incorporate that into your own design philosophies.
“So, what are the trade-offs there in terms of the endplating to the surface of the water, how close can we get the boats to the surface?
“That’s really interesting with these boats; the dynamics of how these boats are sailed – and that will ultimately define which boat has the best performance out of the water.”
This is something that can best be worked on in a simulator, where the sailors can sail a virtual version of each design and learn about its dynamic characteristics.
“The straight-line speed is obviously really important, but then that comes with your overall design; having the right sail shapes and all the rest of it. But the actual hull dynamic performance – how it sits in the water, lifts out of the water, how it performs in the manoeuvring touchdown phases – that’s really what we’re trying to explore and, yes, we can see marked differences between all of the teams in that respect,” concluded Ainslie.
BEN AINSLIE is the most successful Olympic sailor of all time, and Team Principal of the British America’s Cup challenger. INEOS Team UK will be challenging for the 36th America’s Cup in New Zealand in 2021. Each month he talks to MARK CHISNELL about the innovations and technology behind the new AC75 foiling monohulls.