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America's Cup: Emirates Team New Zealand launch radical Cup Defender

by Richard Gladwell/Sail-World.com/nz 19 Nov 02:50 PST 19 November 2020
Emirates Team New Zealand launch Te Rehutai - November 18, 2020 © Richard Gladwell / Sail-World.com

America's Cup champions revealed their Version 2 AC75 today at the team base in Auckland's Viaduct Harbour.

Christened Te Rehutai by Lady Margaret Tindall, the 75ft radical foiling monohull is quite a design departure from most pundits expectations, which was for a development of their team's first AC75, Te Aihe, launched in September 2019.

That boat never sailed a race, and Te Rehutai was designed based on performance analysis of data from Te Aihe, and by using the team's design and sailing simulators. That process was carried on using the team's 12metre test boat, Te Kahu while the Version 1 AC75 was shipped to Europe in mid-January. The AC75 program restarted using Te Aihe in early June.

"It's a fundamental mistake to think you are designing a boat", says team CEO Grant Dalton. "You have to look at the boat as an object - a flying object that makes as little imprint on the planet as it can - like a race car."

Dalton's design philosophy explains much of the rationale for the AC75, which would appear to be an outlier at first blush, but when broken down into various elements, the design synergy becomes quite clear.

The boat borrows heavily from F1 aerodynamic thinking - maybe not surprising given design chief Dan Bernasconi's PhD in Mathematical Modelling and Aerodynamics, and six years at McLaren Formula 1 for six years, leading their Vehicle Modelling team, before swapping fast cars for fast boats.

The hull design was signed off in October 2019, about three months earlier than the Challengers, and with only two months sailing on Te Aihe.

However it is decision to run a 12 metre long test boat - the maximum allowed under the Protocol governing the event - that has kept the Kiwi test program running at near full speed through the COVID19 disruptions.

Like all boats the Kiwi's V2 AC75 has a bow, a centre and a stern.

Breaking those components down, the bow is most noticeable for its pronounced flare, coupled with a deck which rounds down into the nose of the boat. That all provides plenty of buoyancy in the event of a nosedive, and helps keep the crew dry and water out of the pits.

ETNZ design chief explains the bow design thinking as being "about accelerations and touchdowns as well - when you come off the foils at a low windspeed, or when something doesn't go perfectly in a manoeuvre".

In that respect the new AC75 should be a very forgiving boat, which makes for easier handling and encourages the crew to push harder with less risk of the consequences of a small error.

"It's all about trying to minimise the amount of hull that you have in the water," Bernasconi explains.

Moving back into the centre of the boat, there are the high sides that were first seen on the first British AC75. These contain the crew pits and are deep with only the head and shoulders of even quite a tall crew member protruding. That makes for less windage, and again protects the crew when sailing in high apparent winds speeds of around 60kts, and driving spray.

The centre of the boat is completely clean and like the first British boat the sails will endplate onto the deck, below gunnel level. Between the crew pits on either side the deck is clean providing a channel to clear any deck water. The raised centre in Te Aihe's cockpit has gone.

The stern of the boat has a John Spencer-style hard chine which design chief, Dan Bernasconi explains is to give greater hull form stability and achieve the minimum roll-moment required by the AC75 class rule. Underneath the canoe body of the boat is near flat bottomed, with a small centreline skeg - quite similar to American Magic's down aft, and like the US boat becoming more pronounced running into the bow.

The hull volume is carried right into the after end of the AC75, with relatively straight chines back from the maximum beam, around the point where the foil arms intersect the hull.

Put those three elements - bow, centre and stern back together to form a hull - and the unusual shape of the Version 2 AC75 starts to make design sense.

Explaining the different design approaches taken by the four America's Cup teams, Bernasconi says they reflect the different trade-offs taken between aerodynamics and hydrodynamics.

Or put simply: "you've got to accelerate to get out of the water, and then once you are up and foiling it is all about the aerodynamics, " Bernasconi explains.

"The idea is that as the boat accelerates, we quickly reduce hull volume in the water and the boat becomes more like narrow multihull than a big monohull. There is also the aerodynamic benefit, that all the teams have seen, that by having a dividing skeg in the centre of the boat, you create a difference in pressure between the windward side and leeward side."

"We've put a lot of work into the optimisation of the design. With the tools that we have, we are confident in the shapes that we have come up with. Ultimately we do have to make some sort of judgement call on the hydro and aerodynamics and look at the low wind performance, along with foiling performance."

"It's an interesting design problem for the engineers, and it's great for us to have some really open space to play in," he adds. He freely admits that ETNZ's latest is nothing like the design hull shapes they first envisaged, when drafting the AC75 class rule, back in 2017/18.

Overall Bernasconi says he expects the new boat to be better all-round than Te Aihe "through the acceleration region and better aerodynamically".

The foil arms have some significant differences.

The hull indentation to take the foil axis is more refined than on Te Aihe and again is an improvement aero and hydrodynamically. The waterline beam (free area beyond the axis point) appears to be wider providing more stability to help uplift the boat onto its foils.

A design feature which Bernsaconi will not explain lies on the foil arms about where they enter the water where they widen for about half a metre, before tapering back to standard width.

Another area on which we are told to wait and see is the rig, which Bernsaconi says is a significant development area. "It's the only thing pushing the boat forward", he says. "Everything else is slowing it down."

"We have to put as much effort into that side of it as the foils and the hull," he adds.

Te Rehutai is expected to have its first sail on the Waitemata on Friday.

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