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America's Cup Rialto: January 25/26 - Emirates Team NZ reveals slim-line foils

by Richard Gladwell, Sail-World NZ 27 Jan 04:27 PST 26 January 2021
Te Rehutai, Emirates Team New Zealand - January 25, 2021 - Waitemata Harbour - America's Cup 36 © Richard Gladwell / Sail-World.com

Emirates Team New Zealand are testing two foils, one new, that appears to be smaller and quite different from the challengers.

The new foil has probably been on the AC75 for a week or so - we first spotted it looking over the fence into the ETNZ compound last Friday, en route to the media centre.

But it was only on Monday when the Italians passed the Kiwis on the same tack that the difference between the two teams' approach to foil design became evident.

Unlike the Challengers, when the COVID pandemic first hit in late March and the lockdowns started, Emirates Team New Zealand started back into their on the water testing program using their half-size AC75 test boat, Te Kahu.

The foils currently being used on the team's race boat appear to a continuation of the foil concepts initially developed on Te Kahu.

The AC75's are limited to building six wings, and 20 flaps (two are attached to the after edge of each wing) with the common practice being to build and test a new wing against one that is the benchmark, check the results against the performance predictions for the design, and then make a decision one the design direction. Of course, hundreds of designs are tested in the computer, and are never built.

By switching to Te Kahu, the Kiwi's wing development program didn't really stop, while ETNZ's first AC75 Te Aihe, was on its way to Europe, from late January and their race boat Te Rehutai was still under construction, launched in mid-November.

While the AC75 wings take 3-4 months to build (depending on who you talk to), the ones on Te Kahu were half the size, half the cost and we assume a faster build time.

The basic theory with wings is that those which have a bigger surface area will produce more lift, getting the AC75 out of the water in the light, but have more drag (because of their bigger surface area) in the medium to stronger breezes. The smaller wings, in theory don't provide as much lift in lighter winds but are faster in a breeze and when the AC75 starts foiling, as they create less drag.

The theory too, is that the smaller wings will cause the AC75 to fall off the foils more readily - and struggle to get foil-borne again. We saw that in the second leg of the final race in the America's Cup World Series between ETNZ and Luna Rossa - where the Kiwis started 800metres behind and finished 399 metres ahead at the end of the leg. Both boats fell off their foils when the wind lightened, and both struggled to get airborne again. But the kiwis were the first to do so, and got the win .

The giveaway with the smaller foil, compared to others AC75's, lies in the ballast bulb. The designers have to achieve a total foil weight of 1378-1385kg and the wing itself must weigh 921kgs.

The tradeoff with smaller foils is that while they have less drag, they also have less opportunity to "lose" the required 921kgs, and the designers strategy is to take up the weight difference in the ballast bulb, and shape the bulb in such a way that it holds the maximum required weight, and offering as little drag as possible.

Working out these trade-offs to get the fastest boat is where the design time goes. And then there is weather moding - and whether the design should be optimised for the expected wind conditions in say March. Or if an all round performer is best.

We saw this in Fremantle in the 1987 Challenger Final where the all-purpose KZ-7 won 44 out of 53 races prior to the Challenger Final, but she was beaten by Stars and Stripes which was moded for the heavier breezes expected in late January/early February 1987..

Adding to the complexity is whether the foils have anhedral wings as discussed in sail-World for foiling Moths back in 2014. The new (port side) foil for ETNZ appears to have more of a downward angle, than the starboard foil.

There are no clear answers on foil design and which is best. All the teams have highly qualified (usually at Ph.D level) hydrodynamic designers - and all have produced quite a different solution to the wing design question.

Emirates Team New Zealand has always been innovative, and latterly they have got it right - so what are they up to this time?

Certainly they have taken a route that is quite different to the other three teams.

The training session on Monday was conducted in fresh winds - which had the AC75's at full pace. Occasionally a hookup was offered, but unlike other teams the invitation was not accepted.

Luna Rossa dropped a buoy - about where the top mark was off Tamaki Yacht Club in the fifth and last race of the Round Robin for the Prada Cup.

They went through their routines, and it's interesting to see the Italians now training on the new combined B, C, D course area, rather than just towing through it before heading north through the Rangitoto Channel, or east to The Paddock or Course E.

In the end Luna Rossa headed off to Takapuna and could be seen doing some windward-leewards and were obviously speed testing.

The New Zealanders, made some use of the Italians racing mark while it was there, but after the Italians had gone, to towards the end of the session went further up into the harbour, making some brave high speed runs.

Brave, because Burling and friends started off in a position where they were aimed at the "picket fence" that used to protect Okahu Bay, when it was a mooring area.

In 18kts plus of breeze the AC75 is very fast - probably sailing in the high 40kts maybe seeing 50kts - and on their course had to gybe to avoid hitting the wall. That's certainly putting some pressure into the training, with no option to pull out, or have second thoughts about getting the timing right.

They did a few of those - it was impressive to watch.

The now "Picket Fence" used to be called "The Wall" and over the years has been a very formative part of the development of young, and not so young, 12 and 18ft skiff crews, as they sailed the traditional leg downwind from the former Compass Dolphin to a rounding mark off Orakei Wharf which always required a high-speed gybe to make the Orakei Wharf mark.

It was always great spectator sport to see the crews three on the wire, fully powered and often over-powered, as they tried to get as far down the wall and as close to it as they dared. Having an AC75-style rudder ventilation could result in the crew literally hitting the wall, or getting their bowsprit skewed through it. More likely was a regulation capsize - and lengthy recovery. On a good day the crew would just it just right - getting far enough down the wall to be able to make the Orakei mark in one gybe - and maybe pick up a race changing lead on your rivals.

Against that backdrop, it was heart in the mouth stuff as the Kiwis turned at the top of their windward work, and Burling aimed at the wall, sprinting through the Valley of Death - as the AC75 accelerated from 35kts upwind to close to 50kts downwind.

The period to watch after an AC75 is 10-5 secs after a gybe/tack. That seems to be the transition period as the foiling forces transfer to the new foils, and the old foil comes out of the water.

Each time they ran this manoeuvre Te Rehutai, the Kiwis did a flawless performance - but then they had to as the consequences of failure to too terrible to contemplate.

It's a hell of a way to put your crew under pressure - and full marks to the Kiwis for putting themselves under pressure in this way.

The debate continues over whether Luna Rossa with their twin helmsmen setup is working, or if the Italians should go with the flow and have the helmsman switch sides each tack/gybe.

In the Bermuda context we recall Jimmy Spithill, after Oracle Team USA won the Qualifiers, telling the media that ETNZ should switch from having Burling helm the boat, and calling tactics to the way it was done on Oracle - who had Tom Slingsby dedicated to calling the shots and helping with grinding (including the hybrid grinder downwind - behind the helmsman).

We also remember the sight of Nathan Outteridge slipping during a tack in the Challenger Finals and going feet first into the Great Sound - and it was race over - and maybe Final over for Artemis Racing.

On the water, to our eyes, the traffic around the back of the boat, the better. It reduces windage, and aero-drag. There is less chance of a crew member going over the side.

The legs are quite short in the AC racing and the less crew movement the better. It also means that the helm positions can be customised for the different helmsman. All teams keep a man to leeward in the aft cockpit to handle the helm transition, to keep a lookout, and provide tactical and wind spotting input - plus warning of a leeward same tack boat that shifts into high mode.

You don't see these nuances on TV - simply because they never hold the shot for long enough.

And to our eyes the choreography of changing from stand-in helmsman to regular helmsman always looks slightly awkward, and there doesn't seem to be a compelling reason for doing this while the AC75 is travelling at 40-50kts.

All teams have made design decisions, made a long time ago, around how they will handle helm transition, grinding positions and a host of other crewing dynamics.

The margins between the three Challengers at least don't indicate that one system is any better than another. Maybe we will have a more definitive answer come the Ides of March.

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