America's Cup Rialto: November 20 - Kiwis throw down the gauntlet to Challengers
by Richard Gladwell, Sail-World NZ 21 Nov 2020 06:13 PST
22 November 2020
In flight Te Rehutai appears to be an aircraft - Emirates Team New Zealand AC75 - Te Rehutai - November 20, 2020 © Richard Gladwell / Sail-World.com
America's Cup champions, Emirates Team New Zealand, turned on an impressive first sail performance in their Version 2 AC75, Te Rehutai just over 18 hours after her Thursday evening launch ceremony.
The radical hull design proved to be quick out of the box, easing the fear of many Kiwi Cup fans that the defending champions had gone a step too far this time.
In a briefing given pre-launch to a handful of Kiwi sailing media, design chief Dan Bernasconi was asked about the light airs performance of their new AC75 Te Rehutai - given the aft end of the hull sitting above the group appeared to have a flat wetted surface area the size of a tennis court.
ETNZ's Bernasconi was very coy about giving a windspeed when their AC75 would start foiling. "I don't want to give an exact wind speed, but I think at the bottom end of the range at where we are allowed to race, it is going to be really tricky conditions for sailors, and the differences will be most obvious if one team gets up on foils and the other doesn't."
On Friday morning that very point was well demonstrated as the large fleet of chase boats and jogged around the Rangitoto Channel in windspeeds that varied from 1-5kts. Certainly too light to race.
Surprisingly Te Rehutai was moving relatively easily in these conditions. Still, after a couple of criss crossings of the Rangitoto Channel, the red and black hulled AC75 got a sniff of an advancing southwesterly - around 6.5kts and was up and off out accelerating the following chase boats - going from 6kts to over 25kts in less than a few seconds.
There is no denying that the latest AC75 to be launched has a complex hull shape, with a flared bow somehow evolving into the flat tennis court after underbody.
A cleverly worked contour line marks the perimeter of the flared bow and then swoops down to the waterline to form a hard chine running right aft.
It is this chine which seems to be the design signature of Te Rehutai. At rest and fully loaded with crew she floats perfectly with just a perceptible gap between it and the surface of the water. Up close the noise of water slapping the flat underbody is clearly audible - indicating that there is some airspace under the back end of the boat. It should not be sticky.
Sailing with just a few degrees of heel - either to windward or leeward - the chine clears the surface very readily and daylight can be seen running right into the bow.
When slightly heeled, the boat drops quickly sheds a big chunk of her wetted surface and hull drag. Te Rehutai then runs on her immersed chine and skeg - looking eager to get onto her foils, while staying in level trim fore and aft - essential to stop her stern sections dragging during lift-off.
She gets airborne in what appears to be 5kts of windspeed and steady pressure, maybe less. A look to windward at another AC75, a mile or so away - and in what should be the same wind pressure - reveals it is still sailing in displacement mode, and with a Code Zero set.
Helmsman Peter Burling rather cheekily heads up in their direction - just to make a point. He gets halfway before pulling away into what looks, from bow-on, to be a dry foiling gybe, before doing a fast run down the Rangitoto shore in a steady breeze that must be all of 6kts.
After running out of wind off Takapuna, the breeze of the day fills in, again at a steady 6kts or so. Te Rehutai sets off foiling down the East Coast Bays and then out into the Hauraki Gulf, sailing fast and looking like she has just been handed the baton in the America's Cup relay from the team's V1 AC75, Te Aihe.
Despite a break of five weeks, Burling and his crew don't miss a beat all day and look like they could race in the Christmas Cup if it were sailed tomorrow.
Their performance is a marked difference to the other teams' first sail, punctuated by frequent stoppages while adjustments were made. ETNZ's breaks today seem to be more to give the grinders a rest/switch over than to effect running repairs.
Deck and cockpit design explained
One of the more obvious features is the hard chine - most noticeable where it terminates at the aft end of the AC75.
"The chine is to maximise the stability of the boat at rest," explains Dan Bernasconi, and alludes to an AC75 rule requirement regarding minimum volume.
Moving above deck level he is asked to explain the design rationale with the rounded nose, the "spillway" running through the centre of the boat, and the crew being housed in two deep pits each side, and well wide of the centreline.
"When Boat 1 came off the foils, there was a huge amount of spray, and we got a lot of white water into the boat which hit crew displays. It's not a huge consideration with this boat, but we think it will be a lot better behaved in that respect."
Asked whether the design team had leaned more towards aerodynamic performance than hydro, says it is probably more aero than hydro.
"Ultimately, it is about lap times. Maybe in the perfect race, you enter the starting box on the foils and never come off - so maybe we prioritise aerodynamics more. But then we try and make the best hydrodynamics that we can within the window of good aerodynamics."
All Version 2 AC75's have a different approach to crew trenches and protection. Te Rehutai's are shoulder-height deep raising the issue of whether crews will swap sides between tacks and gybes, given the depth of the crew trenches.
"Shoeb and I went down there the other day, and they didn't find us until the next day", Dalton quips.
Helmsman Peter Burling says from what they have been able to see, all the crews except Luna Rossa have the same approach to crossing the boat - where usually three of the afterguard move across, leaving the grinders behind. "We won't know until the Christmas Cup when there are several cameras in each cockpit and another five looking at other bits and pieces on the boat - so it will be all out in the open"
"Everything on this boat is a lot more optimised than the first as we have had a lot more time to think about what is required. Having sailed the first boat before the design for this one was finalised, we had a lot more to lean on when prioritising the trade-offs."
"The bow has more volume than Boat 1", Burling continues. "Every team has broken it down differently. The cockpits are designed more for aerodynamics than coping with water issues", he says.
Water coming over the bow has a clear flow down the spillway - the wide channel that lies between the two crew areas. It also serves as a landing zone, or end plate for the mainsail and jib - which now make contact with the hull, below gunnel level. In Te Aihe the foredeck was extended back through the cockpit, forming a centre console - which is now no more.
Asked how much Te Aihe has been cannibalised for parts to be fitted onto Boat 2, Bernasconi responds: "a lot of electronics came off Boat 1, and some of the hydraulics - but not a lot."
It becomes evident that ETNZ has no intention of sailing Te Aihe against Te Rehutai. "It would take a lot of work to put it back together", says Burling.
Chief Operations Officer, Kevin Shoebridge runs the sailing side of ETNZ - leaving the commercial side to a long-term buddy, Grant Dalton.
"We learned a huge amount from Te Kahu", says Shoebridge. "It's a really cool boat for developing because everything is smaller and is quicker to develop."
Asked about crew security on the AC75 given the expected 50kt plus speeds, Shoebridge responds that these boats are very safe compared to the two catamaran classes used in the 2013 and 2017 America's Cups.
"This boat has been through a complete structural testing at full load," he explains, "so we can go out with confidence on Friday. We'll do some checks and then go sailing.
"These are not the sort of boats on which you can you can take it easy - once you sheet on, you're off - there is no half pace."
Shoebridge says the team lost 8,000 manhours in the five weeks of COVID19 Alert 4 level, which a significant portion of the 75,000 manhours to build, but the team have still managed to hit the same launch deadline that was set a year ago - pre-COVID19.
"We ran 24 hour shifts at the boatyard to get those hours back we went from 40 builders to 50. It caused a big rejig of our program, and the lost racing time meant we had to make more use the design tools and simulator."
"The lucky thing for us was having the 12 metre test boat (Te Kahu) - that kept everything rolling, even though Te Aihe was stuck on a ship for five months. We didn't really stop the program."
Interestingly he adds that they learned more about the performance of Te Aihe, while she was on the ship, via the various simulators, to add to their four and a half months of sailing data from the Version 1 AC75.
A Cup winning design
What design will win the Cup?
Dan Bernasconi puts the deciding design differences down to the three principal factors.
"The Cup will come down to three things, the foils, the hull and the mainsail."
"The rule allows you to have six wings, or three pairs, but they don't have to be identical port and starboard," he explains.
"We are locked in for our race wings now, because they take four months to build. All teams will very soon be launching their race wings."
There are a diversity of wing designs in the fleet, and clearly, there is no ideal solution. Otherwise, the highly experienced, multi degreed, and multi-disciplined design teams would have settled on a near-identical solution. The wings and flaps are all made from milled steel, and aside from the shape the other trick is to get as much lead into as small a volume as possible to achieve the rules specified weight.
"The wings could easily determine it", Bernasconi continues.
"If the wind is light, then hull hydrodynamics could be a big part of it. If it is a stronger breeze, the focus will shift to aerodynamics. They could be a big part of it."
"The mainsail design approach is also different between the four teams."
"The rig is a big development area", he explains. "It is the only thing pushing the boat forward - everything else is pushing it back. We have to put as much effort into that side of the design as much as the foils and hull." There are reports that the Kiwis developed a full size test platform for their mast and mainsail, inside the shed to see it under full load at ground level.
The double-skinned mainsail was first announced for the AC75 class, marking a departure from the hard wingsails used in the 2010, 2013 and 2017 America's Cups, and the soft sails used prior to 2007.
The AC75 move has pulled sail designer back into the America's Cup teams. Soft sails (mainsail, job and Code Zero), to an open design within size restrictions, have not been a part of the Cup for the last 13 years. Clearly, there is a lot of catch-up that can be done. But the experience of previous campaigns of foil wings, hydrodynamics and aerodynamics are a knowledge extension, to a large extent from the previous three America's Cups.
"The hard wing sail really gains when you are trying to generate a lot of power, at say, take-off, and at wide apparent wind angles," Bernasconi explains, when asked about the benefits of a hard wing sail used in the 2010, 2013 and 2017 Cups compared to the double skinned mainsail used on the AC75.
"But once you are up and foiling there is little difference in power between the hardwing and double skinned mainsails. The hard wing has a high lift coefficient because of the two split elements, which is not the case with a soft wingsail."
"Once you are up and going, there's not much difference at all", he adds.
That assertion would appear to be confirmed by the fact that the hard winged AC72 and AC50 foiling catamarans were hitting the 50kts top-end speed - which has been hit by all four AC75 foiling monohull teams - using double-skinned mainsails.
From what we saw as Te Rehutai took off in light winds, it would seem that the New Zealand design team may have made a breakthrough in the performance of the double-skinned mainsail. The engine above is generating substantial power - explaining the boat's easy speed through the water when off the foils. Of course, the extra horsepower comes in very handy at lift-off and transitioning into foiling mode when the AC75 travels at four times the true windspeed.
Bernasconi wouldn't explain whether the team used a main boom, or what was different in that regard. A day later, on the water, the change was evident with very solid, an angled boom structure clearly designed to apply more pressure to tension AC75's mainsail.
Given the design variations in the Class of 2021, the 2021 America's Cup looks set to be decided by the design team have got the right answer to the AC75 questions. It would be surprising if the various design packages all had the same performance.
While the 36th America's Cup will be sailed in the fastest and most radical boats in its 170yr history, we can expect to find out the answer to the likely winner in the traditional way - who has the advantage five minutes into the first Cup race.
That moment occurs at 4.05pm on March 6, 2021.
Until then the guessing games continue.
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