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Coastal Turmoil - Winds, waves and tidal races by Ken Endean
Coastal Turmoil - Winds, waves and tidal races by Ken Endean

America's Cup Rialto: AC75 leaping and nosediving in the big breeze

by Richard Gladwell Sail-World NZ 15 Sep 16:06 BST 15 September 2020
American Magic - Waitemata Harbour - September 14, 2020 - 36th America's Cup © Richard Gladwell / Sail-World.com

American Magic's AC75 Defiant performed some spectacular gymnastics on Monday - when training in winds well above the racing limit.

Leaving the dock in the Wynyard Marina at 11.00 am an hour after Emirates Team New Zealand's Te Aihe, the American Magic's first AC75 Defiant set up off Browns Island in the area of America's Cup Course C. The breeze kicked in as forecast - building to 23kts gusting over 25kts.

It did ease away briefly - giving way to a rain squall gusting over 35kts in the early afternoon.

American Magic started another beat towards North Head, as Emirates Team New Zealand had called it a day, and packing the boat away off Motuihe Island - about 5nm away from North Head. In this phase, the breeze eased away, being recorded as low as 10kts on the Northern Leading recorder before increasing over a short time to 25kts gusting 35kts.

In the buildup to the "sky leap" incident, American Magic looked a bit wobbly 7 secs after coming out of a gybe, slowing down and going into windward, before recovering and sailing fast and under control, albeit a little high.

From timings taken off a frame by frame replay, it shows that a few seconds later, the fine high speed spray off the rudder started showing solid white indicating the rudder was ventilating. It all came apart very quickly as the rudder ventilation continued - losing lift. The stern sank with an impressive rooster tail of solid white spray emitting from the rudder. It is a familiar scenario seen on at least three of the AC75's - caused by a design flaw that will be fixed in the second generation boats.

With the boat rotating skywards as the stern sank, the angle on the foil arm and wing became more acute, and the boat lifted like an aircraft in take off. Defiant continued in that stance for just a few seconds, before the boat slowed, the lift from the foil arm and wing reduced, and gravity took over as the AC75 sank back into the water, near upright, but with a massive shower of spray to leeward.

The chase boat closed in. No doubt there was a quick chat about what happened, and then Defiant, true to her name, sailed off.

She crossed ahead of a packed away Te Aihe, before hardening up and starting another lap of racing practice, and heading into what was going to be the rain squall of the day, gusting to 35kts.

Arriving at her rounding mark about four minutes later, Defiant decided that caution was the better part of valour, and opted to ride out the still building rain squall, lying head to wind with her mainsail hoisted and attached to her chase boat. That in itself was quite a feat of seamanship in the conditions.

Apologies for the manual focus error in the next two shots - but they show some of the key points of the action

Back in action

Forty-five minutes later, Defiant was back into her race training, doing a run along the western side of Motuihe when she appeared to bear away, aiming for a race mark. Again there seemed to be a control issue this time caused by lack of downforce from the rudder. The wind readouts show the breeze looked to be a steady 20kts but with a relatively sharp and sudden gust to 25kts or so - which was probably the root cause of the ensuing nosedive. A bear-away is the most high-risk manoeuvre in the AC75.

AC75 rudders, unlike AC50's, are allowed to rake and yaw without limitation, to control the angle of attack of the rudder wing. In this situation, it appears that the rudder was not able to generate sufficient downforce to offset the force pulling down on the bow. AC50's generated around 500kg of downforce from their rudders - which is suddenly released if the rudder breaks the surface - as happened with Defiant.

Initially, there was a big shower of spray to leeward as the foil arm dug in and the bow went down, with the rudder unable to generate sufficient downforce to pull the boat out of a nosedive. Sailing physics took over and in she went, in a most spectacular fashion, lifting the rudder wing clear of the water.

This time it was the buoyancy in the bow that ended Defiant's spectacular display, assisting her in regaining directional control.

The photo sequence below was shot from about three or four miles distant

As with the previous incident, Defiant returned to a nearly upright position of her own accord while the crew collected their wits, followed by another conflab with their coaches and design team, while the other three team's recon hung in close.

Then she was off for a couple more runs, without further incident, before Defiant turned and sailed home - getting swiped by another rain squall as she entered the harbour.

While the shots of the two incidents look spectacular, we have seen it all before across several of the teams and their first-generation AC75's - none of which we expect to see sailing in the Prada or America's Cup.

Having watched many hours of AC75's sailing in Auckland for the past 12 months, it is apparent that class rule works well and has produced a high-performance boat that is both jaw-droppingly spectacular and seaworthy.

Despite the best attempts at nautical acrobatics from the AC75's of several teams, it would seem that the worst that can happen is that the boat will capsize - and can be recovered with the aid of a chase boat in less than five minutes.

At best it will return itself to an upright position and wait for the crew to decide what they are going to do next.

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