Concept testing of a new mainsail design to be used in the America's Cup is underway.

After months of designing and simulating, the concept was put on the water in Auckland's Waitemata Harbour last week with a scaled down version of the sail rigged into a 22-foot non-foiling trimaran. The prototype was about a third of the size it would be on the AC75 vessel.

"Six or even of us working on this for the past six months have done a lot of simulation work but to see it in reality even in a small scale like this is a big step," North Sails and veteran America's Cup sail designer Burns Fallow said.

"We've done enough work on it to know it's a fast concept, but it's got to do the basic things like tack and gibe and make it do up and down – just silly little things like that before we commit to this for the next three years. It's quite a big step to take."

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The testing comes as Team New Zealand work toward finalising the AC75 rule ahead of the March 15 deadline, at which point they were expected to be able to share the rule with prospective challengers. It would then enter the public domain on March 31.

The concept being tested has a large "D" shaped section mast, developed and built by Southern Spars, with separate mainsails on either side, providing a smooth transition from the mast to sail in its aerodynamic shape.

The concept being tested has a large
The concept being tested has a large "D" shaped section mast, developed and built by Southern Spars, with separate mainsails on either side. Photo / Emirates New Zealand

Once the mast is in, the sail can be raised and lowered in conventional fashion, eliminating the daily work required to attach the hard one-piece wingsails used in the last two America's Cups.

Designers from Italy's Luna Rossa syndicate, the official challenger of record, and representatives from Southern Spars and North Sails we there to watch Team New Zealand's Glenn Ashby man the tiller for the two days of testing.

"Obviously this is a very early concept and test," Ashby said, "but the main purpose is validate that our thinking is heading in a realistic direction.

"It's a huge step towards the finalisation of the class rule."