Yachting World — February 2018

the new design was a compromise, two concepts in one – a

foiling machine, but a monohull at the dock.

And of the many questions that arose, among the most

common was: ‘How will the boat remain upright?’

From displacement to foiling

At rest, the 5.5m wide hull provides a certain amount of

inherent stability just as any normal monohull would.

This stability is supplemented by lowering both of the

curved foils, which will weigh around 1.5 tonnes each, to

provide a twin 3-tonne keel. It is claimed that this will be

sufficient to ensure that the AC75 is self-righting.

Generating stability under normal sailing conditions

will be achieved largely by the vertical lift that the leeward

T-foil generates as the boat moves through the water. As

the speed increases, so does the lift, and the main hull

raises clear of the water.

Like the move from displacement to planing, the point

of transition from displacement to foiling is central,

according to Dan Bernasconi, chief designer for Emirates

Team New Zealand and one of the key architects of the

new machine.

“Although the boats will be foiling most of the time it is

going to be incredibly important to have a hull which gets

you foiling as quickly as possible”, he said in a recent

interview with New Zealand website Sail-World. “That’s an

interesting design problem and a big part of the design

conundrum – how do you design a hull that gets you

accelerating well and then gets you up on to the foils

before your competition?”

Reducing the drag to improve your acceleration points

towards a narrower hull to reduce wetted surface area, but

this will also reduce the form stability of the boat.

Unlike the foils on the AC50s, which were not allowed

moving surfaces, the new AC75 will have a moving

element on the trailing edge of the T-foils like an aircraft’s

aileron or elevator, often referred to in the marine world

as flaps.

These elevators will control the amount of lift that the

foils are generating in much the same way as a T-foil does

on an International Moth. They may even have a self-

levelling device such as a wand to automatically control

ride height. The rudder will also have a trimming system,

again much like the Moth and the AC50.

“You still need control on both the main lifting surface

AMERICA’S CUP

‘It is vital to

have a hull

that Foils

as quickly

as possible’

But laying the foundations for the 36th America’s Cup

hasn’t been that straightforward.

The New Zealand team set the foiling agenda in the

build-up to the 2013 America’s Cup in San Francisco

before going on to win in 2017, so the incentive to

maintain their advantage is clear to see. Keeping a boat on

foils, at speed and in stable flight while turning sharp

corners took a great deal of clever thinking and

technology, something the Kiwis have in spades. Surely,

they would want to maintain this advantage?

Yes, but there was a twist.

They accepted assistance from Luna Rossa boss Patrizio

Bertelli in order get to Bermuda, and rumour had it that

his support came with a condition that if the cash-

strapped Kiwis were to win the 35th America’s Cup then

the next event would be in monohulls and the Italians

would be Challenger of Record.

Having achieved the victory, Team New Zealand

appeared to be caught between continuing what they had

proved to be so good at and being forced (apparently) to

turn the clock back to monohulls.

So when the first images were revealed it was clear that

Dan Bernasconi

Stable sailing mode

Maximum stability

in manoeuvres and

difficult conditions

Self-righting

in the event

of capsize

Above: the twin

1.5-tonne foils will

act as a 3-tonne

keel, providing

righting stability

Hamish Hooper/ETNZ

52 February 2018