Boating New Zealand — January 2018

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The class rule will leave hull design open to the teams to

explore. And given the predicted ‘take off ’ speed will be in

around nine knots of wind (rather than six knots for the

AC50s), there will in fact, be design effort directed at striking a

balance between a low drag (low stability) hull form.

A form that accelerates quickly and let the foils work sooner

versus a high righting moment (higher drag) form that allows

the power of the rig to be harnessed to drive the hull faster.

Thus was ever the dilemma in monohull design.

While there will be differences in structural loadings

(primarily a catamaran platform twisting transversely vs a

monohull bending longitudinally), and in the way the foils

attach and articulate, the fundamentals of an immersed foil

supporting the weight of the vessel while opposing the heeling

moment and side-force from the rig remain the same for both

cat and monohull. So why fix it if it isn’t broken?

The AC75 as presented has no keel in the traditional

sense, but two curved, hinging foils with a T-foil on the tip

of each and a single, centreline T-foiled rudder. The proposed

design stated no dimensions – that’s to come with the release

of the class rules in March.

But scaling off the images provided, and assuming 75ft/23m

hull length, this indicates the beam to leeward out to the T-foil

tips to be around 10m off centre and to windward its about

8m off centre. So, a nominal 20m beam on 23m length. Those

are catamaran proportions. But let’s look at existing foiling

monohull configurations.

The foiling IMOCA 60s have a canting keel, with a large

percentage of their total mass in a bulb on the end, and derive their

stability partly through the bulb pulling down on the windward

side, and partly from the foil lifting up on the leeward side.

If the AC75 were to adopt a similar configuration it would

require the yacht to carry a lot of ballast, and that would be

counter to effective foiling, which requires the vessel mass to be

reduced as much as possible.

There had been concepts of a 75ft version of a foiling Moth

LEFT The foiling IMOCA 60s have

a cantling keel and derive their

stability partly through the bulb

pulling down on the windward side.

Whether you’re racing up the coast, or around the globe, you

can rely on North Sails cutting edge technologoy to outperform

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