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TOP LEFT The ‘conventional’ Moth has a
foil-equipped centre keel.
TOP Normal sailing mode – the leeward foil.
provides lift, the windward foil righting moment
ABOVE Stable sailing mode – maximum stability in
manoeuvres and difficult conditions.
LEFT Close-quarters tacking duels and rounding
marks will require great care.arm between the T-foil’s lift acting vertically and the
vessel mass acting downwards at the centre of gravity.
The slight differential in beam of the retracted and
deployed foils may shift the vessel’s centre gravity to
windward, which combined with the indicated 1,000kg
crew weight being offset to windward should increase
lever arm and hence the righting moment further.
It looks as if the T-foils are (nominally) set at about
20 o off horizontal to oppose the result of the rig’s
heeling force and the vessel mass acting vertically
downward. A component of that T-foil lift is also acting
horizontally, opposing the rig’s side-force to reduce
leeway. These main foils can be canted up and down, in
a gull-wing like motion, with the T-foils having trailing
edge trim-tabs to vary their lift rather than raking the
entire main board as was the case in the AC50 cats.
The T-foil on the rudder provides the fore-and-aft
pitch stability, presumably being trimmed underway
to maintain overall pitch control but, being only a
single rudder, the AC75s won’t have the option for
differential rudder forces to increase righting moment,
where the windward one pulls down and the leeward
one pushes up.
In stable sailing mode both foils are deployed such
that the T-foils are horizontal, producing maximum
vertical lift and, importantly, equally on both sides- so the yacht can’t fall over sideways, as long as it is
going forward.
With the rudder T-foil working to control pitch we
have a high-speed, three-legged milking stool that can
corner aggressively in the same way that ETNZ’s AC50
could in Bermuda.
We may see this mode either in the very light
airs when maximum lift is required, or in the very
heavy airs and lumpy sea state when high stability is
prioritised over top speed.
What we don’t yet know is to what extent the
control of the foils will be manual or automated, or
what sensors/inputs might be allowed in an otherwise
manual operation to maintain ride height. Given
the tightrope to be walked between embracing the
future and constraining cost, ETNZ will recognise that
measures to limit cost through limiting complexity
can actually drive up cost as design effort is directed to
circumventing the very rules intended to reduce cost.
So, the debate shifts to whether there is a tangible
difference between the foiling AC50 cats and the
AC75s, and should the cats have been retained
given the investment and experience gained.