7 tonne machine becomes a lever off the T-foil, which is
around 5m to leeward. Multiply the weight of the boat by
the distance from its support and you arrive at a ballpark
figure for the righting moment around 35,000kg.m.
Then you have a crew of 12 who might collectively
weigh, say, 1,000kg riding in the flying hull. That adds a
further 5,000kg.m.
The third component is the additional righting
moment generated from canting the windward foil up to
weather. That’s like a trapezing crewmember on the racks
of an 18ft skiff; it doesn’t take a great deal of weight placed
a long way out of the boat to have a big effect.
Each elaborately shaped T-foil is expected to weigh
around 1.5 tonnes apiece. Canted out to its maximum
horizontal reach, the tip of the foil will be around 15m to
windward of the leeward T-foil. Once again, weight times
distance of this element could provide 15,000 kg.m.
Add the three righting moment components together
and you get a whopping 60,000kg.m.
That is double the potential power of an IMOCA 60, a
design that generates around 30,000 kg.m yet weighs
around the same at 7-8 tonnes.
In reality, the stability issue is more complex as therighting moment that the T-foil generates varies with the
speed of the boat, but the ballpark figures and simplified
analysis go some way to illustrating the enormous power
that is on tap for the new breed.
There is another key element in making these new boats
go quickly: drag reduction. Imagine the boat fully
airborne and riding on just two foils, the T-foil to leeward
and the single T-foil rudder astern. It doesn’t take a
mathematical genius to envisage the reduction in
hydrostatic drag compared with a 60ft monohull with the
a wetted surface area of a whale.
But the issue of drag plays on both sides of the
performance equation.
“Although an AC75 will sail at up to 50 knots and
potentially over 50 knots, it’ll come down to how you
reduce aero-drag at those really high speeds, and
particularly with the hybrid soft wing,” said Land Rover
BAR’s new CEO and latest recruit, sailor, designer and
all-round Cup expert Grant Simmer.
“The rig height is still being decided, but we are told that
the preferred solution is a D spar with a double
membrane sail behind it with a camber inducer at the top
of the rig. It will be halfway between a soft sail and a solid
wingsail.”
With ten Cup campaigns under his belt of which four
resulted in victory, Simmer is used to thinking big
and radical.
“The really big catamaran Alinghi V [he led the
Alinghi design team] and Oracle’s the big trimaran [a team
he would eventually run], they were really at
the limit of the engineering in the yachting world at
that time.AMERICA’S CUP
Above: The AC75
will have a moving
element on the
trailing edge of
the T-foils
Right: Peter de
Savary’s 1988
America’s Cup
project Blue Arrow
was an attempt to
dispense with a
keel and generate
righting moment
using foils on
outriggers‘IT WILL COME
DOWN TO HOW
YOU REDUCE
AERO-DRAG AT
HIGH SPEEDS’
HarryKH/ Land Rover BAR Grant Simmer
Nick Rains/PPL54 February 2018