Yachting World — February 2018

and the rudder,” said Bernasconi. “Whether we rake the
whole rudder as we did on the AC50 or whether we control
trim tabs on the rudder, which is fi xed in rake, is still
something we are considering. We are looking at both
options for that.”
But for many people, the complexities of achieving
stable fl ight still don’t address the big issue. How quick
will the new boats be?
Here the answer at both ends of the scale is surprising.

Topping 50 knots
The AC50s weighed just under 2.5 tonnes and could pop
up onto their foils in just 6 knots of breeze – at least the
Kiwis could. A 7 tonne boat like the AC75 is going to
struggle to match this, but nevertheless it is anticipated
that it will fl y in 9 knots of wind.
According to Bernasconi, in 9-15 knots of breeze the
AC75 could be quicker than an AC50. And when it comes to
top speeds, the monohull looks like it will be capable of
speeds in excess of 50 knots.
So how come a heavy monohull can be faster than a
fl yweight multihull?
The answer lies with the huge righting moment that the
AC75 generates compared with her displacement. And
there are three key aspects that make up this huge
potential power output.
Just like the physics of a catamaran when the windward
hull lifts clear of the water, the moment the AC75s
main hull breaks the surface the boat will be
generating its maximum righting moment as the i

Head to head: the AC50 foiling catamarans raced in Bermuda

1,500kg

2.7m

1.5m

Righting moment = 5.7m

9.9 x 1,500

Righting moment =

14,850 kg.m

Righting moment =

5.7 x 1,000

Righting moment =

5,700 kg.m 5.7m

Crew

1,000kg

Boat

7,000kg

Righting moment =

5.7 x 7,000

Righting moment =

39,900 kg.m

Lift

2.7m

3.0m

5.7m

RIGHTING MOMENT = 1+2+3

RIGHTING MOMENT = 60,450KG.M

1 RIGHTING MOMENT (BOAT)

2 RIGHTING MOMENT (CREW)

3 RIGHTING MOMENT (FOIL WINDWARD)

Dock Configuration

Both foils down:

maximum stability;

minimum beam

Sander van der Borch/ACEA 2017

February 2018 53