June-July 2020

The Foiling
Revolution
Words by ROY ESPIRITU
Photographs as Credited
The science of watercraft is being pushed to the
limits, allowing sailboats to sail more than twice
the speed of true wind, and all sorts of watercraft
getting sustained higher speeds more than ever
before. All of this is because of foiling, the revolution in
water propulsion that started in the early 21st century and
is now gaining popularity and used
on all kinds of water craft, from surf
boards to 75 foot America’s Cup racing
yachts.
In Fluid Dynamics, a foil is a solid object
with a shape such that when placed
in a moving fluid at a suitable angle
of attack, the lift (force generated
perpendicular to the fluid flow) is
substantially larger than the drag (force
generated parallel to the fluid flow). If
the fluid is a gas, the foil is called an airfoil or aerofoil, and
if the fluid is water the foil is called a hydrofoil.
Hydrofoils are all about reducing drag, reducing the vessel’s
wetted surface allowing it to go faster. The International
moth, a developmental class of singlehanded sailing dinghy,
has seen the most widespread use of lifting foils in sailboats
and was instrumental in technological development as well
as popularizing foiling watercraft. Being a developmental
class and not a one design class meant that innovations
came quickly and were adopted widely. Without the
innovations that came from the moth class, foiling on
sailboats probably would have progressed much slower.
Hydrofoils are all
about reducing drag,
reducing the vessel’s
wetted surface
allowing it to go
faster.
Lift using foils have been the basis of flight for airplanes
since the Wright brothers, a wing after all is an aerofoil.
While for sailboats the principle of lift usually applies to its
sails creating lateral lift and allowing it to sail at a tighter
angle into the wind. While underwater foils like center
boards and rudders provide a similar lateral lifting force.
Hydrofoils lift boats out of the
water, thereby reducing drag, less
surface friction means more speed.
Traditional sailboats like the Laser
and the Optimist have been fitted
with foils with varying degrees of
success. Flying above the water on
foils is simple in principle but not
that easy to implement, there are so
many variables, it can be a challenge
to keep track of them all on complex
boats like the International Moth
The Moth has two centerline T-foils, the forward foil is
located where the centerboard is, while the aft foil is on
the rudder. Ride height is controlled by flaps on the leading
foil that is connected to a surface sensor wand. The moth is
Hydrofoils as we used to know them,
the Aldebaran a fast passenger ferry
Marc Weiler
Hydrofoils have been around since the early 20th century,
in 1909 a hydrofoil called the Miranda IV was able to reach
a speed of 65 km/h with a 60hp engine. Commercial
applications started appearing in the 1950’s being used as
fast passenger ferries as well as military attack craft such as
torpedo boats and fast patrol boats.
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