Professional BoatBuilder - April-May 2018

24 Professional BoatBuilder

HIGH SPEED: Foiling, Part 1

performance], you’d have to eliminate

mechanical control types and human

interaction,” he said. “Control really is a

software issue. It’s a programming

problem.”

The pioneers

Air and water are fluids with vastly

different densities. A foil running

through them produces lift based on

similar principles—Newton’s Laws of

Motion and the Coanda effect. It’s not

surprising that many flight pioneers in

the early 20th century also experi-

mented with hydrofoils. In Italy,

Enrico Forlanini, an aeronautical

engineer, was perhaps the first to

achieve full flight on ladder foils in

his surprisingly modern-

looking craft Idrottero,

which zipped across Lago

Maggiore at 37 knots as

early as 1906. Photo-

graphs reveal that he

experimented with vari-

ous propulsion systems,

including two counter-

rotating air screws and a

strut-drive that extended

downward into the water

about amidships to drive

a propeller. In the U.K.,

John Thornycroft used

foils to boost speed in his

designs.

On the U.S. side of the

Atlantic, brothers William

and Larned Meacham

tested hydrofoiling con-

cepts during the first

decade of the 20th cen-

tury, including a surface

follower that acted as

a height sensor. They

applied for and received

several patents for their

work, which they called,

strangely enough, a hydroplaning boat.

Inventor Alexander Graham Bell and

his partner Frederick W. “Casey”

Baldwin experimented with stacked

kites and hydrofoils on the Bras d’Or

Lakes in Nova Scotia. After meeting

Forlanini in Italy in 1911 and getting a

ride on his foiler, Baldwin designed

a few inches or a couple of feet at most.

“Complete (optimal) control is not

possible with a mechanical system that

is based on angle input or visual input

from the operator,” said Chris Pappas,

an engineer at Naiad Dynamics, a

U.S. company that develops and imple-

ments stabilizing and control systems

for large military and commercial ves-

sels. “The analogy is fly-by-wire sys-

tems used on airplanes today. The pilot

moves a stick, but the computer is cor-

recting to steer the plane in the direc-

tion requested. A human cannot sense

rate, because by the time he is feeling a

change in attitude (angle), the motion

has already occurred, while computers

can use the rate and acceleration input

from a sensor and do the math to pro-

vide a solution.” Pappas’s sentiment is

shared by Paul Bieker, a Seattle-based

yacht designer who has been instru-

mental in all of Oracle’s America’s C u p

campaigns. “Once you go to a fly-by-

wire control system, input and output is

controlled by data bus. [For optimum

For a long time, the so-called air-

plane configuration dominated hydro-

foil design, with the main foil(s) for-

ward and small stabilizing foil(s)

under a narrow stern. This works well

on boats with a forward center of grav-

ity. Beginning in the late 1950s, the so-

called canard system gained traction.

It puts one foil forward to support the

bow, to initiate turns when foilborne,

and to control foiling height; the main

foil aft carries most (70% to 80%) of

the boat’s weight and provides lateral

stability and roll authority.

Control systems

The third major factor determining

hydrofoil performance is motion and

ride control. The wilder the

water and the smaller the

craft, the more challenging

the task. Small sailboats

rely on passive control via

foil shape (the Olympic

Nacra 17 foiling catama-

ran or the A-class cata-

marans), or active mechan-

ical controls where a wand

determines the foils’ angle

of attack and the hull’s

elevation above water.

(For more on wand con-

trols and the development

of sailing foilers, see

“Takeoff Window,” PBB

No. 139.) While simple

and cost-effective, such sys-

tems require user input and

response, which is not as

smooth and consistent as a

computer program med to

manage pitch, heave, yaw,

heel, and elevation based

on the inputs from sen-

sors—gyroscopes, accel-

ero meters, IMUs (iner tia

measuring units), radar,

optical or acoustic sensors, knot-

meters, GPS, and positioning sensors.

It’s easy to imagine that modern foil-

ing boats are destined to operate

less like conventional boats and

more like airplanes, though the latter

have thousands of feet of altitude to

play with, while a foiling boat has but

Designer Alexander Sahlin drives an early prototype of the Swedish

Foiltwister. It uses mechanically controlled foils, including a wand

at the bow to manage foiling height.

“Control really is a software issue.

It’s a programming problem.”

—Paul Bieker

COURTESY FOILTWISTER

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