We've talked about catamarans and

trimarans, different hull configurations

that try to solve the speed and

stability problem in different ways.

Hydrofoils take a completely different

approach. Instead of adding more hulls,

they get rid of the hulls' contact with

water entirely.

At low speed, a hydrofoil boat looks

normal. It floats like any other vessel.

But once it picks up the speed, the hull

lifts out of the water. It rises up on

underwater wings and flies just above

the surface.

A conventional hull sits at the surface,

pitching and rolling with every wave.

The foils pass through below the

turbulence. The hull barely moves.

To understand how hydrofoils work, it

helps to start with planing hulls. A

displacement hull pushes through water.

It's supported by buoyancy, the water it

displaces. The faster it goes, the more

drag it creates. There's a practical

speed limit when adding more power. Just

creates bigger waves instead of more

speed. A planing hull works differently.

At low speeds, it floats like a

displacement hull, but as speed

increases, the hull starts to ride up on

its own bow wave. Water flowing under

the hull creates hydrodynamic lift. The

hull rises partially out of the water

and starts skimming across the surface.

Less hull in the water means less drag,

which allows for higher speeds.

But even a planing hull has limits. The

hull is still pushing water. It's still

creating waves. And at very high speeds,

the forces on the hull become enormous.

Slamming into waves, structural stress,

fuel consumption. Hydrofoils take that

concept further.

Instead of relying on the hull itself to

generate lift, they use underwater

wings. Foils attached to the hull by

struts. The foils are designed

specifically to generate lift as water

flows over them. As the foil moves

through water at an angle, it deflects

water downward. The foil pushes water

down and the water pushes the foil up.

The faster the boat moves, the more

water gets deflected and the more lift

the foils produce. At low speeds, the

foils generate some lift but not enough

to raise the hull. The boat operates

normally either as a displacement hull

or if it's a planing design, it might

start to plane. But as speed increases,

the foils generate more and more lift

and eventually the lift force exceeds

the boat's weight. The hull begins to

rise and it keeps rising until only the

foils and their struts remain in the

water. The hull is flying, completely

clear of the surface, supported entirely

by the foils.

This is why hydrofoils are so much more

efficient than planing hulls at high

speeds. A planing hull reduces drag by

lifting part of the hull out of the

water. A hydrofoil eliminates hull drag

entirely. Hydrofoils show up in

different forms depending on what

they're designed to do. The simplest

versions appear on surfboards and

kiteboards. A foil attached to the

bottom of the board with a single strut.

As the rider picks up speed, the foil

generates lift and the board rises out

of the water. The rider is flying above

the surface, carving turns with much

less drag than a conventional board.

These use surface-piercing foils. The

foil penetrates the water surface. As

the board rises, less of the foil stays

submerged, which reduces lift. If the

board climbs too high, the foil breaks

the surface, lift drops and the board

settles back down. If it drops too low,

more foil enters the water, lift

increases and the board rises. The

system is self-stabilizing.

The rider controls it by shifting weight

and adjusting speed, but the foil itself

naturally regulates height. Small

recreational hydrofoil boats work in the

same way.

Surface-piercing foils, simple

mechanical systems, and no computers.

They're effective in calm water, lakes,

protected bays, but they struggle when

waves get large. The varying depth of

foil submergence in choppy water makes

the ride inconsistent. As hydrofoils

scale up to larger crafts, like racing

yachts, high-speed ferries, military

patrol boats, their design changes.

These boats use fully submerged foils.

These foils sit entirely below the water

surface, operating at a fixed depth

beneath the hull.

This solves the rough water problem

because the foils are deep enough that

the surface waves don't significantly

affect them. Lift will remain relatively

consistent. A ferry running in 2-m seas

can maintain a smooth ride because the

foils are passing through water below

the turbulent surface layer.

But fully submerged systems need active

control. Sensors constantly measure the

boat's attitude and motion. A computer

processes that data and adjusts the

foils in real time. Hydraulic actuators

change the angle of attack or move

control surfaces to keep the boat level

and at the correct flying height.

There's also the propulsion problem.

When the hull lifts out of the water, a

conventional stern-mounted propeller

would be too close to the surface or

even break through it. So, the solution

is to mount the propellers on the foil

struts themselves, keeping them

submerged even when the hull is flying.

Some designs use water jets instead.

These are pump systems that draw water

in and expel it for thrust, which can be

positioned to work regardless of hull

height. The Candela P-12 is the current

benchmark for this. It runs in open

coastal waters at 20 to 25 knots,

smoother than a conventional ferry doing

half that speed. But, the system is

complex.

Racing hydrofoils, like the ones used in

the America's Cup, push the technology

even further. Fully submerged foils,

active controls, aggressive foil shapes

that are designed for maximum lift and

minimum drag.

These boats can exceed 50 knots, flying

a meter or more above the water.

But, they require skilled crews and

constant adjustments to keep them

stable. So, we have two types of foils

here. Surface-piercing foils for calm

water and simplicity, fully submerged

foils with active control for rough

water and performance. The idea of using

hydrofoils on ships isn't new. In the

1960s and '70s, the US Navy saw them as

a solution to a specific problem. Soviet

submarines were getting faster.

New nuclear-powered submarines could

reach over 40 knots submerged, fast

enough to outrun torpedoes and evade

most surface ships. The Navy needed

ships that could keep up. Hydrofoils

seemed like the answer.

They allowed high speed, the ability to

operate in rough seas, and the potential

for anti-submarine warfare.

The Navy built several prototypes,

including the USS Plainview. At 320 tons

and over 200 ft long, it was the world's

largest hydrofoil at the time. The

Plainview could reach 50 knots

foil-borne, powered by these gas turbine

engines, driving propellers mounted on

the foil struts. The ship used fully

submerged foils with an automatic

control system, maintaining level flight

even in 10-ft waves. But, the program

didn't last. Aircraft turned out to be

more effective for hunting submarines.

And the hydrofoils themselves were

expensive. The Navy did put one

hydrofoil class into production, the

Pegasus class.

Small air patrol boats designed for

coastal operations, but only six were

built and they were retired after 10

years. High operating costs and no clear

mission justified keeping them. While

the military moved on, commercial

operators found applications where

hydrofoils made sense. The Soviet Union

operated hundreds of hydrofoil passenger

ferries starting in the 1960s. Designs

like the Raketa and Meteor ran on rivers

and coastal routes, carrying passengers

at speeds that cut travel times

significantly. Some are still in service

today. Passenger ferries remain the most

common use for hydrofoils. Predictable

routes, consistent speeds, and

passengers who value the smooth ride.

And now, some are even electric. The

efficiency of foil-borne flight makes

battery-power viable in a way that isn't

right for conventional hulls. The

Candela P-12 can carry 30 passengers at

25 knots with a range of over 50

nautical miles on electric power. A

conventional hull at that size couldn't

come close. Racing is the other area

where hydrofoils dominate. America's Cup

yachts use hydrofoils to reach speeds of

over 50 knots. Foiling dinghies and

moths, which are small racing sailboats,

have become standard in competitive

sailing. The performance advantage is so

significant that conventional designs

can't keep up. But for cargo shipping,

for naval combatants, for general

purpose vessels, hydrofoils haven't made

inroads there. Their complexity, cost,

and operational limitations outweigh the

speed advantages for most applications.

Catamarans, trimarans, and hydrofoils,

all of them are trying to solve the same

problem, but none of them solve it

completely. The right one depends on

what you're actually trying to do.

Thank you all for watching. Hopefully

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