How Does a Hydrofoil Work? Lift, Speed and Foils Explained
How does a hydrofoil work? Learn how an underwater wing lifts a board out of the water, why speed matters and what it means for your eFoil. Simply explained.
Key Takeaways
- A hydrofoil is an underwater wing. Moving water flows over it, the wing turns that flow downward, and the water pushes the wing up.
- Lift grows with the square of the speed: twice the speed gives four times the lift. That's why a foil only lifts a board out of the water above a certain speed.
- Wing size, wing shape and the angle of the wing to the water all change how much lift you get.
- The popular "equal transit time" explanation is wrong. Even NASA says so.
- On an eFoil, a motor supplies the speed, so you can fly without waves, wind or a boat.
You've probably seen a foil board gliding above the water and wondered what holds it up. The answer isn't magic and it isn't complicated. It's the same physics that keeps an airplane in the air, just in water.
A hydrofoil is a wing that works underwater: when it moves through the water fast enough, it produces an upward force called lift that raises the board, boat or rider above the surface. In this article, you'll learn where that force comes from, which factors control it and how it applies to an eFoil.
Only the mast and the wings stay in the water. The board itself hovers above the surface.
What Is a Hydrofoil?
A hydrofoil is a wing shaped to lift when water flows past it. Attach one to a boat or board with a mast, and once you're fast enough, the hull rises out of the water. Less hull in the water means less drag, so the ride gets smoother and more efficient.
Engineers use two basic layouts, as the overview of hydrofoils on Wikipedia describes:
- Surface-piercing foils are V-shaped. Part of the V stays above the water, so the lift changes by itself as the boat rises or sinks.
- Fully submerged foils are shaped like an upside-down T. They stay completely under water, are less affected by waves, but don't stabilize themselves. Someone, or something, has to control them.
Foil boards, foiling sailboats and eFoils use the second layout. That's why a foil board feels so different from a normal board: you steer the lift yourself, with your weight and your feet.
How Does a Hydrofoil Create Lift?
A foil creates lift by pushing water downward. Because the wing is tilted slightly against the flow and curved on top, it turns the passing water downward. By Newton's third law, if the wing pushes water down, the water pushes the wing up. Pressure differences around the wing are the visible result of this turning.
Think of holding your hand out of a car window. Tilt it slightly upward and the air pushes it up. A foil does the same thing with a much better shape and a much denser fluid: water is about 800 times denser than air. That's why a foil the size of a small tray can carry a whole person.
Where many explanations go wrong: you'll often read that water has to travel farther over the curved top of the wing and therefore must go faster to "catch up" with the water below. This "equal transit time" idea is a myth. NASA's page on the incorrect lift theory points out that a flat or symmetric wing also lifts, and that the water on top moves much faster than this theory would require.
Pressure and flow turning are two views of the same event. The water above the wing speeds up and has lower pressure, the water below has higher pressure, and the wing is pushed from the high-pressure side to the low-pressure side. The MIT report on how a hydrofoil works shows how both explanations fit together.
What Controls How Much Lift a Foil Makes?
Four things decide how much lift a foil makes: how fast it moves, how big the wing is, how steeply it's tilted against the water and how it's shaped. Speed matters most, because lift grows with the square of the speed. Double the speed and the lift quadruples.
| Factor | What it does | What you notice on an eFoil |
|---|---|---|
| Speed | Lift grows with speed squared | Below takeoff speed you stay on the water, above it you rise |
| Wing area | Twice the area gives twice the lift at the same speed | Large wings lift earlier, small wings need more speed |
| Angle of attack | The tilt of the wing against the flow | Pitching the board up or down makes you climb or sink |
| Wing shape | Long and narrow wings glide efficiently, short and wide wings are more agile | Different wings feel calm, sporty or playful |
The tilt has a sweet spot. According to the MIT report, a foil works most efficiently at an angle of attack of about three to four degrees, where it can produce roughly 20 to 25 times more lift than drag. Tilt it more, to around 15 degrees, and the flow separates from the wing. The foil then loses lift, a condition called a stall.
Why does speed matter so much?
A small example shows it. If you want the same lift at half the speed, you need four times the wing area. The other way around, a wing with just under half the area, say 900 instead of 2000 cm², needs about 50 percent more speed to carry the same weight. This is why large wings suit beginners and small wings suit fast, experienced riders. We explain the choice in our guide to front wing sizes.
Why Does a Foil Need Speed Before It Lifts?
A foil needs speed because lift is zero when the water doesn't move. As speed builds up, lift grows until it matches the weight of the rider and the board. At that moment, the board leaves the water. On an eFoil, that takeoff happens at around 20 km/h.
Below takeoff speed, the board sits in the water like any other. In the transition, the drag of the hull is at its highest and the foil lifts only a part of the weight. Once the hull leaves the water, drag falls sharply and the ride becomes smooth. That's the moment riders describe as "flying". More about speeds and takeoff in our eFoil speed guide.
On a conventional foil board, the speed comes from a wave, a kite, a wing or a towing boat. Every one of them needs conditions that you can't control. A motor removes that dependence.
A Short History of the Hydrofoil
The hydrofoil is more than 150 years old. The first known patent for lifting inclined planes under a boat dates from 1869, and Italian inventor Enrico Forlanini worked on hydrofoil boats from 1898 and patented his ideas in Britain and the United States. Both are listed on the hydrofoil page of Wikipedia.
- 1919: Alexander Graham Bell's HD-4 hydrofoil boat set a world marine speed record of 114 km/h (71 mph). The record stood for two decades.
- 1952: The first commercial hydrofoil, the PT10 "Freccia d'Oro", entered service. Hydrofoil ferries then ran on many coasts and lakes.
- Today: Most ferry routes use catamarans, which carry more and cope better with rough seas. Hydrofoil ferries still operate on some routes, and foils are back at the top of sailing: the America's Cup boats of 2013 and 2017 flew on foils.
- Since the 2010s: Foil boards, kite foils, wing foils and eFoils brought the technology to everyday water sports.
How Does the Hydrofoil Principle Work on an eFoil?
An eFoil uses the same physics as every other hydrofoil, with one addition: a motor in the mast drives the board forward, so the foil can reach lift-off speed on flat water. The front wing lifts the rider, the small rear wing, the stabilizer, keeps the board level. You shift your weight to control the angle of attack.
A full-carbon front wing. Its size and shape decide how early and how calmly you lift off.
Here's how the parts of an Aerofoils eFoil share the work:
| Part | Job | Read more |
|---|---|---|
| Board | Carries the rider and stays above the water in flight | Hardboard or inflatable |
| Mast | Connects board and drive, holds the wings at the right depth | Mast length |
| Drive with jet | Pushes the water backward and drives the board forward | Jet vs. propeller |
| Front wing | Creates most of the lift | Front wing sizes |
| Stabilizer | Keeps the pitch stable, like the tail of an airplane | Stabilizer explained |
The front wing and the stabilizer bolt directly onto the drive in the mast. That keeps the underwater shape compact and lets you swap wings to change the ride.
Because an eFoil is a fully submerged foil, it doesn't stabilize itself. The board reacts to your weight, your feet and the motor. If you lean forward, the front wing tilts down and you sink a little. If you lean back, it tilts up and you climb. Small movements are enough, and the stabilizer calms the whole system. Learn how to control your height in our guide to flight height control.
Once the foil carries the weight, the board glides above the surface almost silently.
What Does the Foil Mean for Your Ride?
For your ride, the foil decides how early you lift, how stable you glide and how fast you can go. A big, wide wing lifts early and forgives mistakes. A small wing is faster and more agile but needs more speed to fly. Choose your wing for your level, not for the top speed.
Three examples from our wing range, with the area of the front wing:
| Wing set | Area | Character |
|---|---|---|
| Easyride 2000 × 510 | 2000 cm² | Largest wing, earliest lift, maximum stability |
| Freeride 1350 × 400 | 1350 cm² | Balanced all-round set, our standard |
| Freerace 900 × 300 | 900 cm² | Smallest wing, built for speed |
Every front wing ships with a matching stabilizer, so choose them as a set. Not sure which board and wing fit you? Try the Board Match Quiz or read our eFoil for beginners guide. If you'd like to see the principle in action, you can test an Aerofoils board near you, and the full step-by-step lesson is in our guide on how to eFoil.
Where Does a Foil Reach Its Limits?
A foil works well in a window of speed and depth. Outside it, three things can go wrong: the wing stalls, it breaks through the surface, or, at extreme speeds, the water cavitates. Only the first two matter for riders, and on our inflatables the second one is even turned into a safety feature.
- Stall: too steep an angle makes the flow separate from the wing and the lift drops. In practice, you avoid it by keeping the nose position calm.
- Breaching: if the foil gets too close to the surface or leaves the water, it loses the water above it. Lift collapses and the board drops. The MIT report names a foil breaking the surface as one of the main risks for hydrofoils.
- Cavitation: at very high speeds, vapor bubbles form on the wing and disturb the lift. According to Wikipedia, this starts above about 110 km/h (60 knots), far beyond any eFoil.
How does the Anti-Breach System keep you at a steady height?
The Anti-Breach System (ABS) uses air on purpose to stop a breach before it gets out of hand. Our inflatables, the Hyperfly and the Airfly, come with a 60 cm mast that includes it, built to make the first flights as simple and forgiving as possible for beginners.
Here's how it works: the mast has an air opening above the water line of the drive. As long as it stays under water, nothing happens. If you rise so far that the opening leaves the water, the jet drive sucks in air. The air bubbles disturb the flow in the jet, much like cavitation does, and the thrust drops. The board then sinks back down by itself, and you stay at roughly the same flight height. It works through physics alone, without electronics. Learn more about height control in our guide to flight height control.
Frequently Asked Questions
How does a hydrofoil work in simple terms?
A hydrofoil is a wing under water. When it moves through the water, it is tilted slightly against the flow and pushes the water downward. In return, the water pushes the wing upward. This upward force is called lift. Once the lift is as big as the weight of the board and the rider, the hull rises out of the water.
Why do hydrofoils need speed before they lift?
Lift depends on the speed of the water flowing over the wing, and it grows with the square of that speed. At a standstill there is no flow and no lift. Once the speed is high enough for the lift to equal the weight, the hull rises out of the water. Larger wings need less speed for that, smaller wings more.
What's the difference between a hydrofoil and a planing boat?
A planing boat skims over the surface and still has its hull in the water, so it keeps pushing against waves and drag. A hydrofoil lifts the hull completely above the surface and only the mast and the wings stay in the water. That means less drag at the same speed, and a calmer ride.
Why were hydrofoil ferries replaced, and do hydrofoils still exist?
Hydrofoils still exist, but there are fewer hydrofoil ferries than in the past. Many routes switched to catamarans, which carry more and handle rough seas better. Some operators still run hydrofoil ferries, and a new generation of electric foiling ferries has appeared in recent years. Foils are also used in sailing and in water sports.
Why does a foil lose lift when it breaks the surface?
A foil needs water on both sides to build up the pressure difference. When it breaks the surface, it loses the water above it, and air reaches the wing. The lift collapses and the board drops back into the water. On our inflatables, the Anti-Breach System catches this early by cutting the thrust, so the board sinks back to a steady height.
Why does a foil ride smoother in choppy water?
A fully submerged foil works below the surface, where the water moves less than at the surface. The hull flies above the waves, so small chop doesn't hit it. Big swells and wind-driven waves can still disturb the ride, which is why we recommend checking the weather and tide conditions before you go.
Note: the physics in this article follows NASA's educational material, the MIT report on hydrofoils and the Wikipedia overview of hydrofoil history. Speeds and wing data for Aerofoils come from our own product pages as of October 2026.
