How do hydrofoil boats rise above the water?

1. An ordinary boat has to shove water aside all the time. The faster it goes, the harder the water pushes back. What if the boat could rise above the water?
2. Look under this boat. Long legs called struts reach down to small wings called foils. When the boat is still, it floats on its hull like any other boat.
3. As the boat moves, water flows over and under each foil. The foil is curved and tilted, so it turns the water downward. The water pushes back up on the foil. That push is called lift.
4. The faster the boat goes, the bigger the lift. When the lift grows bigger than the boat's weight, the hull rises out of the water. The boat is flying on its foils!
5. Now only the thin struts and foils are in the water. There is much less water to push aside, so the boat goes faster on the same power, and the waves pass underneath.
6. Small flaps on the foils tilt up and down to keep the boat at a steady height. When the boat slows, the lift shrinks and the hull settles gently back onto the water.

Tap each part of the flying boat. Watch what it does!

• Hull. The hull is the body of the boat. At low speed it floats in the water. At high speed the foils lift it clear.
• Cabin. Passengers ride in the cabin. Because the hull flies above the waves, the ride is smooth even when the sea is choppy.
• Struts. Struts are the thin legs that join the foils to the hull. They are shaped like knife blades so they slice through the water easily.
• Foils. The foils are the underwater wings. Water flowing over them makes lift, just as air flowing over an airplane's wings does.
• Flaps. Flaps on the back edge of each foil tilt down for more lift and up for less. They hold the boat level and help it bank into turns.
• Propeller. The propeller is deep under the water at the bottom of the rear strut, so it stays in the water even when the hull is flying.
• Height sensor. A sensor at the front measures how far the hull is above the water. It tells the flaps when to add or take away lift.

Push the speed up and watch the green lift arrows grow. What happens when they are as long as the red weight arrow?

Experiment: What will happen to the hull as the boat goes faster and faster?

1. Start slowly. Is the hull in the water? Look at the arrows.
2. Speed up a little at a time and watch the lift arrows.
3. Keep going until the hull is clear of the water. Then slow down again.

What we found: At low speed the foils make only a little lift, so the hull floats in the water. As the speed rises the lift grows quickly. Once the lift matches the boat's weight, the hull rises out of the water and the boat flies on its foils.

Turn on JavaScript to try it yourself.

Get the ferry flying before it reaches the choppy waves. Choose the speed, the foil angle and the load.

Water is much harder to push through than air. Try running in a swimming pool and you will feel it. A boat's hull has to push water aside all the time, and the faster it goes, the harder the water pushes back. That push is called drag.

A hydrofoil boat has a clever answer: get the hull out of the water. Under the boat, on long thin struts, are small wings called foils. They work just like airplane wings. Each foil is curved and tilted so that it turns the water flowing past it downward, and the water pushes the foil up. That upward push is lift.

At low speed the lift is small, and the boat floats like any other. But lift grows very quickly as the boat speeds up. At takeoff speed the lift equals the boat's weight, and the hull rises clear of the water.

Now only the struts and foils are in the water, so there is far less drag. The boat can go much faster using the same engine, and it glides smoothly above the waves. Flaps on the foils keep it steady. Hydrofoils are used for fast ferries, racing sailboats and even surfboards.

Key words

Foil

A wing that works under water.

Lift

The upward push of water on the foils.

Drag

The force of the water (and air) that holds a moving boat back.

Strut

A thin, streamlined leg that joins a foil to the hull.

Foilborne

Flying: when the foils carry the whole boat and the hull is out of the water.

Takeoff speed

The speed at which the lift first equals the boat's weight.

Check your thinking

1. What lifts a hydrofoil boat out of the water?

   Show the answer

   Water flowing past its underwater wings. The foils turn the water downward, and the water pushes the foils up.

2. Why does a hydrofoil only fly when it is going fast?

   Show the answer

   Lift grows with speed, and it must be bigger than the boat's weight. At low speed the foils make too little lift. Above takeoff speed the lift can carry the whole boat.

3. Why can a hydrofoil go faster than an ordinary boat with the same engine?

   Show the answer

   Its hull is out of the water, so there is less water to push aside. Only the thin struts and foils stay in the water, so much less water holds the boat back.

4. Why are a hydrofoil's wings so much smaller than an airplane's?

   Show the answer

   Water is much thicker than air, so a small wing gets a big push. Water is about 800 times denser than air, so even a small foil makes a lot of lift.

5. What do the flaps on the foils do?

   Show the answer

   They add or take away lift to keep the boat level. Tilting the flaps changes the lift a little, which keeps the boat steady.