How do hovercraft travel over land and water?

1. A hovercraft speeds across the water and drives straight up the beach without stopping. It has no wheels and it does not float like a boat. So what holds it up?
2. A big fan on the deck pulls air in from above and blows it down, into the space under the craft.
3. A flexible rubber skirt hangs all around the edge and traps the air. The air squashes together under the craft and pushes up on it. The whole hovercraft rises.
4. A little air keeps leaking out under the bottom of the skirt. The craft rides on that thin layer of moving air, so almost nothing rubs on the ground. Rubbing is called friction.
5. A propeller at the back blows air backward, and that pushes the hovercraft forward. Flaps called rudders swing in the stream of air to turn it left or right.
6. Because it never touches the surface, a hovercraft can cross water, sand, grass, mud and ice. But with so little grip it slides sideways in turns, so the pilot has to steer early.

The side is cut away so you can follow the air. Tap each part to see what it does.

• Lift fan. The lift fan pulls air in from above and blows it down under the craft. It has to keep running, or the cushion leaks away.
• Air chamber. The air from the fan spreads out in this chamber, so it reaches every part of the skirt evenly.
• Skirt. The skirt is made of tough rubber fabric. It traps the air and bends over waves, rocks and bumps, so the craft can ride high without losing its cushion.
• Air cushion. This is the cushion of trapped air. It is squashed a little, so it pushes up on the whole bottom of the craft and carries its weight.
• Thrust propeller. The thrust propeller blows air backward to push the craft forward. A ring around it guides the air and keeps people safely away from the blades.
• Rudders. The rudders sit in the wind from the propeller. Swinging them pushes that wind to one side, which turns the hovercraft.
• Engine. The engine turns the lift fan and the thrust propeller. More power to the fan lifts the craft higher.
• Cabin. The pilot sits at the front with a steering control for the rudders and levers for the fan and the propeller.

Slide the fan power up and follow the air into the cushion. Then press Push! on different surfaces, with the fan on and with it off.

Experiment: The hovercraft sits on sand with its fan off. You turn the fan up high. What will the same push do now?

1. With the fan off, choose sand and press Push! How far does it go?
2. Slide the fan up until the craft lifts. Watch the air go under the skirt.
3. Press Push! again. Then try grass, water and ice.

What we found: With the fan off, the hovercraft sits on the ground and rubs, so a push hardly moves it. With the fan on, trapped air lifts it and it rides on a thin layer of air. Now the same push sends it gliding a long way, on any surface.

Turn on JavaScript to try it yourself.

Switch on the lift fan and press Go! Steer between the flags on water, sand, grass and ice. The hovercraft slides, so steer early.

Push a heavy box across the floor and you feel it rub. That rubbing is called friction, and it slows down everything that slides. A hovercraft beats friction in a clever way: it does not touch the ground at all.

A big fan blows air under the craft. A rubber skirt around the edge keeps most of that air from getting away, so the air underneath gets squashed. Squashed air pushes back in every direction, and that includes up. It only pushes a little on each bit of the craft's bottom, but the bottom is huge, so all those little pushes add up to enough to lift the whole craft.

The skirt does not seal tight. A thin sheet of air keeps sliding out under its edge, and the fan keeps topping the cushion up. The hovercraft rides on that moving air. It is just like a puck on an air hockey table, except that the hovercraft brings its own fan along.

Since it does not touch, a hovercraft does not care what is underneath: water, sand, mud, grass or ice. That makes it useful for rescues on mudflats and thin ice. But a craft with no grip is hard to steer. It keeps sliding the way it was going, so pilots turn early and gently.

Key words

Air cushion

The trapped, slightly squashed air under a hovercraft that holds it up.

Skirt

The flexible rubber wall around a hovercraft's edge that traps the cushion of air.

Friction

The rubbing that slows things down when they slide over each other.

Pressure

How hard something pushes on each bit of a surface.

Thrust

The forward push made by blowing air backward.

Check your thinking

1. What holds a hovercraft above the ground?

   Show the answer

   A cushion of trapped air under it. The fan blows air under the craft, the skirt traps it, and the squashed air pushes the craft up.

2. What is the rubber skirt for?

   Show the answer

   It traps the air and bends over bumps. The skirt holds the cushion of air in, and because it is bendy it can pass over waves and bumps.

3. Why can one small push send a hovering hovercraft a long way?

   Show the answer

   It rides on air, so there is almost no rubbing to slow it. Friction comes from surfaces rubbing. The hovercraft does not touch the ground, so very little slows it down.

4. A hovercraft has a mass of 3,000 kg and a hull area of 30 square meters. About what cushion pressure does it need? (Use g = 10 N/kg.)

   Show the answer

   About 1,000 pascals. Weight = 3,000 x 10 = 30,000 N. Pressure = force / area = 30,000 / 30 = 1,000 Pa, about 1 percent of atmospheric pressure.

5. Why must a hovercraft pilot start turning early?

   Show the answer

   With no grip on the ground, it keeps sliding the way it was going. A car's tires grip the road in a turn. A hovercraft has nothing gripping, so it drifts until the propeller's push changes its path.