Hovercraft
Last updated: March 11, 2009.
People have been building boats now for thousands of years, but
engineers are still finding better ways to carry us over the water. One of the things
that slows boats down is the choppy waves brushing underneath them so, if you want
to go faster, you need to go higher too. How can you make a boat race over the waves?
One way is to use a hydrofoil: a kind of underwater wing
that makes a boat fly, very slightly, like a plane. Another option is to use a giant
fan and ride your boat on a cushion of air. Boats that work this way are
called hovercraft (or, in the military, as LCAC, Landing Craft Air Cushion vehicles).
Let's take a closer look at how they work!
Photo: A US Navy hovercraft (LCAC) photographed in 2008. Picture by Chad R. Erdmann courtesy of
US Navy. Much of the deck is empty space, suitable for carrying huge amounts of drive-on, drive-off military cargo.
What is a hovercraft?
Hovercraft are among the world's most versatile boats. Because they
are amphibious (they can travel equally well over land or water), they
can ride right up onto the shore. They can also carry massive amounts
of cargo. A US military hovercraft called the Landing Craft Air Cushion
(LCAC) can carry a 70-ton (64-metric ton) tank at speeds up to 74 km/h
(46 mph) and can land on roughly three quarters of the world's
coastline. Lighter hovercraft can reach speeds of 130km/h (80mph) or more.
Photo: A coastguard hovercraft photographed in 1971.
Photo courtesy of NASA
Ames Research Center (NASA-ARC)
How does a hovercraft work?
In a hovercraft, a giant fan creates a massive down-draft of air
that pushes the hull upward anything from a few centimeters/inches to
a couple of meters (5-6 ft). A cushion of air is trapped underneath the
craft by a flexible rubber skirt that can bend around obstacles on
water or land. Small secondary fans mounted on top, and driven by separate
diesel engines, create a backward force that pushes the hovercraft forward.
Rudders behind the fans swivel this backward draft of air from side to
side to provide steering.
Small "fingers" of rubber attached to the bottom of the skirt
improve the seal between the skirt and the waves beneath it. This
maintains the cushion of air, keeping the hovercraft above the water
and making the ride smoother for passengers.
Photo: A typical hovercraft has two or more fans. The main fan blows air downward to push the craft above the water. Two or more other fans blow air backward tomake the craft go forward.
A side-wall hovercraft has two rigid sides that extend under the
water and so needs a skirt only at the front and back. Although it
cannot travel on land, it can use propellers or water-jet engines,
which makes it much quieter than a traditional hovercraft.
How much can a hovercraft carry?
The actual amount of lift depends on how much weight the craft is carrying. A fan of a given power will
create a certain amount of pressure under the craft. Now since:
pressure = force / area
it follows that a bigger hovercraft (one with a bigger overall area) can carry more weight than a smaller hovercraft
with a fan the same size.
Looking closer at a hovercraft
These pictures, courtesy of the US Navy, show some key features of a hovercraft in close-up:
Left: Close-up of a hovercraft skirt making a tight seal with the water beneath. Photo by Cody D. Lund.
Middle: Vertical rudders behind the fans steer the hovercraft by directing air to the side. Photo by Brian P. Biller.
Right: The fans are driven from engines in the side by giant axles. Photo by Christopher A Newsome.
Further reading
Books
- Ships and submarines by Chris Woodford.
Facts on File, 2004. My own book about the history of ships, from ancient wooden craft to the very latest wave-piercing catamarans.
- Discover the Hovercraft by Kevin Jackson.
Flexitech, 2004. This short, hands-on book about hovercraft technology includes experiments and activities you can try.
