Hovercraft and hydrofoils
by Chris Woodford. Last updated: March 6, 2011.
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.
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 Cushioned
(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 a hovercraft works
In a hovercraft, a giant centrally mounted 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. Smaller secondary fans mounted on top, and driven either by the same
diesel engine (or separate 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 in the center blows air downward to push the craft upward, above the water. Two or more other fans at the back blow air backward to make the craft go forward. This is an example of action-and-reaction (Newton's third law of motion) at work!
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 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 courtesy of US Navy.
Middle: Vertical rudders behind the fans steer the hovercraft by directing air to the side. Photo by Brian P. Biller courtesy of
US Navy.
Right: The fans are driven from engines in the side by giant axles. Photo by Christopher A Newsome courtesy of US Navy.
Click the US Navy links to see further details and download hi-res versions of these photos.
Hydrofoils
Walking through water takes much more effort than walking through
air and this explains why ships travel much more slowly than
automobiles and aircraft. Water is almost 1000 times more dense than
air, so most of the energy produced by a boat is taken up overcoming
drag (water resistance). Hydrofoils travel much more
quickly than ordinary boats not by pushing through the water but by
raising the hull (main body) of the boat upward so it can glide above
the waves.
Photo: This US navy hydrofoil has one foil at the
front and two at the back. Note how the entire hull lifts clear of the water as the boat picks up
speed. Photo of the USS Taurus (PHM-3) patrol missile ship by Mark S. Kettenhofen,
courtesy of Defense Imagery.
What are hydrofoils?
Hydrofoils are among the fastest boats on the water, with top speeds of around 100-110 km/h (60-70 mph).
The most powerful hydrofoils have three different
engines, two diesel engines for pushing the boat through water at low
speeds and a powerful gas turbine engine to lift it onto its hydrofoil
and power it along at top speed. Hydrofoils have been widely used as
high-speed ferries and as fast military patrol boats.
How does a hydrofoil work?
A hydrofoil is like a cross between a boat and an airplane. It has
three wings on stilts called "foils" just beneath the water level.
As the boat begins to pick up speed, water accelerates over the curved top surface of the wings
and is then forced downward behind them. Since the wings push water down,
Newton's third law of motion tells us the water
must push the wings up. That's what creates an upward force called lift, strong enough to raise
the entire boat above the waves. Sharks have a pectoral fin on the
sides of their bodies that produces lift in the same way.
Photo: You can clearly see the wing-shaped hydrofoil under the
surface of the sea in this shot of the missile hydrofoil USS Hercules (PHM-2).
Photo by Mark S. Kettenhofen courtesy of
Defense Imagery.
What are jetfoils?
The fastest hydrofoils are pushed forward not by propellers but by
massive jets of water forced backward at high speed. A gas-turbine
engine pumps out up to 180 tons (164 metric tons) of water per minute,
roughly the same as 75 fire engines working together. A typical boat of
this sort, the Boeing Jetfoil, speeds along on three inverted T-shape
foils. Each foil is fitted with sensors wired to an on-board computer.
This constantly adjusts flaps on the foils to maintain lift and keep
the ride smooth.
Photo: You can see how the hull of this hydrofoil boat rides completely clear of the water on three foils, two at the back and one at the front. This boat's the USS Taurus (PHM-3). Photo by Mark S. Kettenhofen courtesy of
Defense Imagery.
Further reading
On this site
News articles
- Hovercraft still afloat 50 years on by Brian Milligan, BBC News, 10 June 2009. A news report celebrating 50 years of the hovercraft, with video footage of early hovercraft experiments.
- Hovercraft genius dies BBC News, 3 June 1999. An obituary for hovercraft inventor Sir Christopher Cockerell.
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.
- The Hydrofoil Mystery by Eric Walters.
Puffin, 2003. A blend of history and fiction for young readers.
- Hydrofoils: Design, Build, Fly by Ray Vellinga.
Peacock Hill Publishing, 2009. Ray's been building hydrofoils since the 1960s and, in this book, he explains how to design, build, and fly your own craft.
Other websites
Hovercraft
Hydrofoils
