Hovercraft and hydrofoils
Last updated: December 11, 2007.
Walking through water takes much more
effort than walking through
air and this explains why ships and boats 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). Hovercraft and 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: A US marines hovercraft photographed in
2001.
Picture courtesy of Defense Visual Information Center
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 46 mph
(74 km/h) and can land on roughly three quarters of the world's
coastline.
A hovercraft photographed in 1971.
Photo courtesy of NASA
Ames Research Center (NASA-ARC)
In a hovercraft, a giant fan creates a massive down-draft of air
that pushes the hull upward. A cushion of air is trapped underneath the
craft by a flexible rubber skirt that can bend around obstacles on
water or land. Small fans at the back, 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.
Side wall hovercraft
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.
Hydrofoils
Hydrofoils are even faster than hovercraft, with top speeds of up to 60-70 mph
(96-113 km/h). 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.
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 travels faster over the curved top surface of the wings
than under the straight bottom surface. This reduces pressure above the
wings and produces 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: 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.
Public domain photo of the USS Taurus (PHM-3) patrol missile ship, shot
in 1990,
courtesy of Defense
Visual Information Center.
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.
Further Reading
Favorite websites
Have a go!
- Hovercraft Scotland: You can read about hovercraft—or you could have a go on one. Hovercraft Scotland run neat little experience days and taster sessions.
