Hovercraft

The invention: A vehicle requiring no surface contact for traction
that moves freely over a variety of surfaces—particularly
water—while supported on a self-generated cushion of air.
The people behind the invention:
Christopher Sydney Cockerell (1910- ), a British engineer
who built the first hovercraft
Ronald A. Shaw (1910- ), an early pioneer in aerodynamics
who experimented with hovercraft
Sir John Isaac Thornycroft (1843-1928), a Royal Navy architect
who was the first to experiment with air-cushion theory
Air-Cushion Travel
The air-cushion vehicle was first conceived by Sir John Isaac
Thornycroft of Great Britain in the 1870’s. He theorized that if a
ship had a plenum chamber (a box open at the bottom) for a hull
and it were pumped full of air, the ship would rise out of the water
and move faster, because there would be less drag. The main problem
was keeping the air from escaping from under the craft.
In the early 1950’s, Christopher Sydney Cockerell was experimenting
with ways to reduce both the wave-making and frictional
resistance that craft had to water. In 1953, he constructed a punt
with a fan that supplied air to the bottom of the craft, which could
thus glide over the surface with very little friction. The air was contained
under the craft by specially constructed side walls. In 1955,
the first true “hovercraft,” as Cockerell called it, was constructed of
balsa wood. It weighed only 127 grams and traveled over water at a
speed of 13 kilometers per hour.
On November 16, 1956, Cockerell successfully demonstrated
his model hovercraft at the patent agent’s office in London. It was
immediately placed on the “secret” list, and Saunders-Roe Ltd.
was given the first contract to build hovercraft in 1957. The first experimental
piloted hovercraft, the SR.N1, which had a weight of
3,400 kilograms and could carry three people at the speed of 25 knots, was completed on May 28, 1959, and publicly demonstrated
on June 11, 1959.
Ground Effect Phenomenon
In a hovercraft, a jet airstream is directed downward through a
hole in a metal disk, which forces the disk to rise. The jet of air has a
reverse effect of its own that forces the disk away from the surface.
Some of the air hitting the ground bounces back against the disk to
add further lift. This is called the “ground effect.” The ground effect
is such that the greater the under-surface area of the hovercraft, the
greater the reverse thrust of the air that bounces back. This makes
the hovercraft a mechanically efficient machine because it provides
three functions.
First, the ground effect reduces friction between the craft and the
earth’s surface. Second, it acts as a spring suspension to reduce
some of the vertical acceleration effects that arise from travel over
an uneven surface. Third, it provides a safe and comfortable ride at
high speed, whatever the operating environment. The air cushion
can distribute the weight of the hovercraft over almost its entire area
so that the cushion pressure is low.
The basic elements of the air-cushion vehicle are a hull, a propulsion
system, and a lift system. The hull, which accommodates the
crew, passengers, and freight, contains both the propulsion and lift
systems. The propulsion and lift systems can be driven by the same
power plant or by separate power plants. Early designs used only
one unit, but this proved to be a problem when adequate power was
not achieved for movement and lift. Better results are achieved
when two units are used, since far more power is used to lift the vehicle
than to propel it.
For lift, high-speed centrifugal fans are used to drive the air
through jets that are located under the craft. A redesigned aircraft
propeller is used for propulsion. Rudderlike fins and an air fan that
can be swiveled to provide direction are placed at the rear of the
craft.
Several different air systems can be used, depending on whether
a skirt system is used in the lift process. The plenum chamber system,
the peripheral jet system, and several types of recirculating air systems have all been successfully tried without skirting. Avariety
of rigid and flexible skirts have also proved to be satisfactory, depending
on the use of the vehicle.
Skirts are used to hold the air for lift. Skirts were once hung like curtains around hovercraft. Instead of simple curtains to contain the air,
there are now complicated designs that contain the cushion, duct the
air, and even provide a secondary suspension. The materials used in
the skirting have also changed from a rubberized fabric to pure rubber
and nylon and, finally, to neoprene, a lamination of nylon and plastic.
The three basic types of hovercraft are the amphibious, nonamphibious,
and semiamphibious models. The amphibious type can
travel over water and land, whereas the nonamphibious type is restricted
to water travel. The semiamphibious model is also restricted
to water travel but may terminate travel by nosing up on a prepared
ramp or beach. All hovercraft contain built-in buoyancy tanks in the
side skirting as a safety measure in the event that a hovercraft must
settle on the water. Most hovercraft are equipped with gas turbines
and use either propellers or water-jet propulsion.
Impact
Hovercraft are used primarily for short passenger ferry services.
Great Britain was the only nation to produce a large number of hovercraft.
The British built larger and faster craft and pioneered their
successful use as ferries across the English Channel, where they
could reach speeds of 111 kilometers per hour (160 knots) and carry
more than four hundred passengers and almost one hundred vehicles.
France and the former Soviet Union have also effectively demonstrated
hovercraft river travel, and the Soviets have experimented
with military applications as well.
The military adaptations of hovercraft have been more diversified.
Beach landings have been performed effectively, and the United
States used hovercraft for river patrols during the Vietnam War.
Other uses also exist for hovercraft. They can be used as harbor pilot
vessels and for patrolling shores in a variety of police-and customs-
related duties. Hovercraft can also serve as flood-rescue craft
and fire-fighting vehicles. Even a hoverfreighter is being considered.
The air-cushion theory in transport systems is rapidly developing.
It has spread to trains and smaller people movers in many
countries. Their smooth, rapid, clean, and efficient operation makes
hovercraft attractive to transportation designers around the world.