Hovercraft

Also known as: Air cushion vehicles, Surface effect ships
Definition: Vessels that float above the surface of the water on a cushion of air.
Significance: Hovercraft have allowed aircraft vessels to operate at very high speeds and become fully amphibious.

Background

Humans have sailed ships since the beginning of recorded history. These ships have always been designed and built based on the ancient mathematician Archimedes’s principle of displacement. More recently, using eighteenth century Swiss mathematician Daniel Bernoulli’s principle of dynamic lift, humans learned to build vessels that lift out of the water and fly above it.
A vessel that flies over the surface of the water is interesting for a number of reasons, the first of which is the concept of displacement. A ship floating in water displaces, or pushes aside, a weight of water equal to its own weight. This is Archimedes’s principle. If the displacement is reduced by flying the vessel above the water, the drag, or the friction acting against the vessel’s hull to slow it down, is reduced. Secondly, if the drag is reduced by reducing displacement, the speed can be increased dramatically using the same horsepower. Most conventional floating vessels have a top speed of 35 to 40 miles per hour. Hovercraft have a top speed of 100 to 150 miles per hour. A final advantage to flying over water is that hovercraft can be fully amphibious. That is, they can travel over land as easily as they travel over water.


History

The earliest experiments with hovercraft were undertaken in 1716 by Emanuel Swedenborg, a Swedish designer and philosopher. Swedenborg designed and built a vessel that looked somewhat like an upside-down dinghy. It had a cockpit in the center with air scoop openings on either side. These scoops were rotated by the operator, and air forced under the vessel lifted it above the water’s surface. The problem with the design was that the horsepower required to maintain the lift was greater than the operator could create.
The first successful operation of a hovercraft was made by another Swede, Hans Dineson. Dineson built a vessel with rigid sidewalls and a flexible skirt both fore and aft, or front and back. By 1916, Austrian engineer Dagobert Muller von Thomamuhl had built another rigid sidewall hovercraft torpedo boat that was capable of a speed of 40 miles per hour.
Over the next half-century, many other designers and builders experimented with hovercraft and other types of “flying” vessels. In 1959, Christopher Cockerell tested his vessel. This vessel not only floated on a cushion of air, as many others had done, but it also used air jets, rather than fans, to maintain this air cushion. These air jets reduced the leakage from the vessel and increased the height of clearance from the sea. The first full-scale hovercraft was built by Saunders-Roe in England and christened the SR.N1. The vessel made its maiden voyage from Calais, France, to Dover, England, on the morning of July 25, 1959. This date was chosen to commemorate French aeronautics pioneer Louis Bleriot’s epic cross-Channel flight in a heavier-than-air craft fifty years earlier.
Another Englishman, C. H. Latimer-Needham, added skirts to Cockerell’s invention in 1961. This adjustment dramatically increased the height of the vessel above the water. The increase in the depth of the vessel’s air cushion allowed it to function in much rougher waters.

Hovercraft Design

Vehicles that either ride above the surface of the water or are partially lifted above the water’s surface by a cushion of air have been called by many names by their various designers and builders. Although “hovercraft” is certainly one of the most familiar of these names, the vehicles are also known as air-cushion vehicles (ACVs), trapped-air-cushion vehicles (TACs), captured-air-bubble vehicles (CABs), ground-effect machines (GEMs), surface-effect vessels (SEVs), and surface-effect ships (SESs). In all cases, the design of the hovercraft follows a few basic principles. First, since air is 815 times less dense than water, it is easier to push something through the air than through the water. Second, increasing the amount of the hull that is lifted out of the water decreases the amount of drag the vehicle experiences and increases the speed of the vessel.
Hovercraft, or ACVs, are lifted out of the water in one of two ways: by eitherstatic ordynamic lift. Hovercraft are said to be aerostatic or aerodynamic. “Aerostatic” means they can be lifted, generally by fans, even when they are not moving. “Aerodynamic” means that the hovercraft’s forward motion creates the lift. These vessels settle back to the surface when their forward motion is stopped.
Aerostatic vessels have developed in two different ways. The first of these is through use of the plenum chamber. The plenum chamber is the large area under the hovercraft that contains the air cushion. Fans push large volumes of air down into this area. Skirting around the edge of the vessel helps contain the cushion of air under the vessel. These vessels are designed so that a large amount of air escapes around the bottom of the skirt and helps to lift the vessel clear of the water.
The second type of aerostatic hovercraft is the annular jet type, such as that designed and built by Cockerell in 1959. In the annular jet type hovercraft, the inner skirt and the outer skirt are pinched together at the base, and this pinching creates a jet of air. These jets are focused inward and downward around the edges of the vessel. In this way, less air from the cushion is lost and greater lift can be generated with the same power.
Aerodynamic hovercraft depend on their forward motion to create enough lift for the vessel to rise clear of the water’s surface. There are two types of aerodynamic vessels. The first of these is the ram-wing design. As the name implies, the speed of the vessel forces, or rams, air under the hull, lifting the vessel clear of the water’s surface. At slow speed or when stopped, the vessels float in the water, only lifting as they accelerate. Many of these vessels use rigid sidewalls to contain or focus the airflow under the hull when moving.
The second type of aerodynamic vessel is the wing-in-ground type, also sometimes called ground-effect vessels. The Soviets built a class of this type of vessel, called Ekrano plan, for their military. In ground-effect vessels, the surface of the sea and the underside of the vessel create a tunnel in which air is trapped, lifting the vessel clear of the water’s surface. These vessels also usually have rigid side-walls to support the air cushion.

Skirting

In 1961, when skirts began to be added to hovercraft, a number of things occurred. First, the skirt deepened the air cushion, so the vessels rose higher out of the water. Second, drag was reduced by lifting the vessel, so speeds increased using the same amount of horsepower. Finally, the skirt reduced the size of vessel required to operate in rough water by 75 percent, because the skirt effectively lifted the vessel above the waves rather than running through them.
Skirts are of two types. The first is the flexible skirt, resembling large rubber inner tubes that extend down from the sides and ends of the vessel. Even though this skirt extends all the way around the vessel, it is usually made up of more than one piece and contains the cushion of air. Side sections and front and rear sections are placed very close together to appear as one piece. The skirt on a vessel is designed so that its depth is twice the significant, or average, wave height for the area in which the hovercraft is to be used. In this way, the waves do not wash over the vessel when it is moving. Some vessel designs contain not only a flexible outer skirt, but also smaller skirts within the larger one. These inner skirts are called petticoats. In this way, the air cushion can be maintained even in rough sea conditions.
The second type of skirting is rigid. These are often called sidewalls. Vessels with this type of skirting generally have flexible “finger” skirts at the front and back of the vessel and along the vessel’s rigid sidewalls. The sidewalls are constructed of the same metals as the hull of the vessels, whereas the fingers are of the same rubber as the flexible skirts.


Propulsion and Lifting Systems

Hovercraft are different than other types of seaborne vessels in that they need two different types of power systems. One system creates the lift required to form the cushion of air under the vessel. The other system is used to develop the thrust to drive the vessel through or over the water.
Propulsion systems for most types of hovercraft involve propellers driving the vessel over the water. These propellers are driven by gas-turbine or diesel engines. A small percentage (10 percent) of hovercraft, mainly surface-effect ships (SESs), are driven by water jets that extend down from the rigid sidewalls. Such vessels can be problematic in very rough seas, although water jets control the steering of the vessel better than do the propellers.
Lifting systems involve pushing a very large volume of air under the vessel to create a cushion. The simplest type are the axial fans, in which the air moves in the same direction as the axis of the fan. This system works well for smaller vessels and vessels that do not require a high-pressure air cushion. The other type of fan is a centrifugal fan, in which the air is thrown out at 90-degree right angles to the axis of the fan. Centrifugal fans appear to work better in larger vessels, in which higher pressures are needed and fans may be spaced along the length of the vessel.
On some types of vessels, side thrusters are used for maneuvering at slower speeds. At speeds of less than 15 miles per hour, the cushion fans can release small amounts of air out of the side of the vessel, causing it to turn. These are called “puff ports” or “thrust ports.” At higher speeds, however, these ports are ineffective.

Application of Design

Hovercraft, or ACV, designs have been adapted by a variety of users in different areas over the years. One of the first groups to exploit the application of hovercraft design was the military. Hovercraft were fast, maneuverable, and completely amphibious. They could be made small in size and armed as patrol boats or gunboats. They could be used in coastal areas, swamps, and even over open ground. They could be adapted and made larger to carry people and equipment ashore from naval vessels lying offshore.
The civilian adaptation of the hovercraft concept was no less effective or diverse. The most widely advertised uses of hovercraft were as passenger and vehicle ferries in congested urban areas. The ferries that cross the English Channel, large vessels carrying both passengers and vehicles, have for a number of years been successful on several routes.
An interesting use of hovercraft is in areas of sensitive terrain. Hovercraft are used in the Arctic over frozen tundra, over frozen ocean surface, or on frozen rivers. They are also used in swamp or marsh areas or on beachfronts where sand may be too soft for other types of vehicles. Recently, they have been used for heavy lifting of industrial equipment such as oil-field or mining equipment. Of course, no vehicle that travels at great speeds can escape the sporting enthusiast. Groups have developed that race different types of hovercraft depending on size, horsepower, and the skill of the driver.

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