Hovercraft

A hovercraft is a vehicle that can be used to journey over water and land. Unlike a boat, which floats on the water, a hovercraft is suspended above the water on a cushion of air. This also allows a hovercraft to move over land and float over small depressions such as a ditch or over waves. A powerful and specially designed fan creates the air cushion that is part of the hovercraft. For this reason a hovercraft is also called an Air Cushion Vehicle or ACV.

Englishman Christopher Cockerell invented the hovercraft in 1956. For this accomplishment and his other efforts, which included being part of the research team that developed the radar, he was later knighted in 1969.

Cockerell's main idea involved a vehicle designed to that float on a cushion of air, with another power source that would move the vehicle horizontally over the surface. The feasibility of the idea was tested initially using tin cans and the nozzle end of a vacuum cleaner. Initially, cans of different sizes were modified so that air could be blown down through the sealed end. Cockerell found that single cans did not produce sufficient air pressure. However, positioning one can inside another and forcing the air through the narrow cylinder of space between the two cans created a zone of high air pressure created in the region between these cans. It was this basic design that was used for the first commercial hovercraft in 1959, the Saunders Roe Nautical One (SRN1), and for subsequent versions of the hovercraft.

In the SRN1 and other models of hovercraft, the narrow cylindrical space between the tin can test system is referred to as a plenum chamber. The word plenum is from the Latin word meaning full. Large fans, which are analogous to the vacuum cleaner nozzle on Cockerall's tabletop developmental model, blow air down through the plenum chamber.

The fan used on hovercraft differs from the standard propeller type of fan. Propeller blades generate back-pressure as they rotate. In hovercraft, backpressure would decrease the efficiency of the air cushion.

The fan of a hovercraft is called a centrifugal lift fan. The fan appears like an inverted funnel positioned inside a donut-like chamber that has angled slats around the outside edge of the chamber. When the funnel shaped assembly rotates at high speed, air is sucked into the chamber and is expelled out through the slats. This design creates a more powerful airflow than does a conventional propeller.

The airflow must be constant and powerful in order to compensate for air that escapes from the edge of the hovercraft. The airflow must also be even around the edge of the hovercraft, to prevent one region of the hovercraft from lifting higher off the surface of the ground or water than other parts of the hovercraft.

The edge of a hovercraft contains a flexible curtain of material. This material, which is known as a skirt, helps prevent the escape of air from the plenum chamber, which in turn lessens the mount of energy that is needed to generate the suspending airflow. The skirt must be durable and flexible to accommodate the different heights of terrain or waves that the hovercraft passes over. Additionally, the skirt must be light, yet resistant to flapping. Originally, a skirt was a single piece of material; now it is typically made of rubber. When one region of the old-design skirt wore out, the entire skirt had to be replaced. The cost of this replacement, often in the millions of dollars, prompted a redesign. Nowadays, damaged rubber portions of a skirt can be removed and replaced without the necessity of replacing the entire structure.

As a hovercraft operates, the rubber skirt is inflated outward by the air pressure generated by the centrifugal lift fan. This effect produces an air cushion that is about 3 meters in depth beneath the hovercraft. Propellers that blow air out horizontally provide the power that moves the hovercraft over the surface of the ground or water. The blades of these propellers can be moved, or pitched, to control the speed of the hovercraft. When the propellers have a zero pitch, the hovercraft is not moving. Positive pitch moves a hovercraft forward and negative pitch is the braking system that slows the hovercraft down.

The hovercraft has proven to be useful for applications where passengers or cargo are transported over water and where the loading and unloading can be done on land. For example, hovercraft have been used for decades to ferry people back and forth across the English Channel between the United Kingdom and France.