So how does a hovercraft work? The principle is pretty simple, in order to hover, a hovercraft needs to sit upon a cushion of pressurised air. The tricky bit is to get enough pressure between the hovercraft hull and the surface of whatever it is you wish to hover over, to enable the craft to lift above it. Not enough air pressure and the craft will not hover or not gain enough height to hover efficiently. Too much air pressure will create excess water/dust spray from all around the craft. A happy medium needs to be found.
To create the air pressure beneath the hull of the craft you need two things.
Firstly you need a way of generating the air beneath the hull. This is achieved by using a multi-bladed fan, known as the lift fan. Some small hovercraft use what is known as an integrated system to supply lift air but more on that later. The fan is enclosed within a shroud which diverts the air beneath the hovercraft’s hull at a rate of many litres per second. If we take a quick trip back in history to when Christopher Cockerell carried out his experiments with an air blower, set of scales and tin cans, he discovered by having one can inside the other and diverting the air between them air pressure was increased significantly compared to using a single can.
So drawing on this principle as a way to significantly increase air pressure, by forcing air through a restricted space we can generate enough through air that with the aid of another item called the skirt create enough air pressure that will lift the hovercraft hull above any surface. Without the skirt not enough air pressure would be generated.
Now what is the skirt? How does it work? What properties does it need to have?
The skirts main function is to contain the correct amount of air beneath the hovercrafts hull, which will increase the air pressure to the correct level. Although the skirt looks a fairly simple thing, its design is critical to enable the hovercraft to function correctly. It also determines what height the hovercraft will travel above the surface. As mentioned above too much air pressure and you have an excess dust/water spray depending on the surface you are operating on. Not enough air pressure & the craft will not hover efficiently.
The skirt also needs to be tough enough to handle the rigors of everyday use and be flexible enough to follow the undulations of the surface the craft is travelling above. In an ideal world all surfaces would be perfectly flat & level, but in reality they rarely are. Usually the skirt is made of a vinyl type material as it is reasonably tough flexible and easily obtainable.
We now have a vessel with the correct amount of air pressure that is capable of hovering above the surface at a predefined height. We now need a method to propel the vessel forward. This is achieved in one of two ways. Some small hovercraft use multi bladed fans set inside a shroud diverting the air flow to the rear of the craft. This fan is known as the thrust fan. These fans are usually around one metre in diameter. This method is fine for smaller craft, but it does have its limitations. Most commercially available small hovercraft in the UK use this method to create sufficient thrust and is more than adequate and can propel small highly tuned racing hovercraft to speeds around 80 mph.
Above I mentioned about the integrated system. This is a way in which both the lift air and thrust are created by the same rear facing fan. A portion of the fan duct is partitioned off to allow a percentage of the air generated by the thrust fan to be diverted beneath the hull to create the lift.
Diagram of an integrated system
The other common method of creating thrust is to use aircraft propellers as used on micro-lites. This method uses a rear facing 2 or 3 bladed propeller at the rear end of the craft, as in the picture below. The propeller is surrounded by some sort of guard for safety reasons.
Rear end of a Sevtec Surveyor Hovercraft fitted with a 2 bladed propeller.
It is all very well being able to hover and have a method of propulsion, but neither is of much use if you cannot control the direction the craft travels. So we need a method to steer the hovercraft. Steering the hovercraft is by a simple method of using rudders as used by early aircraft. These rudders are connected to a steering wheel/handle bars at the front of the craft, by means of cables known as control cables. By generating a large airflow across the rudders via the thrust fan/propeller, we are able to divert the air in either a left or right direction enabling us to steer the hovercraft. The picture above shows the rudders on a Sevtec Surveyor hovercraft being test fitted for alignment.
To generate power to operate the lift & thrust systems of a hovercraft an engine is used. Early small hovercraft used two stroke engines, but today most manufacturers have moved to using four stroke engines. Four stroke engines are slightly heavier than two strokes, which is the main downside of their use. On the positive side to using four stroke engines they are usually more robust and reliable, they offer better fuel economy and are much quieter than two strokes.