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Hovering reduces drag and friction between the bottom of the hovercraft and the ground or water. Air is blown into a chamber (plenum chamber ) under the craft by the lift fan and this air is held in place with a flexible skirt that acts as a seal. The air lifts the craft off the ground and acts as a lubricant so that a craft can travel faster with a smaller engine. Leakage from the base of the skirt forms a small air gap so that there is very little friction between the craft and the ground, although in practice the edge of the skirt is usually in contact with the surface. A boat has to push water aside to move, but a hovercraft skims over the surface and doesn't have to waste energy in moving huge volumes of water. A hovercraft exerts the same downward force on the surface as a seagull standing on one foot.
At low speed the craft will travel in a cloud of spray as it literally blows a hole
in the water, but as it speeds up, it climbs out of the depression -
The skirt acts as a seal to trap the air and hold it under the craft but also enables
the 'hard structure' of the craft to hover at greater height and thus avoid obstructions
A conventional boat would require much more power to run at hovercraft speeds and
would be less stable. Another important factor is the wash -
Large craft have a lift fan dedicated to blowing air into the chamber under the hull and also into the skirt, and one or more fans or propellers for thrust. In some cases full size craft also use a single set of fans for lift and thrust ( like the CC5 above ) and use ducted air for propulsion jets. Others use one engine to drive lift and thrust systems through gearboxes. Bigger craft have multiple engines and drive systems.
Steering is achieved using rudders, swivelling propellers with variable and reverse pitch blades, moveable outlet ducts which literally blow the craft sideways or backwards and also by altering the balance of the craft and moving the skirt to adjust the centre of gravity and the centre of lift.
Another factor that has been essential in the success of hovercraft is the ability
to come out of the water to load and unload. Being amphibious, there is no need
for complex docking and mooring required by a ship. The craft can operate from any
flat area and passengers and cars merely drive on and off using built in ramps. Whereas
a ship needs jetties and moving ramps to deal with tidal conditions, a hovercraft
just drives ashore, drops the ramp and loads the cargo. This means that you can
get a lot more work out of a hovercraft -
Small craft use a different system that enables them to hover and move with only
one engine. The fan is placed at the back and pushes the craft along, but some of
the air -
Driving a hovercraft is like nothing else. The nearest you can imagine is a tea tray on an ice rink. The craft will travel as easily sideways as forwards and will slide away on any slope or in a wind. You have to use the rudders and thrust of the engine to point the craft in the correct direction and then to push it where you want to go using the engines while taking any ground conditions, slope and wind speed into account. Turning is done by combining power, steering and balance.
Stopping can be done by gradually reducing power or in an emergency a small craft can be stopped by spinning round and opening up the engines while travelling backwards. Bigger craft have reverse thrust, small ones don't! 'Dumping' the lift is not an option as the craft will stop dead when it hits the ground and do a lot of damage to the passengers and cargo. It might also rip the skirt off and smash in the hull! Almost all craft are designed to survive a loss of lift on water, but it is not a pleasant experience and is like hitting a wall at full speed. In an emergency, such as an unexpected sandbank or gully, the driver has to open up the lift engine to give more cushion air to ride over the obstruction. This is against all natural instinct which is to shut down and stop! This makes for some interesting moments until the controls are mastered!
Craft are all designed to float and they can stop on water and take off again. They
have landing pads to alight on hard ground without risk of hull damage. However the
skirt will wear rapidly on dry surfaces and operating on concrete or tarmac is not
suitable for long periods. All craft have buoyancy tanks to keep them afloat and
can operate as an air propelled boat if the lift system fails -
The design of the craft makes them extremely stable and despite a rather strange way of driving, they are easy to operate and very safe. They are also great fun to drive.
A model hovercraft.
Note how the craft is skimming over the water and not pushing through it.
A 'chip tray' sized radio control 'integrated' model hovercraft.
Visit the Hovercraft museum Website http://www.hovercraft-