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Hovercraft are also known by the term ACV (Air Cushion Vehicle). Other terms sometimes used are "surface skimmer" (although this also applies to some high speed boats), SES or "Surface effect ship" which is a non-amphibious ACV having side walls or thin hulls which remain in the water, but with a skirt front and rear to contain the air cushion.

What does the skirt do?

The main purpose of the skirt is to help maintain the air cushion. It also allows the cushion of air to be deeper thereby letting the hovercraft pass over taller obstacles.

Does the skirt go right under the craft?

No, the skirt runs around the periphery of the craft leaving a space under the hull, which is surrounded by the skirt. It is within this space that the air cushion is formed between the hull and the ground.

What different types of skirt are there?

There are five main types of skirt, although some of these have several variations of design.

Curtain skirt:

This is simply a straight length of material, which hangs from the edge of the craft down to ground level. At the top, it is held in position by a continuous fixing to the edge of the hull. At the bottom it is prevented from being blown outward by cord, straps or chain, fixed at intervals between the bottom edge of the skirt and the lower hull.

The curtain skirt is now very rarely used on anything but model hovercraft.

Segmented skirt:

Sometimes called a finger skirt. It is formed from a large number of individual pockets of material, which are mounted side by side around the periphery of the craft. The upper edge of each segment is fixed to the craft with screws or clips and the sides of the pocket are attached lower down on the hull with cord or plastic ties. The pocket is shaped a little like a paper carrier bag with the open end attached to the hull. The side which faces downward and inwards towards the cushion is cut away to form an open pocket. The outer wall of the segment which touches the ground is set to meet the ground at 45 degrees.

The segmented skirt is used on small and medium sized craft. Each segment is normally supplied with air from a plenum, which is built into the hull.

Bag skirt:

Also called a loop. It can be formed from a tube of material, which is attached to the craft as a continuous loop around the periphery of the hovercraft. On most craft, this tube is formed partially by the hull itself and partially by the fabric of the skirt.

Bag skirts are separated into two distinct types, which differ more in use than in structure.

• Full flow (or serial feed), in which all of the lift air is fed into the bag and exits from the bag into the cushion. Variations of this design include adding small holes around the bag at the contact point with the ground to aid lubrication of the skirt and fitting a jupe ( a very short curtain skirt ) at the contact point which prevents unwanted vibration of the bag caused by aerodynamic forces.

 

• No-flow (or parallel feed), in which only a small percentage of the lift air is fed into the skirt at pressure. The main lift air is fed directly to the cushion. Once inflated the skirt only passes a small volume of air, which exits through drain holes or possibly lubrication holes.

Bag skirts are used on small, medium and some commercial hovercraft. One advantage of the bag skirt is the lack of the need for a plenum built into the hull.

Loop and segment:

This is essentially a combination of the bag skirt (Full flow type) and the segmented skirt. The segments are fixed to the underside of the bag and fed with air through holes in the wall of the bag or loop.

The loop and segment skirt is used on medium sized and large hovercraft. It is the preferred type for most commercial hovercraft.

Pericell skirt:

Originally of French design and sometimes called a Jupe skirt. It consists of a number of self-contained cells. Each cell is constructed using a curtain skirt, shaped to form a truncated cone, the wide end attached to the underside of the hull. A craft may have six cells fitted three on each side.

Pericell skirts operate at much higher pressures than other types and tend to be less reliable. They are only used on medium and large sized craft, but have generally fallen out of favour.

What sort of engine does a hovercraft use?

Large hovercraft, normally use gas turbines and/or diesel engines. Small recreational craft use a variety of engine types but mostly small 2-stroke or 4-stroke petrol engines.

Racing craft tend to use engines developed and manufactured for micro-light aircraft, snowmobiles, or ex-motorcycle engines. Small cruising craft use either the snowmobile type of 2-stroke engine, or commercial 4-stroke utility engines of the type fitted to generators etc.

Larger cruising craft normally use car engines. These are chosen for their low weight and good power output, mostly the modern all-aluminium types fitted to small saloons and sporty hatchbacks.

How many engines does a hovercraft have?

Medium and large commercial craft can have 3 or more engines. The light-hovercraft used for recreational purposes, usually have just one or two engines. If two engines are fitted one is used for thrust and drives the thrust fan or propeller. The other drives the fan used to provide the lift air to the cushion. If a single engine is fitted this drives the thrust fan which is partitioned so that some of its air is directed under the hull to provide lift. In some small craft the engine drives the thrust fan via a belt drive and also another shaft connected to a second fan which provides the lift air.

How is the air cushion generated?

 

In most craft an engine drives a lift fan which supplies the cushion with a constant volume of air at the cushion pressure. In an integrated craft, the thrust fan is divided into two sections by a splitter plate. The larger part is used for thrust, the smaller part (about 15-25% of the area) is used to provide lift air. This is ducted into the plenum or bag skirt and then to the cushion. Most small hovercraft use axial fans for both thrust and lift. Some large hovercraft use centrifugal fans for lift.

Can you use a hovercraft on the road?

No. A hovercraft has no brakes and no steering (in the accepted sense). Because there is very little friction with the ground over which it flies, when the steering is turned the craft does not turn as a car would. The craft rotates but continues travelling in the direction it was going until the thrust takes effect and propels the craft in the new direction. The only way to stop a hovercraft is to stop the thrust and allow the forward motion to be scrubbed off by drag on the skirt. This takes a long time. Alternatively the craft can be spun through 180 degrees and full thrust applied to bring the craft to a stop rapidly. This makes a hovercraft unsuitable for use on public roads. In addition they are often quite noisy, and could not be made to comply with roadworthiness legislation.

Where can you use a hovercraft?

Providing that there are no local by-laws or statutes prohibiting their use, they can be used on rivers, harbours, and in estuaries, where they are capable of crossing deep water, shallow water and mudflats. For this type of use, the craft should be constructed in accordance with the cruising craft standards. These craft have more freeboard than racing hovercraft and are less likely to swamp. Racing craft are only suitable for inland waters and can be used at events organised and run by organisations in Australia such as the NSW Sporting Hovercraft Club. Hovercraft can be used on any suitable private land with the landowners permission. The user has a responsibility to ensure that he does not create a nuisance (eg with noise), or endanger other people or animals.

Do you need a licence to fly (use) a hovercraft?

Yes - in NSW Australia, a recreational boat driver's licence from Waterways will suffice, as the Air Cushion Vehicle (hovercraft) potentially can exceed eight knots. If used in public places, or on common land, the craft should be adequately insured.

Can a hovercraft go backwards?

Some hovercraft are fitted with devices to allow them to direct thrust forward and thereby move backwards slowly for manoeuvring. Without devices of this type a hovercraft cannot move backward under power.

What are hovercraft made of?

It is possible to construct hovercraft, from many different materials. The most important factor in choosing these materials is low weight. Commercial craft are normally constructed from aluminium and magnesium alloys and plastic composites.

Small hovercraft hulls are commonly made from Plywood, jointed with epoxy resins, moulded glass re-inforced plastic (GRP), or carved foam covered GRP and epoxy resin. Other materials can be used for some items including sheet polypropylene, carbon fibre, expanding polyurethane foam and closed cell foam in sheet form for buoyancy.