Hovercraft construction

Designing and building a hovercraft 

The basic principle

The basic principle of an air cushion is that if you lift an object off the ground using air you reduce the force needed to move the object.

Because the object is no longer in contact with the ground the friction this causes (surface friction) is reduced, making it easier to move the object.

This also implies that the smoother the surface the less the friction. This is also true. Uneven surfaces with rocks and other obstacles can significantly slow down or even stop and damage an air cushion vehicle if the chamber containing the air cushion (the plenum chamber) was made of rigid materials.

Hence the use of a flexible skirt and the need for sufficient lift. This allows the craft to pass over (smaller) rocks and other uneven terrain obstacles without (too much) trouble or damage or slowing down.

The basic parts

That said with a hovercraft we can identify four basic construction necessities - 1) the hull, below which is attached the skirt system, 2) the carriage or cabin part which needs to hold the passenger(s), crew or freight, 3) a propulsion system to move the craft, and 4) the lift system to feed air into the plenum chamber below the craft in order to create the air cushion.

Some hovercraft use a single engine system to provide both the air for the plenum chamber and propulsion. The difficulty in using one engine is to provide optimal efficiency for both systems, dividing the power for propulsion as well as for the fan to produce enough air for the lift.

Many modern air cushion vehicles use separate systems for air and propulsion. But advances in engines have made it possible to choose between one and two engine systems, especially for smaller, recreational hovercraft. Bigger, industrial, commercial or military hovercraft can have anywhere up to eight engines for propulsion and lift.

Power-to-weight ratio

Although an air cushion vehicle does not require the critical power-to-weight ratio precision as does an airplane in order for it to still operate, it is nonetheless necessary to consider the power-to-weight ratio at the design stage of an air cushion vehicle, rather than find out later that there isn't enough power to lift or move the craft.

The power-to-weight ratio determines in large part the amount of ground clearance between the skirt and the ground surface. The greater this ground clearance the more efficiently the propulsion system operates. That is not to say that the higher the hovercraft lifts into the air the better. Lifting it too high will cause instability.

Such is the power of the lift that even a severely overloaded and miscalculated power-to-weight ratio hovercraft construction will still work, but it is far from ideal.

Power-to-weight-to-strength - The hull of the hovercraft

The next consideration for a properly functioning hovercraft is the power-to-weight-to-strength ratio. This deals with the structural strength of the raft to be light enough to be lifted by the air cushion created underneath, yet strong enough to carry the weight of the engine, its passengers or payload.

Air cushion vehicle hull construction is more closely based on aviation rather than marine constructions for the simple reason that aviation hulls are a combination of strength and lightness as opposed to strength as a priority.

Although wood and plywood are often used, many hovercraft hull structures are made of aluminum (aluminium) skin, welded or riveted onto an aluminum web or frame. Enclosed spaces are sealed to provide airtight compartments for natural buoyancy.

A hole in the center of the raft can be made to feed air to the plenum chamber beneath the craft. However, the use of new skirt techniques makes peripheral jets, led in from the edge of the raft through ducts, more sensible and more common.

Other craft use aluminum honeycombed paneling to provide the buoyancy, and fiberglass and composite materials, such as PVC, are becoming more popular as they combine strength, lightness and buoyancy in a single material.