Hump performance
When a hovercraft is at rest on water, it floats like a boat – that is, it is in ‘displacement mode’. When the craft goes to move off, it must transition from displacement mode to non- displacement (hovering) mode. The transition occurs, for a small craft, at about 8ph, and is known as ‘going over the hump’, because the craft must ride over a wave that forms in front of the craft just before ‘hump speed’.
Many otherwise excellent craft will not go over the hump. This is bad. It means that you cannot stop on water, because if you do you cannot get back into hovering mode. You will have to go back to land at 8mph, enclosed in your own torrential rainstorm! Not pleasant, and possibly dangerous in poor conditions.
You should if possible have the hump performance demonstrated to you before you buy. If you buy a craft that will not go over hump, it may not be possible to improve it.
Fuel consumption
Hovercraft can consume huge amounts of fuel. The worst culprits are 2 strokes, a 50 hp 2 stroke can easily consume 5 gallons per hour. Frankly, for cruisers, 2 strokes are best avoided, look for a 4 stroke craft.
Craft size
The size of craft you need will depend mainly on your intended use, and the number of passengers you wish to carry. The table below is a rough guide – other factors do affect this, so seek advice. For example, a well designed 12 foot craft may well take 3, but when poorly designed will only manage 2.
Length (feet) Passengers 9-11 1+ 10-13 2+ 12-16 3+ 14+ 4 and up
Craft types
There are several types of craft in common use today. A knowledge of these is vital if you are to choose a craft that will not disappoint you.
Integrated
This is the simplest type of craft – most craft that come up for sale are integrated. They are simple, cheap and easy to fly.
A 2 seat integrated craft from Flying Fish. One engine, one fan, simple and easy to fly. (www.flyingfishhovercraft.co.uk) Photo courtesy of www.russpullen.com
In this design there is one engine and one fan. A portion of the fan air is directed under the craft to provide lift, whilst the rest is directed for thrust.
The integrated design is a compromise – the requirements of a lift fan are the opposite of the requirements of a thrust fan. This makes the craft inefficient, noisy and sometime difficult to operate if set-up by an inexperienced builder.
However, there are some very successful integrated craft out there, typically small craft.
Integrated craft with more than about 35hp installed may have difficulty passing the HCGB noise regulations, so you will need evidence of the noise levels if you are thinking about buying such a craft for use on HCGB events.
Twin fan craft.
The twin fan craft uses a small fan to provide the lift air to the cushion, and a large fan or aircraft propeller to provide thrust air, with or without a duct. The fans will be powered from a single engine via an arrangement of belts or gearboxes. Typically these are larger craft, although there are successful small craft of this type.
The advantage of this arrangement is that each fan can be properly designed for its function, and therefore are more efficient and quieter then integrated craft. The disadvantage, of course, is the additional complexity and potential for breakdown.
A home-built twin fan SevTec Scout. This craft is capable and extremely quiet. (http://members.aol.com/sevtec/sev/skmr.html)
A twin fan craft will be easy to drive if properly set-up.