Hull construction
There are a number of methods of hull construction, each with advantages and disadvantages.
Aluminium
The first hovercraft were constructed from aluminium using aircraft methods.
Disadvantages: Tends to be heavy. Expensive.
Advantages: Very strong.
GRP
Glass reinforce plastic, or glassfibre, is the most common hull material for professionally built craft.
Disadvantages: Needs a mould, lots of space and some skill. Suited to professional manufacturers. Can be heavy.
Advantages: Complex shapes can be manufactured easily, good strength to weight, robust and repairable, doesn’t corrode. Excellent pedigree.
GRP-Foam composite
A layer of high density PU foam has a lightweight GRP cloth bonded on both sides.
Advantages: Extremely stiff, light and strong material. Suited to home-build or professional build. Provides the best strength to weight of the common construction methods.
Disadvantages: Labour intensive process with expensive materials means these hull are not cheap.
Wood
This is the traditional home-build material.
Advantages: Can be a light, cheap and easily constructed material, suited to moderate skill levels.
Disadvantages: Construction quality dependant on the skill of the constructor. Can be heavy if the wrong grades are chosen. Will rot in time.
Polystyrene foam – Epoxy resin
Large blocks of polystyrene foam are carved into the required shape and covered with glass fibre cloth and epoxy resin.
Advantages: Complex shapes can be made as one-offs. Light and strong.
Disadvantages: Expensive and labour intensive.
Provenance
What does this mean? Essentially, where did the craft come from... there have been many hovercraft manufacturers, ranging from excellent to poor, and in addition many craft have been home-built, again to variable standards.
A professionally built 4 seat cruiser from K&M Hovercraft (www.avdw32.dsl.pipex.com)
High quality professional cruising craft or 3 seats or more do not come up for sale very often, and when they do, they command a price starting at several thousands of pounds. By contrast, many ‘Challenger’ style home built craft come up for sale, and whilst they can be had for a few hundred pounds, they are probably best avoided, unless you simply want a fun toy to fly around the paddock on! In between are a few small craft which can make a sensible ‘starter’ craft, such as the Osprey 1, and will probably cost somewhere between one and two thousand pounds.
Homebuilt craft designed by Universal Hovercraft (http://www.hovercraft.com/content)
Occasional larger home-built craft come up for sale, and where built to established plans, such as the well known Universal Hovercraft or SevTec, these can be a very good buy, provided that the build quality is good. As ever, join the club and seek advice before parting with your money.
What to look for?
There are a number of points that should be looked for in a second hand craft – here are a few things to out look for.
Skirt
The skirt on a hovercraft has a limited life – like the tyres in a car. Many craft will come up for sale with well worn skirts. This isn’t a disaster, but the cost of a new skirt can be high - £350 and upwards, so be prepared for the expense.
Old bag skirts will be recognised by the presence of multiple repairs and patches, together with wear on the ground contact line.
Worn-out segment skirts will be seen to have a frayed and torn lower edge (where it meets the ground) with the backing material showing through the rubber.
Hull
Hulls have frequently suffered damage from minor collisions, abrasion and so on. Look for the signs of repairs. The place you really must look is underneath, as damage here is likely and you don’t want to find it when floating without power in a choppy estuary! The good news is that GRP can be repaired fairly easily – but it has to be worth a negotiation with the seller.
Engines
Checking the engine is similar to any motorbike or car – but more so. Hovercraft engines get a hard life, so take someone who knows engines with you.
In addition to the usual things, look out for salt corrosion if the craft has been used on the sea.
Fan or propeller
Old and perished fans can be dangerous. If the blades have turned green, this indicates deterioration due to UV light, and they will have to be replaced, at around £70 - £150 per set.
With propellers, check for signs of erosion on the leading edge, caused by sand and grit in the air flow literally sand blasting the edge.
Transmission
Belts are often used – and should be checked carefully for fraying, cracking or other damage. You may choose to replace it anyway once you own the craft – they are well known for breaking, I like to know that mine is in good condition.
Check for play and roughness in bearings.
Controls
Controls often rely on long bowden cables, which can suffer from salt corrosion. Check for free operation and frayed ends, and replace if in doubt.
Guards
Check guards for security and strength, and if you intend to use the craft at HCGB events, that they meet the current construction regulations.