Performance Priorities
There are a number of things that you need to know about the way hovercraft works before you can make a sensible choice of craft. They are unlike any vehicle you have driven before, and the pitfalls will be unfamiliar.
Safety
Unsurprisingly, safety comes at the top of the list. You will need to look for the obvious, like properly guarded rotating assemblies, adequate buoyancy in case the engine fails on water, good ‘freeboard’ (the height of the sides above the water surface), etc.
There are many other factors in the performance that contribute to a ‘safe’ cruising craft in real life, and these are covered in the sections below. Here you will become familiar with many of the terms that experienced hovercrafters talk about all the time – “plough-in”, “the hump”, to mention just a couple.
Noise
Noise is a critical issue for hovercraft, some are incredibly noisy.
If you own a noisy craft you will be banned, pilloried, have stones thrown at you and generally made to feel unwelcome, wherever you go! There are some craft out there that are so noisy they make a 747 seem like a whisper. You do not want to own one of these!
The HCGB limits cruising craft noise to 87dB at 25m, but encourages new craft to be quieter than this – 82dB, or about the noise of a small van passing by, would be considered to be good. The reason is simple – many hovercraft venues have been lost due to excessive noise.
The good news is that newer designs have been evolving that are much better in this respect.
Ask the seller if they have measured the noise – if it is an HCGB registered craft, they should know the dB figure. If it is above 87dB, leave it unless you wish to do the work required to bring it within limits. Take advice – some older designs may never meet the limit and should probably be scrapped!
Things to avoid:
ß 2 stroke engines ß Ducts smaller than 800mm ß Very powerful engines (90hp+) ß Poor silencers
Things to look out for:
ß Separate lift and thrust fans (may be driven from same engine) ß Double silencers ß 4 stroke engines ß Low revving engines ß Large thrust fans/propellers
Reliability
When you are out, miles from anywhere, the last thing you need is to breakdown. There’s no RAC on the mudflats! Recovering a stricken craft can be difficult, so reliability is a big issue.
Professionally built craft may be more reliable than homebuilt – but not necessarily so, if they are old and neglected. You may need to look very carefully and judge for yourself.
Things to look for are low stress engines – small industrial engines, car engines or some motorbike engines can be reliable. 2 stroke engines can be unreliable – the exception to this is the Rotax microlight engines, so long as they are unmodified and in good condition.
Any craft that has been used in a marine environment (most cruisers have) may well suffer from salt corrosion – often all the controls will require overhaul before reliability can be assured. Likewise, bearings and transmission belts may need replacing.
The good news is that an unreliable craft can be made reliable, but it will take considerable effort and plenty of money to do so. So if in doubt, join the HCGB and ask for advice!
Freeboard
In simple terms, freeboard is the height of the lowest part of the hull above the water when you are floating with the engine stopped. If you don’t have enough freeboard, the craft will flood very easily.
Cruiser with good freeboard
Race craft have very little freeboard – they don’t need it. A cruising craft should have at least 6 inches. If it hasn’t then it won’t be much use as a cruiser! Look for high sides and front bodywork.
Racer, not much freeboard here!
Plough-in
A plough-in occurs when the front (usually, but it can happen to the side) of the craft suddenly dips down into the water, causing the craft to violently decelerate. Sometimes passengers can be thrown out of the craft – in any event, it is not good.
Contrary to the assertions of some manufacturers, all hovercraft can and do plough-in. It is caused by hydro- dynamic forces acting on the skirt, causing the skirt to be dragged under the craft, and this effect is practically inevitable. Most small hovercraft are very susceptible to ploughing in, and the driver will soon become skilled at predicting and preventing it.
The difference between craft is how frequently and how violently they plough in, and whether it can be recovered.
Good craft have a hull designed to reduce the effect of plough-in, so that when it happens you are not ejected from the craft – although there is still an uncomfortable deceleration period before control can be re-gained.
The best craft have active control of plough-in using a compartmented cushion and responsive skirts. In this case, the plough-in phenomenon can be brought under control and used to effect a kind of emergency brake.