Before we get going, just one foreword – be careful to avoid any obviously partisan advice. Members of the Australian Hovercraft Association can offer good, impartial advice and you will find your local members to be friendly and approachable, so drop a line and start your hovercraft career!
Here’s a few questions you should be asking yourself:
How many passengers?
A small (2.5-3.5m) hovercraft can be great fun, easily handled and stored, but will obviously have less capacity for equipment and people. So if you’re wanting to take just yourself, and maybe a small child, then this could be for you. If you want to take the family out, and need two or three seats, then you will need a mid sized (3-5 m) craft, but if your plans include 4 or 5 seats, then you’ll be looking for a 6m+ craft.
The maximum size of craft you can operate easily in Australia is 1000kg, but you’re unlikely to come across something this big – it would be in the region of 8m+
Marine Authorities limit the number of persons on a marine craft based on hull length. Check your state regulations. In Victoria the limits are:
|Less than 3||2|
How much equipment?
Taking people is one thing, equipment is another – it doesn’t sit quite so neatly on seats! If you’re thinking about day trips only, then you’ll need fairly small storage – enough for some tools, emergency equipment, sarnies and so on. But – if you want to go overnight camping on longer trips then you’ll need space to store lightweight camping gear- the type of equipment you might take back-packing or motorbike camping.
When it come to the capacity of any craft, be careful of poor advice. It is common to rate the number of passengers as the number that can physically fit into the craft, and this will be unsafe on water – even if the craft will lift them on a field, it is quite different on the water. Subtract 1 or 2 from this number for safe on-water operation.
Remember that the sea-keeping ability is directly related to size, bigger is better in this respect. Buy the largest craft that you can afford, unless you expect to trade up once you have gained a year or so experience.
BeachesOperation on beaches is usually a bad idea – if it is in any sense public, then you will undoubtedly attract unwanted attention pretty quickly if you are operating on the beach. Usually the local authority will be along to shoo you away. Having said that, launching from the beach may be OK, which means turning up, offloading and making a single pass out onto the water and away to sea. This generally gets quite positive comment, so long as you choose a quiet beach and do not then spend all day going round in small circles in the bay. This last point is really important – under no circumstances take a craft onto a popular beach – it just isn’t safe. Check with local members to find out what is Ok and what is not.
In general, if you are on the water and at least 100m from the shore, then you will be OK, although there will be people who will tell you otherwise. Again, refer to the Federation for advice.
Operational area and Craft Size
Obviously open water and estuaries favour a larger craft, whereas sheltered waters will be fine for a smaller craft.
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. Put simply, its how big a wave would have to be to come over the side.[img width=600]http://hoverclub.org.uk/gallery/9_18_02_12_5_46_59.jpeg[/img]
A cruising craft should have at least 300 mm. If it hasn’t then it won’t be much use general use! Look for high sides and front bodywork.
This really is a key safety issue. When the weather turns against you, or you cross the wake of a large ship, waves over the front or back can turn a Sunday outing into a life-threatening Mayday situation. Larger is better for open waters.
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. All control will be lost, and sometimes passengers can be thrown out of the craft – in any event, it is not good.[left][img width=300]http://hoverclub.org.uk/gallery/9_18_02_12_5_47_49.jpeg[/img][/left] All hovercraft are susceptible to plough-in unless fitted with effective anti-plough devices. It is caused by hydro-dynamic forces acting on the skirt, causing the skirt to be dragged under the craft. Unless fitted with effective anti-plough devices, most light hovercraft are very susceptible to ploughing in, and the driver will need to become skilled at predicting and preventing it. However, there are well known solutions to plough-in, so look out for craft fitted with such a device. I personally wouldn’t buy a craft without.
In addition to effective anti-plough devices, good craft have a hull designed to reduce the effect of plough-in, so that if it ever happens you are not ejected from the craft – although there is still an uncomfortable deceleration period before control can be re-gained.
Any craft that ploughs in so badly that the Pilot is ejected from the seat is dangerous and should be scrapped.
Some craft have passive control of plough-in using a compartmented cushion and responsive skirts, while other use one or elevators for active pitch control. In this case, the plough-in phenomenon can be brought under control.
Hump performance[left][img width=300]http://hoverclub.org.uk/gallery/9_18_02_12_5_48_12.jpeg[/img][/left]When a hovercraft is at rest on water, it floats like a boat – that is, it is in ‘displacement mode’ or ‘boating’. 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 12 kph, 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 – you could liken this to driving a car on the motorway with no clutch – at the first traffic jam you are stuck with no way of getting restarted. This is a key safety issue. It means that you cannot stop on water, because if you do you cannot get back into hovering mode. You will have to ‘boat’ the craft back to land at about 6 kph, enclosed in your own torrential rainstorm. Often the spray is so bad that vision is impaired and secondary engine failure occurs, whereupon the situation can take a turn for the worse.
You should 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.
There are several craft types, and a discussion on this subject is well worthwhile.
Most race craft that come up for sale can be recognised as such because they are small and have two stroke engines. There are very few of this type of craft in Australia.
Cruising Craft[img width=600]http://hovercraft.org.au/forum/gallery/9_18_02_12_5_49_07.jpeg[/img]
It is absolutely vital to have good advice when buying a cruiser, there are excellent craft out there, and some poor craft, but you just couldn’t tell one from the other without some inside knowledge. A dodgy cruiser could turn out to be a dangerous liability that leaves you stranded at sea, making a mayday call.
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 Craft[left][img width=300]http://hovercraft.org.au/forum/gallery/9_18_02_12_5_49_31.jpeg[/img][/left]This is the simplest type of craft – most craft that come up for sale are integrated. They are simple, cheap and easy to fly.
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 different to the requirements of a thrust fan. This can make the craft relatively inefficient and noisier. In general, the integrated design is used on small craft and the craft tend to be quite agile.
Integrated craft with more than about 35hp installed may be very noisy and should be avoided.
Twin fan craft[left][img width=300]http://hovercraft.org.au/forum/gallery/9_18_02_12_5_49_50.jpeg[/img][/left]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 to achieve its function, and therefore may be more efficient and quieter then integrated craft.
Twin engine[left][img width=300]http://hovercraft.org.au/forum/gallery/9_18_02_12_5_50_10.jpeg[/img][/left]The twin engine is similar to the twin fan, except that each fan has its own engine. The transmission is simpler than the twin fan type, but there are now two engines to maintain.
The twin engine design provides independent control of lift, which is useful in some circumstances.
There are two methods of providing thrust, ie a ducted fan, or a propeller. The choice of fan or propeller is largely a matter of opinion. However, size is important. Do not consider a fan of less than 900mm, with 1100mm fast becoming the preferred size. This is because the larger fan or prop will be quieter and produce more thrust – a win-win situation.
There are two main skirt types, segmented and bag, but there are many variations and hybrids. Both types are successful, so it seems to come down to personal choice in the end.
Segment skirts[left][img width=200]http://hovercraft.org.au/forum/gallery/9_18_02_12_5_51_34.jpeg[/img][/left] This type of skirt consists of many individual segments, often 70 or more to make the complete skirt. Usually the segments will be the same around the front and sides of the craft, and different at the rear, although there could be up 5 types around the craft. The rear segments include a flap to prevent the free edge scooping water when going over hump.
The advantage of the segment skirt is that is that they are stable and damage tolerant – should a segment be damaged, the neighbouring segments expand to fill the space, and in any case it is easy to change a damaged segment in the field. The downside is that the segments sometime catch on debris and a partly pulled off, requiring re-attachment in the field – not a major issue though.
Bag skirts[left][img width=200]http://hovercraft.org.au/forum/gallery/9_18_02_12_5_51_59.jpeg[/img][/left] There are a range of types of bag skirt, but all share a similar bag-like appearance. The skirt can be thought of as a large inner-tube that is formed around into the shape of the craft.
Bag skirts are cheaper and quicker to make and replace, and are reliable, as there are less ‘edges’ to snag on obstacles. Minor damage can be tolerated, and field repair is easy if needed taking just a few minutes. Bag skirts have more drag than segment skirts and are not favoured by faster craft.
Bag or loop and segment skirt[left][img width=200]http://hovercraft.org.au/forum/gallery/9_18_02_12_5_52_43.jpeg[/img][/left] This is a hybrid, designed to have the best of both worlds. A bag skirt is mounted to the craft, and a set of segments are attached to the bottom of the bag. This is the most expensive type of skirt, and is found on all large craft but few small craft.
A partitioned skirt has one or more skirt dividers that run underneath the craft. These divide the cushion into compartments, usually front and rear, or front, rear and sides.
Large craft rely on a compartmented cushion (or skirt shift mechanisms) to provide stability, in a small craft with a single compartment, the driver must frequently shift body-weight in order to provide the required stability.
A compartmented skirt is very effective for controlling plough-in and properly designed means the craft will trim up properly regardless of the forward/aft weight distribution of the craft.
Partitioned cushion systems are very common on racing craft in Europe.
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’ 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 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 AHA encourages new craft to be quiet in operation – 80dB, or about the noise of a small van passing by, would be considered to be good.
The good news is that newer designs have been evolving that are much better in this respect.
Look out for designs that have twin fans, one for lift and one for thrust. The use of two fans allows the manufacturer to design efficient and quiet systems. These may be driven from one or two engines.
Ask the seller if they have measured the noise. If it is above 87dB, leave it unless you wish to do the work required to bring it within limits. Take advice – older designs may never meet the limit and should be scrapped. But be careful – some manufacturers issue quite misleading noise figures.
Things to avoid:
- 2 stroke engines
- Ducts smaller than 900mm
- Very powerful engines (90hp+)
- Poor silencers
Things to look out for:
- Efficient silencers
- 4 stroke engines
- Low revving engines
- Large thrust fans/propellers
When you are out, miles from anywhere, the last thing you need is to breakdown. Recovering a stricken craft can be difficult, so reliability is a big issue.
The reliability of the craft is perhaps the most difficult thing to judge, the best thing to do is ask an experienced Federation member. One might expect professionally built craft to be more reliable but that does not necessarily follow – maintenance is the key here. A craft known to Federation members will be a known entity, anything else should be viewed with some suspicion.
Things to look for are low stress engines – small industrial engines, car engines or some motorbike engines can be reliable.
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 usually be made reliable, but it will take considerable effort and plenty of money to do so. So if in doubt, join the AHA and ask for advice!
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.
The most important thing is weight. This cannot be over-emphasised, no amount of power will make up for the extra weight of some enormous automotive V6.
2 strokes have favourable weight – they are incredibly light and ideal for racing. But, for general use today, the disadvantages of poor reliability, high fuel consumption and high noise levels discount the weight advantage.
Two strokes are best avoided for cruising use.
4 strokes are heavier than the 2 stroke alternative, but are quieter and more efficient, and generally more reliable.
A few lightweight car engines and some motorcycle (not high revving) engines are suitable. Look for all-aluminium engines.
The Subaru flat fours continue to be popular, whilst the BMW ‘R’ series motorbike engines are used a lot.
Recent developments in 4 stroke microlight engines are very suitable for hovercraft, but may be a little expensive.
Whilst many agree that diesel reliability would be an advantage, they remain heavy just at present so there are few small diesel powered craft around. There are a number of developments in lightweight diesel engines, so this may change in the future
The Briggs and Stratton, Kohler or similar industrial/garden tractor engines in 25-40hp are suitable for small craft, say between 3 to 4 metres. Car engines are likely to be too heavy for this size of craft.
Craft of 5 metres and upwards may be suited to the lightweight car engines, such as the Subaru flat fours, or Suzuki 3 cylinder engines.
The first hovercraft were constructed from aluminium using aircraft methods.
Disadvantages: Tends to be heavy. Expensive.
Advantages: Very strong.
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.
A layer of structural 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. This is probably the best construction method suited to home construction of medium to large craft (3 metres and upwards).
Disadvantages: Labour intensive process.
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.
There a no modern designs for wooden construction, and the potential constructor is best advised to look at GRP-foam composite instead.
Polystyrene foam – Epoxy resin
Large blocks of polystyrene foam are carved into the required shape and covered with glass fibre cloth and epoxy resin. This method can be used for home-built craft given a suitable design.
Advantages: Complex shapes can be made as one-offs. Light and strong.
Disadvantages: Expensive and labour intensive.
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.[left][img width=300]http://hoverclub.org.uk/gallery/9_18_02_12_5_54_31.jpeg[/img][/left] High quality professional craft or 3 seats or more do not come up for sale very often, and when they do, they command a price starting at $10000. If you simply want a fun toy to fly around the paddock on, go ahead and have fun! In between are a few small craft which can make a sensible ‘starter’ craft, and will probably cost around $5000 for a good one. [left][img width=300]http://hoverclub.org.uk/gallery/9_18_02_12_5_54_53.jpeg[/img][/left]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.
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 – $500 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.
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.
A major problem with many Vortex craft is water seepage into the foam on the underside. Saturated foam can add 50kg or more to the craft. Check to see if there are open cracks and if water is present around them.
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 or pink, this indicates deterioration due to UV light, and they will have to be replaced, at around $200 – $300 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.
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 often rely on long bowden cables, which can suffer from salt corrosion. Check for free operation and frayed ends, and replace if in doubt.
Check guards for security and strength, and if you intend to use the craft at AHF events, that they meet the current construction regulations.