Hovercraft trailers are a bit of an art - there are some abominations out there. Some people seem to spend almost as much time getting their craft loaded and unloaded than they do hovering.
The problem lies with the fact that hovercraft are relatively lightweight (even a big cruiser probably weighs less than 500Kg) for their size. It's difficult to buy a lightweight trailer that is large enough to take the hovercraft body! Boat style trailers aren't really suitable as they have no structure to support the hovercraft hull. Large flat bed trailers are far to heavy for the job!
Often the best place to start is with a caravan. A basic caravan trailer base is usually pretty lightweight and can be as big as you want! It's also a pretty cheap way of getting a trailer - you can pick up an old caravan for £50-£200 with a good chassis and then just remove the body. You will probably have to shorten the back end of the chassis to get the balance right for a hovercraft - make sure you get a longer caravan than you need!
There are two basic styles of hovercraft trailer - flat bed and open frame. Flat bed are usually the hover on/off or lift on/off types and may be tilt bed or fixed bed with ramps. Open frame can be almost any shape - the craft has to lifted or winched onto these trailers on rollers or bunks.
My basic guide to hovercraft trailer design is:
1. Place the axle at the craft centre of gravity (obvious, I know, but you'd be surprised how many trailers are badly loaded!). The weight of the drawbar should be give enough hitch nose weight for stable towing.
2. Make the bed as low as possible (even make it slightly tilted to lower the back end). You could also hang the chassis frame under the axle rather than on top to lower the height another 50mm or so. This is probably the number 1 tip - a high bed makes a tilting deck too steep to get a hover back onto the trailer and also makes the ground transition angle too high to make unloading easy. In addition, the lower the bed the more stable the trailer is when towing (and the lower the wind resistance!)
3. Use side guides to stop the craft drifting over the trailer edges during loading/unloading (also keeps it on the trailer when driving! Use either short trailer sides (keeps the skirt in) or a couple of roller posts each side.
4. Build it light! The structural steel frame only needs to be fitted under the hovercraft skids - the remainder of the bed and support structure can be very lightweight. The bed and sides/tailgate, etc can be lightweight ply - all they are doing is keeping the cushion inflated (load = 0.08psi?). You should be able to easily make the trailer and craft under 750Kg (non-braked suspension units and hitches are much cheaper!). Use lightweight box section (20 or 25mm) to make angled bracing to stiffen the frame rather than adding heavy steel section (in other words, design it properly :-).
5. The simplest way to get the trailer bed to tilt is just to hinge the drawbar where it joins the axle cross member (a 12-16mm bolt through a couple of bits of angle welded to the front of the axle).
6. The angle the trailer bed makes to the ground is critical - much over 8-10degrees and you won't be able to power the hover onto the trailer. The cushion will collapse when the nose is on the trailer and the back end on the ground and a gap opens up in the middle.
7. Place the wheels as near to the outside edges as possible - the trailer will be much more stable to tow.
8. Use the correctly rated suspension units. Calculate the final weight of trailer and craft - don't use a higher rated unit as the hovercraft will then have to absorb the bumps rather than the suspension!
I Couldn't find a suitable trailer for the UH18 so I decided to design and build my own
It's a tilt bed trailer hinged by the axle (all that happens is the the single drawbar is hinged where it attaches to the axle bar). The trailer support frame is made to conform the the shape and positions of the landing pads on the hovercraft and the non-load bearing deck is lightweight 9mm ply (it is only there to prevent most of the cushion air escaping). Originally, I made tilt down sides - they tilted to about 45 degrees. They weren't high enough - the craft could just hover right over them. Eventually, I just fixed the sides vertically - they now act as guides to keep the craft centred on the trailer bed. The chassis is made entirely made from 30mm box section with 50mm for the axle and drawbar.
The trailer has been in use for almost two years now - I can now launch the hovercraft in under 5 minutes! Recovery is a bit trickier in that you need to be able to stop the craft slipping back down the trailer bed when you reduce hover power. Usually I can get it to within 12" or so of the bow block - I then tip the trailer bed level and hover the last bit into the block. The trailer has a double folded rear tailgate that unfolds to make a ramp for the last 10" or so to the ground - you do need this as the UH18 is a long craft and you can end up with a 10" gap under the middle with the nose and tail on hover!
Made trailer top deck to hold Scout. First attempt was a four legged tilt down deck (like a parallelogram). Was strong enough but there was no way of lifting the deck from the horizontal position with the 118Kg Scout on it (I had intended using the trailer winch to pull it up). Second attempt involved removing the Scout air drive and fitting it onto a table type structure over the UH18 lift fan (with the Scout hull still sitting on the top deck above the UH18). The trailer nose weight was too high (about 120Kg) with this design. Last attempt was to remove the rudders from the UH18 and fit the Scout air drive partially into the UH18 thrust duct. The air drive is mounted on a two legged table - the legs sit on the back edge of the trailer and the front of the table has two horizontal extended arms which fit into the end of two square tubes of the UH18 engine mount frame (and are locked in by simple cross pins). I re-made the top deck so the Scout hull was nearer to the top of the UH18 roof - it is now lifted onto the UH18 thrust duct and the Scout skids fit onto a roller on the front of the top deck - it's then lifted and rolled up onto the deck. This all seemed to work OK so I took the complete assembly to a local weighbridge to check it - it showed 980Kg as the weight (it was 680Kg without the Scout stuff (about 100Kg) - the weighbridges are obviously not very accurate at the relatively low weight!
I replaced the drawbar, coupling and suspension units/wheels with 1 ton rated versions. I used the trailer several times with the Scout hull on the top deck. It was a bit of hassle re-assembling the air drive to the Scout when unloading so I re-designed the top deck to take the full Scout weight. I use a 3metre long arm pivoted on a cross member on the trailer deck to change the direction of pull of the winch to nearer vertical - after lifting, the bar is attached between the top deck and the front of the trailer to transfer the forward braking load of the the Scout down to the deck (so it doesn't end up in the back of the car!). I also made the deck arms different lengths so that it was still tilted when in the down position
The above pics show the craft half way up and fully down. I used the hand winch to do the lifting - it was hard work and very dangerous when releasing as the winch only had a one direction ratchet. I then bought a small 12v electric winch. The system seemed to work fine although I was worried about the stability of the structure when on the road (the top deck is 6 feet above the trailer bed and the top of the Scout guard is another 6 feet above that!). I used it quite a few times over the following weeks but still took the air drive off before lifting the Scout hull up.
After some more static tests with the full weight I bent the 3m lifting bar (it was only 30mm square tubing) so I replaced it with 25x50mm tube - much stronger). Eventually, I built up the courage to try it on the road (see pic above). It was fine - the trailer seems a little more top heavy as expected but not too bad. The bed flexes a bit under severe bumps and wallows but looks plenty strong enough. The unloading time has increased to about 15 minutes now! The car on the right of the above pic is a 6foot high 4x4 which will gives you some idea of the fully loaded trailer height - I'll need to watch out for low bridges from now on!
Started to design a trailer for Prospector. After much deliberation I've decided to built a custom trailer. Originally, I intended to buy an old caravan and use the chassis but the number of modifications needed make it not worthwhile. I intend to build a 750Kg trailer (no brakes needed) on the roller model specified by Sevtec - the trailer has no deck and just a set of parallel rollers which the craft skids ride on. The back of the trailer will be lowered (using a simple tilting deck) until it is about 9" from the ground. The rear rollers will then be located on the front of the skids and the trailer pulled back down - this will raise the front of the craft to about 15". The hovercraft is then simply winched onto the trailer where it sits on six rubber boat keel rollers.
Getting the craft back off again should mean just pushing it back until the back end touches the ground then pulling the trailer forward to drop the front end. The only problem I can see with this is that the partition skirt may get caught on the rear rollers. A possible solution is to make small ramps in front of each roller to make the skirt edge ride up and over the roller.
I also need to provide some method of holding the skirt up so it doesn't trail on the ground! I intend to make side covers from skirt material that is attached to the inner trailer frame and has a bungee cord around the outer edge. This cord will clip to small cleats attached around the hovercraft hull and will trap the skirt between the lower hull and the cover. These cleats will also be used to attach the lower part of the side cover (the upper is attached to the hood).
The roller trailer was a bit of a disappointment - see the prospector build diary for the result. I eventually build a tilt bed trailer with a flat deck so I can hover the craft on and off easily.
After damaging the trailer while unloading on the first water trip (see Hover Diary) I decided to make some changes. The main problem is the hovercraft is unstable when on hover over the trailer - because of the sloping sides it is always trying to balance at the top of the sides. The result is that it falls down one side and climbs the other - it won't stay in the centre. I have re-made it as a a completely flat deck with 'bumps' over the tops of the wheels. The wheel bumps are just plastic mudguards with a ply plate fitted to the inside to stiffen them and seal the air gap
There are short (10") drop down sides that will also help to keep the skirt inside the trailer on the road. I've also made up four vertical side rollers (two each side) which are extended out from the bed to help keep the craft centred during loading (they also keep the sides in the upright position on the road).
Bed lock pin Bed tipped up and held by stay.
Side angle when tipped up
Mudguard cover support
Side roller in... ... and out (locked by pin through chassis)
Ready to pull trailer out (back end of craft is on ground)
Carrier landing deck!
The trailer seems to perform OK - it's sometimes a bit hard to push the craft off far enough to get the rear skids to hit the ground (and stop the craft being pulled forward as the trailer is dragged out from under it). Loading is very easy now. I may fit a short drop down rear tailgate to block the gap under the back end when loading and unloading (it would also keep the rear skirt in) as I've noticed that the cushion collapses a bit when the partition skirt reaches the trailer edge.
I decide to re-weigh the Prospector again. It weighed 560Kg! This is over 100Kg more than when I weighed it the last time! The trailer weighed 260Kg.
The reason the Prospector weight had increased was due to the "extras" loaded into it - a box of spares: 16Kg, two full fuel tanks: 60Kg, Tool box, first aid kit, extra seats, etc.. The other reason was that the previous times I've weighed the craft I just hovered over my flat bed scales to the balance point and idled the engine to drop the craft onto the scales. This time I shut the engine off - at idle, the lift system was still producing around 45Kg of lift thus reducing the "weight" of the craft!
I've now had to order heavier duty suspension, wheels and hitch and will have to make some serious trailer modifications! At the moment, I'll just remove the fuel tanks and other junk when trailering to keep the weight down.
Re-made the trailer to suit the new suspension units. Eventually I only used the basic top deck frame - the remaining parts were re-built. I used a spare jockey wheel mechanism to tilt the deck. I took the jockey wheel off and bolted the leg to the trailer hitch bar and clamped the body to the deck - you just wind it up to tilt the trailer deck.
The finished trailer works fine. The only slight problem is that I used radial ply tyres rather than cross ply and the sidewalls are softer - this makes the trailer a bit bouncier than it was previously.
- deck down
- deck tilted
- hitch/winch area
- underside of deck