The first hovercraft I built was a Universal Hovercraft UH18SP ( I was initially attracted to this craft for two reasons - it looked great and you could fit wings and fly it! I had not direct experience of hovercraft and had never ridden in one or even seen one in the flesh!

This build diary is a bit lacking in dates - it's mainly a set of notes on what I did and found!

Here is a drawing section:

The UH plans are hand drawn A1 sized sheets, which is a bit dated nowadays. However, they do contain all of the required information and don't have many errors.

UH18 Specifications

Hull size: 18.3ft x 7.5ft (5.6m x 2.3m)

Empty weight: 750lbs (341Kg)

Max payload: 1000lb (454Kg)

Cruise speed: 50mph

Top speed: 74mph

Fuel use: 2GPH

Engine: Subaru EA81

Propeller: 60inch 2 blade wood consuming 55HP @ 350lb static thrust, 1900rpm max

Fan: 34in diameter 4 blade wood fan consuming 10-30HP, variable speed

Hard hull clearance: 8in (204mm)


Build Notes

Started Build on 10Mar04


Used 600mm wide 3" extruded (not white bead foam!) polystyrene foam (couldn't get 1200 wide in UK). This is an advantage as I only needed 16 sheets instead of the 18 (9 8'x4') required by the plan (the sheets are cut short and the remainder used to make the narrow nose - I used a CAD program to plan the layout properly!).

Vacuum bonded foam slabs to make basic hull - left in some long screws to stop foam slippage.

Cut hull edge shape. Used 10mm ply to make skirt attach strips up full depth of sides and back.

Used screws to keep 1/8" ply deck sheeting apart in centre of craft when bonding to foam. Unfortunately I left screws in when vacuum bagging - the gap is to allow the ply to move as it is pressed down onto foam. Luckily, it didn't cause any problems - there was only one small cavity at the compound curve where the hull changes thickness near the front. I drilled two small holes in the ply at each end of the cavity (one to inject and one to let the air out) and injected resin to fill the cavity.

Turned over and bonded foam step and inner attach strip ply to base. I cut the outside edge of the curved ply so it comes to the edge of the foam to make it easier to cut the slope in the foam (using the ply edge and the skirt attach strip as guides for the hotwire)

Cut rear inner attach strip slot in foam using a router. Glued strip into place.

Lift Duct

Used CAD program to draw out elliptical shape for lift duct hole in hull. Cut out hole using a jigsaw then a handsaw - it is very difficult to get the 30degree angle correct.

Cut out disc formers for lift duct using a router bolted to an aluminium bar - bar has a pivot pin (a bolt) which allows the router to be rotated around a perfect circle. I cut out both disks at the same time. Using the router gives a perfect square edge on the discs with no further finishing required.

Cut ply for disk and bent it around former. It was really difficult to form shape - the ply split in a couple of places and I ended up screwing wood straps through the ply into the former to hold the shape. I should have paid more attention to the plans! I cut the ply in one piece meaning that the outer layer grain was the WRONG WAY - this explains why it was difficult to bend around the former! Also the instructions show the WRONG WIDTH of ply strip for the duct - it needs to be about 22" wide, not the 12.5 on the plans!

Fitted duct into hull. I left the wood straps attached to the duct to strengthen it. There were gaps in the foam between the hull and duct - but expanding foam is wonderful stuff! Glassed duct to hull on top. Made foam rings for top edge using ply outside rings leftover from cutting the discs as formers. Bonded and foamed the rings to the top duct - needed quite a few rings. Removed discs. After a couple of days the duct changed shape and was no longer round! I used sections of the ply disc outside rings to pull the duct back into shape - the ply was screwed then glued to the outside of the duct wall edge on (I removed the screws once the glue dried).


Built cockpit sides and centre console - no problems with this. The cockpit sides don't hold their shape vertically (they tend to bow out at the back) as there is no support across the top edge - however, this doesn't matter too much as both sides look the same.

Thrust Duct

Cut discs for thrust duct using router as for lift duct. Made big frame to hold discs on a 25mm shaft and bearings. When mounted and rotated the discs were out of round by about 1/8" - it was also almost impossible to get them to be axially straight. Eventually I used the router the make the edges straight and level. the disc was then about 1/4" too small in diameter so I tacked an 1/8" play strip to the outside edge of the discs all the way around (the ply spanned both discs so I ended up with a drum). The result is very rigid and stiff. With hindsight is would be better to mount the thrust prop hub to the shaft and bolt it through both discs - this would keep the discs level and also prevents the centre hole getting 'worn' as you rotate the former while working on the duct

Joined ply strips for duct. Cut to length and folded around discs. It was misaligned by about 1/2". Used router to make both edges straight. This made the plywood width about an inch less than it should be.

Cut foam circular sections using hot wire on length of string (I used two tables to get the radius required). I used 3" foam instead of 2" plus scrap bits as the plan specified to make it easier to form leading edge. Cut the sloping edge on the foam by mounting the foam on a table about 31" higher than the hot wire string anchor point. It was quite difficult to make this cut and get a smooth surface because it is a compound curve (it must be smooth as it is glued to the plywood duct). I cut slots in the foam every two inches radially to a depth of 2.5" using a wood saw with 1/2" wood blocks attached to the tip and handle of the saw. I also cut intermediate slots to 1" deep as I was using 3" thick foam instead of 2". The foam bent around the duct easily so I didn't fibreglass the outside as the plans specify. I fitted foam to disc - it was difficult to align the different bits of foam and to get the joints to match neatly. Eventually I filled the joint gaps with off cuts of foam cut to the correct shaped. Once it hardened I used the rotating rig and a router to cut the outside surface shape square - very messy! The router cuts the foam nicely if you let it cut slow - too fast and it chews lumps out of the foam. I had installed a small flat table area onto the rotating jig at one end and then fitted a straight edge at the correct distance to move the router along. You need to rotate the duct 360deg while routing a strip, then move the router along by the cutter width and repeat (use as big a face cutter you can get). The end result is very good. I then cut a C shaped disc so it cleared the inner edge and was the correct radius to form the inner duct lip. I attached the router to one end of the 'C' and screwed the other end of the 'C' as a pivot to the rig table. By rotating the duct then moving the router I cut a perfect radial lip! there is a bit of work involved in making the various jigs but it is well worth it - it makes the job very easy and the result good!

A problem was that I had cut too much foam away in straightening the edges! The finished duct is about 1.5" too short front to back. This shouldn't really cause any problems. With hindsight I should have started with a wider strip of plywood and foam! In reality, it would be far simpler just to built up several straight flat layers of foam around the former rather then messing around cutting curves and flat edges. The glued joints in the foam would be easy for the router to cut when forming the shape (obviously you only need to build up the foam thickness as you near the leading edge of the duct.

I filled the small gap between the internal front ply edge and the start of the foam slope. The duct is perfect - it is only out of round by 2mm!

Installing the thrust duct.

I cut a flat on the bottom of the thrust duct as designed - again this was easily done on the jig using the router and a straight edge. I trial fitted the duct to the hull - again the plans are incorrect on this. They show the duct lower back inner edge to be 2.5" up from the hull. This makes the duct centre 32 5/8" from the hull (2.5 + (0.5 x 60 1/4")). Then engine mount from the hull to the bearing centre is:

wood engine mount strip 0.75 + clearance for rubber mount 0.5 + engine mount height 28 + engine mount to bearing centre 2

TOTAL 31.25

The prop shaft will be low in the duct by 1 3/8" - this seems a lot to me! I finally decided that I would leave this as it is - it is relatively easy to move the bearings and/or the whole engine mount up by using packing. I don't have the drive belts yet so leaving this amount of adjustment is probably a good thing.

The plans do not show the design of the trim wing supports very clearly - you need to fit a pretty large (4") triangular spacer to get the support positioned at the correct height and horizontal distance


Next were the rudders/trim wing. These are fairly straightforward to make. I used a belt sander to remove the material to the shape. It's not that clear that you should only do one side of each at a time - the foam is very brittle near the trailing edge. As a guide I used 1/2" (for the trailing edge) and 5/8" (leading edge) wood strips butted against the edge of the 1" foam - this makes it easy to get a consistent thickness on the first side. When the other side was made I used a 1/4" strip for the leading edge and the edge of the table for the trailing edge (the wood centre section was fixed to the bench). I then attached the rudder and trim wings arms to finish them. Note that it is critical that there are no pin holes in the fibre glass matting - paint will not stick to glass fibre! You may need to overcoat the glass with resin afterwards before painting

Engine Mount

I then made up the engine mount - again the drawings are not scaled very accurately. In particular the position of the forward engine bearing mount is incorrect on the views shown. On the subject of bearings I chose to go metric for both shafts and bearings - I used 25mm for 1", 30mm for 1 1/8" and 35mm for 1 3/8".

I then removed the Subaru engine from the pick up (I managed to get a 1993 pickup with only 65K miles for only £50!). The pick up still uses a carbed engine so was ideal for this. The starter motor on this engine is mounted to the gearbox side of the casing - I had to cut away a section of this casing around the starter to keep the starter mounting points. This engine has 8 flywheel bolts and is fitted with a 32mm OD bearing to support the gearbox shaft.

I couldn't source the flexible coupling specified in the UK so I had to use a two part 'spider' coupling. The coupling flywheel adapter is supplied as a general purpose large diameter casting (about 14" x 1" thick) with no mounting faces or edges - the diameter had to be reduced by 4 inches to fit the Subaru flywheel clutch mounting holes. I also reduced the casting thickness on the outside edge to 15mm to reduce the weight (the adapter weighs more than the Subaru flywheel! As this coupling assembly is also slightly longer than the specified one, I have shifted the front bearing mount backwards to behind the vertical strut. I balanced the fly wheel adapter using a car wheel balancing machine (I just removed material from the adapter by drilling blank holes opposite the 'heavy' spot).

Control System

I used the Subaru steering column shaft and bearings cut down to about 4" in length. The universal joint in the steering shaft was used to make the trim wing control movement. The whole assembly is slightly heavier than the version on the plans but is very smooth to operate. I got a twist throttle from an defunct Yamaha scooter. I decided not to implement the remote lift fan cable tensioner - I've just used the threaded rod method. The variator will be controlled using the Subaru handbrake lever - it has a latch so the variator can be left set to a particular speed. I decide to use a single cable for the trim wing pulling the wing down (nose up) and a return spring to pull the wing back up again. Hopefully this will work OK! I have routed the trim wing cable inside the centre belt guard/seat then through a length of plastic pipe to appear level with the back of the duct. I have mounted a roller at this point. The cable then goes upwards at a slight angle to connect to the end of the trim wing. I used a single roller at each side of the cockpit to route the rudder wires.

Engine Control

I need to figure out the wiring for the Subaru engine - I removed most of the wiring harness from the car. Lucky I did! There are several as yet unrecognized control boxes attached to the wiring. The Haynes manuals for the Subaru don't have the correct wiring diagrams for the pick up version. The engine has no engine management system but it does have a fuel pump control unit that seems to monitor the ignition system for spark activity. It may just be a simple timer that runs the pump for certain periods of time depending on the engine speed or activity?

I intend to re-use the Subaru ignition switch, temperature gauge and warning sensors (charge, oil pressure and temperature. I have a rev counter from a boat which I will connect to the engine. The only thing I don't have is a speed indicator. I will either buy a GPS or make a hot-wire type airspeed indicator (using a hot wire airflow sensor fitted to car engines?).

Prop and Fan

I bought these from UH - they looked a bit tricky to make for a newbie! I made a balancing jig frame using two sections of sharpened hacksaw blades as the edges for the shaft to roll on. The prop was pretty well balanced before I glassed it - I only need to add a small piece of fiberglass to one blade to balance in perfectly. The lift fan was well out of balance. I used stainless steel wood screws (of various lengths/weights) screwed into the centre hub as balance weights. The advantage of these is you can remove them or replace them easily and they won't fall out!

Mounting the prop/engine frame assembly onto the hull is pretty tricky. It's difficult to get the prop centred in the duct correctly. You need to centre it perfectly side to side. Leave a larger gap at the top than the bottom (the push from the prop will tip the engine frame up and close the top gap). Once it is running you will need to make adjustments to these gaps - I tightened or slackened the rubber engine mounts to get the prop to run without touching the duct.

The lift fan hits one wall of the duct in spite of there being a 5mm gap when stationary! I have adjusted it back and forward but can't get it to stop hitting the wall. It only does it at a certain engine speed. After looking at the lift fan mounts I discovered that the only way the fan could touch the duct is if the two wood mounts twisted. At first I couldn't understand how they could be twisted as the load on the fan should be equally distributed around the circumference. The only cause I can see is the belt itself - the belt effectively pulls on one side of the drive pulley more than the other. This force may be enough to twist the mounts. I intend to design and fit some kind of brace or stiffener in an attempt to stop this twist. Probably the simplest method would be a couple of horizontal rods at 90degrees to the main mounts attached to the duct wall. I eventually fitted a small (25mm sq) rod between the duct wall and the lift fan mount (right next to the angle iron contact point. I shaped a round onto the end of the rod and drilled a hole in the duct wall to locate it - the other end is held to the mount by glue and a small r/angle bracket. After re-assembling everything the fan now doesn't touch the duct wall anyplace no matter what speed it is running at.


Cutting out the skirt was simple enough if you are careful. Gluing is a bit trickier! The glue I used (Gripsotite PTA) doesn't have a very long tack-to-dry time. this makes it difficult to do the curved corners quickly enough. Just to be sure I re coated inside and outside joints with glue. I also fitted a reinforcing strip over the joint on the inside of the skirt. Actually fitting the skirt was again very simple if the instructions on marking are followed (although putting in the 400 screws is hard work on the inner tack strip).


First Hover!

On 4th July I got the system running (no thrust yet) and the craft hovered - a bit nose high but OK. On 12th July I got the thrust sheave fitted and installed the prop. I moved the craft to a small field 200m away (the field had 1/2 cut grass and 1/2 long grass. With hindsight the field was less than ideal for a learner - it had a small flat area (about 20 x 70yds) at the top of a hill that sloped all the way down to a wire fence. I moved to the bottom of the hill to start and got the craft on hover. The nose immediately drifted downhill towards the fence :( I took to getting out and pushing the nose around to point up the hill. I couldn't get the back end to hover properly - it seemed to be dragging on the ground. I eventually discovered that the skirt pressure was lowish - I hadn't made the splitters deep enough - they stopped about 4 inches below the fan blades. I extended the splitters by adding another bit if 3mm ply and returned to the field. I could not shift the back end around on hover. After a bit of practice at the bottom of the hill I then started up hill - using a lot of thrust it took a while to get up to walking pace but then accelerated steadily. I drove up to the top and backed off and landed on the flat area. I then started off down hill - what a trip!. The craft accelerated down hill with no real thrust (I then discovered that hovercraft have no brakes!). I eventually reduced lift until the skids started skipping into the ground and finally came to a sudden halt with a horrible crunch! I was certain I had damaged the hull or skirt - but, after crawling around the edges I couldn't find any damage at all! I re-started and let the craft drift sideways into the long grass - the drag then stopped the severe acceleration and allowed me to steer better. I eventually practiced by going uphill on the cut grass and downhill on the long stuff! I discovered (like every other air controlled craft driver!) that the rudders don't work at all if there is not airflow over them. The natural reaction to an approaching fence or excess speed is to back off the power - WRONG! If you do this you lose control of the craft - you must keep thrust airflow over the rudders to be able to turn the craft. After a few hours of practice I could eventually get the craft to go where I wanted it to and land when I wanted. I learned to do 180 turns with thrust to stop on downhill stretches and also not to attempt to land on sloping surfaces (as you back of thrust the craft starts to slide down the surface before the lift power decreases enough for it to land on the skids so you risk trapping the skirt under the skids). Over the next couple of weeks I practiced more until it became a bit more second nature to control the craft. The long grass in the field was then cut which made the downhill section a lot more difficult. I had to start the 180 turn to brake about 1/4 to 1/3 of the way downhill to make sure I could safely stop before the bottom fence! The field also sloped sideways a bit and I eventually used too much thrust and couldn't turn the craft quickly enough to prevent me ending up in the hedge along the side of the field. I actually hit a couple of fence posts in the hedge hard enough to knock me off the seat. A small section (about 3") of the side skirt support strip had been pushed in by 1/4" compressing the hull foam in this area and splitting the top ply deck. I cut out the deck ply in the area (only about 4 x 3), packed out the compressed foam by gluing and tapping in sections of ply. I filled the cavities with epoxy/filler mix (letting it run into any gaps then replaced the ply deck section. A coat of paint and it is probably stronger than it was before!

First Water Experience

On 11th August the weather looked good for a trip to a local lake (Keilder Water in Northumberland). This lake can have some serious waves (3ft+) if there is a southerly or westerly wind but this day was forecast 1mph wind. I got there around 1pm. I launched by reversing the trailer into the water down the concrete boat ramp. Big problem! The trailer complete with hovercraft floated around on the hitch! I lowered the trailer side and back boards but they also floated! Eventually I had to stand on the trailer bed and boards at the side to keep in underwater! I started up the hovercraft and, at idle, could push to off the trailer easily. I left it sitting on the water while I parked the trailer. When I started out there was a LOT of spray - the windscreen was plastered and the rear cockpit was very wet. I puttered around in a circle offshore about 30 yards and came back in. After feeling confident about the mechanics I then drove straight out past moored boats and opened up the power. The spray increased dramatically and the craft didn't seems to pick up any speed at all. I kept the power on for about a minute and eventually it started to slowly accelerate - the spray also moved backwards past the cockpit front making it possible to see where I was going! The tail end was still dragging in the water - speed was about 12mph. After about 5 minutes cruising at this speed the back end lifted and the craft accelerated to about 25mph. The spray reduced to next to nothing and all behind the hovercraft! After practicing turns and stops/starts I returned to the launch ramp. This time I drove up onto the ramp before stopping. The problem with spray and slow speed was caused by water in the skirt - the craft had sat in the water for about ten minutes before I started and the skirt was full of water - I only had a couple of small holes at the back of the skirt (both of them caused by damage on the contact line) so it took some time for most of the water to drain out. I started out again directly from the ramp and the craft accelerated straight up to cruising speed with no problems! I cruised around for an hour or so and reached a speed of about 40mph on part throttle. On the last run the top part of the control stick came off in my hand - no throttle or steering! I got back to the ramp by twisting the throttle under my arm and steering by pulling on the wires on the floor!. The control stick rod came off because of my dodgy welding! I hadn't welded enough of the tube onto the old Subaru steering universal joint (I had tried to prevent the bearings in the joint overheating during welding). I taped the rod back onto the remainder of the shaft using duct tape and cable ties and had another run on the lake. That was enough for the first day on water! The launch area had no flat bits where I could get the craft back onto the trailer easily so I tried to do it in the water. Again, the trailer floated, the side boards and tail floated and the skirt filled with water again. After much struggling I got it back on and pulled up the ramp. The amount of water still in the skirt was surprising - it poured out of the small holes for about 5 minutes on a slope! I took the craft home repaired the skirt holes, added proper drain holes and fixed the control stick.

Prop/Fan Balancing Problem

I marked the rotating prop sheave and hub and discovered that they had a small wobble. I bought longer taper lock bushes (50mm instead on 30mm deep). I dismantled the prop shaft assembly and fitted the new taper locks on the bench. While doing this I also fitted prop tape to the end 18" of each prop blade. When I checked the balance of the prop I noticed that the two blades didn't track each other - the track was about 5mm different. I double checked that the hub was true and it was. I then used a sander to remove a couple of coats of paint on one side of the prop hub (where in contacts the alum hub). This reduced the tracking error to less than 2mm. I re-assembled everything and fitted the shaft assembly back onto the engine frame.

I checked and adjusted the centering and level of the engine frame (it needed lifted at the front to adjust the height and align it to the duct). After the usual fiddling around I got the prop centred as well as I could. After re-attaching the ply fins I noticed that there was a smaller gap at the top of the duct compared to the bottom.

The duct is not very rigid - it distorts very easily. By gently pushing on one side you can make the prop strike the duct. I pondered this for a while and eventually came up with a very simple solution to help keep the duct an exact circle - I attached a horizontal wire brace across the front of the duct between the two trim wing supports. This prevents the duct becoming elliptical in the vertical plane. I tensioned the wire to make the duct perfectly round again. The prop now doesn't strike anyplace at any speed!

While running the prop tested I noticed a very strong vibration at about 2200rpm (whole craft shaking). I couldn't tell whether the vibration was coming from the front or the rear so I disconnected the lift belt to determine whether this 'balance' problem was caused by the lift fan or the prop. The vibration disappeared. I took the lift fan out and re-balanced it (it seemed to be out a fair bit). I re-installed the fan but the vibration was still there (although at other speeds it was much smoother). The cause of the vibration was NOT the fan or prop - it was the skirt suddenly releasing because of the high pressure under the craft at this speed - generating a farting/flapping sound at various places around the craft. This caused the craft to drop a little as the pressure was suddenly released then repeating the sequence at a different place. when you looked at the skirt it appeared to be rippling. The causes for this were that I was operating the craft on a flat bed trailer at high lift fan speed - the trailer/skirt were providing a perfect seal and allowing the pressure to build up under the craft to a point where the skirt would be forced out/up at one point and the pressure suddenly released. The result was the craft would shake around at a frequency of about 10Hz and felt like a severe imbalance!

That's it - all done!

To see how the craft performed look into the Hover Diary! For info on any modifications and maintenance see UH18 Maintenance