Engine and fan frames

d6b6df4d18a828b64846c3aa3e62913cOne of my overriding design ideas for Version 2 was that the duct should be clear of obstructions and that the engine was to be as low as possible and out of the way. To achieve this I decided to put it down in the corner of the craft. In order to get it to fit and too make use of standard length drive belts that didn’t give me much height and so the engine frame couldn’t be the usual chunky steel affair.

a8b0c0b3ff33d3cd133b75bb52a1e901Because of this and also because I fancied having a go at it I made the engine frame out of carbon fibre. As you will see, I didn’t think to take a photo (despite it being a thing of beauty), but I did take one of the mould I made it in.9434348ec2baec326d616fb42f536b1e

The hull

The Hull

Firstly an apology. For some reason I didn’t take nearly enough photos of this… Sorry.

In most cases, when you fit a lift duct to a craft that didn’t have one, you cut a large hole in the front deck and fix in a glassfibre duct.

The Moto cyclone doesn’t really have a front deck to speak of so that’s not an option. You can see from the pictures of Random V1 that there was a black fly screen at the front. This was fixed to raised lip that runs all around the cockpit. The new duct is a large moulding that fits over this lip and about a third of the length of the craft.

The inside wall of the cockpit was removed as was the buoyancy up to about half way down the plenum. The new duct was fitted and because I needed the buoyancy and didn’t need a very wide cockpit, a large chuck of the foam was glassed in on each side of the cockpit.

At the back a 900mm duct was fitted in place of the 750mm. This tme there was no need for a splitter plate. The flow straigteners are standard this time and were bought from K&M products as was the duct.

A centre console was needed. The old one wasn’t long enough as there was going to be a prop shaft running the length of the craft and it’s not a good idea to crouch down over an un-guarded prop shaft that’ rotating at 3,500 RPM. A new console was built. This was made from GRP layed up on to a foam plug which was then removed. It wasn’t pretty but it was functional. Other guards were built from steel and aluminium to cover other exposeded bits. More on that later.

Random orbit 2 – The second coming

b68814ddfa8520c9ffb976e0af494753It started with an innocent enough conversation along the lines of “You know Andy, most people with a TZR have a bigger duct”… “Do they? (quick look round) Oh yes.”.

Out with the angle grinder.

Random Orbit version 1 had a 750mm duct and putting 45bhp through that is a bit of a tall order and so I decided to put a bigger duct on. I’d also heard that some time earlier, someone had put a lift duct on a Cyclone and raced it in F1.

I decided I’d have a go at putting a 900mm thrust fan and a lift fan on Random Orbit.

The rules in F3 say that you can run any number of two-stroke engines in any combination up to a maximuim of 250cc but it’s tricky finding a good combination to give lift and thrust so I decided to try a shaft drive.

[note. I’m writing this in 2008 and shaft drive craft are all over the place but when I did this job in 2001ish they were quite rare]

A shaft drive means you can get the advantages of twin ducts on a single engined craft but designing one does make things interesting.

A big note… I didn’t take nearly enough photos and without them it all seems a bit dry so this section is very short.



At the front end, the steering starts with a pair of handlebars and a piece of 25mm steel tube. This runs down through a bronze bush fixed to a plate. a nut has been welded at the bottom of the tube and a bolt fitted through the hull. The tube is simply screwed on the the bolt so the bars can swivel round on the bolt.

I used a ‘U’ bolt to clamp a flat piece of bar to the top of the tube and fitted the end of the morse cable to that. The movement of steering is restricted by two bolts mounted in the centre console so I can’t put too much weight on the morse cable.

At the back, the morse cable ends a right angle bracket. the center of the cable connects to the right hand rudder which is in turn connected to the left by a piece of aluminium tube and couple of rod ends.


0b844b83baabf4892d821ea3a73ac673I have a pair of rudders made of glass fibre on a foam and balsa core. I decided to experiment with vacuum bagging to see if I could make a glassfibre rudder without making two haves and joining them together. With this technique, I made two end pieces from “T” section aluminium, fitted captive nuts to use as mounts. I sandwiched one at each end of two pieces of foam with a piece of balsa down the centre (just to give it some strength whilst I was making it). The next job was to carve the shape. As the foam is very soft, it was a simple matter to sand it down.

I then layed up some glassfibre matting with resin, wrapped it around the foam, covered the lot in pin-pricked halar release film.

Next came a layer of peel ply followed by bleed cloth and the lot went into a plastic bag. Using a compressor from an old freezer, I sucked th air out which pulled the material and resin upto the foam. After a few hours, when the resin has cured, open the bag and trim the trailing edge.

Engine and fan frame


The fan frame is a classic ‘H’ shape made from 25mm square section steel tube. The horizontal is much larger section recangle and is drilled to take the 25mm spindle that the fan hub spins on. This spindle has been welded into place.

The frame is fixed to the front of the duct at four points and it is also fixed to the floor.

Engine frame side view When I started building this frame, I was given a very sensible piece of advice which was to make sure that the fan frame can be removed from the craft without having to go underneath. This makes it much easier to get the frame out if it requires emergency repairs. With this point in mind, I fixed two pieces of angle iron to the floor and then bolted the fan frame to those so that it was easier to remove the frame.

Engine frame front viewThe engine is mounted in a cradle-like sub-frame that hangs the engine from its original mounts. It also carries the plumber block bearings.

This frame is bolted to the fan frame at a pivot on the right. The left hand side is supported by two turn-buckles so that I can quickly raise or lower the drive pulley to tension or change the drive belt.

Drive train

The drive is taken from the crankshaft through a flexible coupling, a couple of plumber block bearings to an 18 tooth H200 pully on a taperlock bush. A 410H200 drive belt takes the drive upto a 54 tooth pulley at the top. This is bolted to a hub which in turn, carries the fan.

I used 6 Multiwing 4Z blades in a 9 blade hub. Although this looks a little odd, it seems to work quite well.


The engine is a Yamaha TZR unit from around 1988. This 2 stroke liquid cooled twin develops around 45HP and revs to 10500 without any trouble. Converting to hovercraft use seems a little daunting but can be done in 2 stages if needed and is not so bad when you get down to it.

The first stage involves removing the contents of the gearbox casing, but leaving all of the existing engine mounts. For the second, you cut the gearbox casing off and bolt mounting bars underneath. I went for option 1.

The first thing to do is to strip down the engine totally and take out the gearbox internals. Now you need to take a drive for the fan and pully straight from the crank shaft. This saves weight and reduces losses but means loosing the oil pump and water pump both of which are driven from the gearbox.

Living without the oil pump it not a problem as this is a 2 stroke and it’s only used to add oil to the fuel, which in turn oils the crank. Just increasing the jets in the carb a little and adding oil to the fuel sorts that one out.

The water pump is slightly more tricky. Several types have been tried in the past including pumps driven by the fan belt but most people have settled on an electric pump. The trick here is to find one that’s rated for around three gallons per minute and can stand nearly boiling water. Don’t be mean on the water pump of you’ll end up paying in engine rebuilds. Also, remember to get the direction of flow through the engine right. In through the bottom, out through the top.


One thing to keep in mind is the problem of vibration. These engines are revving to around 10500 RPM and really do vibrate. If you bolted everything together and to the hull, something would break. So somewhere, there needs to be some rubber mounts. If you go for option one on the engine, as I did, you get the two original rubber engine mounts. With a flexible coupling on the drives shaft, the problem is solved. With option two, cutting off the gearbox casing and bolting mounting bars to the base of the engine, most people bolt this down to the fan/engine frame and then rubber mount that to the hull.

Random orbit 1. The first version.


When I first got the hull, it had a large bench seat across the cockpit. As I wasn’t planning on taking passengers, it had to go. Racing hovers don’t generally have seats at all, you need to be agile and move around from side to side so I went with convention an fitted a centre console.

I made an early decision not to hide anything important inside the console. If anything breaks, it’s important to be able to get at it as quickly as possible.



You can see from the photo that the console has the master kill switch (inside the red triangle), the water temperature gauge, rev counter (on the right), steering cable and limit bolts.

The handlebars are connected to a piece of 25mm steel tube with a nut welded to the bottom end. This nut is wound onto a captive bold that goes through the hull. You can see the top of the tube comes up through a slot in the console, an on through a bush in the folded steel mounting bracket.

The white cable hung over the handlebars is the lanyard. The kill-switch is on the left hand side of the duct.


The duct is 750mm internal diameter and as it’s an integrated craft, there is a splitter plate running horizonaly across the duct about 150mm up from the bottom edge. Any air going below the plate provides lift, any air above provides thrust. In the center is a cone to help smooth out the airflow as it passes the centre of the fan assembly and to hold the flow straighteners.

Flow straighteners.


Flow straighteners and generally regarded as “A Good Thing”. They are supposed to improve steering and help the airflow through the duct. They also do a good job of helping to keep the duct round, which is important, and I’m sure a host of other things.
The flow straighteners I made for Random Orbit are unusual in that they sweep around the duct as well as curve down their length. The idea was that air flowing off the fan blades should be straightened as gently as possible as each time you poke it you knock our some of the energy and I want as much as possible. It’s quite hard to explain a shape so take a look at the pictures.

In the beginning


It was 1998 when I decided to build a hovercraft. I’d joined the Hovercraft Club of Great Britain (HCGB) and been along to a few meeting and met a few people. After a while I started to notice the differences between craft and formed an opinion of what I was after. I spotted an advert in Light Hovercraft, the HCGB’s magazine for an old hover. It didn’t have an engine but it did have a duct, steering, a skirt and engine frame. At this point I didn’t have a tow bar or trailer so a quick phone call to Dad to see if he was up for a trip to Maidstone in Kent for the day.

When we got there the craft was just as I’d expected so a quick look round and money changed hands. At this point I was told it had been used as a corporate craft, giving rides and training people to drive a hovercraft for fun. It was a Moto-Cyclone, around ten years old, originally powered by a Rotax 447,  had a duel bench seat, side bar steering and a 750mm duct.

From these beginnings I’ve had a great amount of fun. So much so that. I built this craft up from the hull twice, hence the version 1 and version 2 labels I given these pages.



Just imagine, you’re racing down the back straight toward the water. You’ve got the throttle wide open, the engine is buzzing at ten thousand revs. As you get to the edge, shift your weight back a little and head out over the lake to the right of the buoy. There’s a craft stuck just off your line in the middle of the lake so you keep one eye on him to see which way he’s drifting. Round the buoy and heading down wind so you get your weight further back. Two craft come up behind on the outside, an F1 and an F2. They’re much more powerful and so get past. There’s a marshal on the headland waving a yellow flag frantically to warn you of something round the corner, just out of sight. As you speed into the inlet, you can see two F25s that didn’t make it up the bank. The two guys that just passed you have got the line wrong, gone wide and are trying to recover and there is just enough room for you. You back off the gas – but not much – shimmy the back round, open it wide again and get up the bank in front of the lot of them!

This is dedicated to my experiences hovercraft racing in the UK.

Andy Collins.

Here in the UK, the racing season lasts from around May to September and a dedicated band visit various venues around the country from Fawley in the south to Acrington in the north, with various stops in between. Racing takes place over weekends with plenty of action from start to finish. For details of when and where we’re racing, check out the Hovercraft Club of Great Britain’s (HCGB) website. I joined the HCGB at the end of 1998 and soon after bought my hull (See below). It was well into 1999 by the time I’d finished building it and so it was at the end of the season before I got a race in. All racers must take part in a series of races as “Novices”. These are mixed formula, so you may be racing against craft with vastly different capabilities but are a great learning ground before you head out into the more fiercely fought world of the National Championships. 2000 was my first complete season and I spent it in Novices, battling against other Newbies who were also trying to qualify. You can read about my craft and how I put them together by working your way down the menu.