Ever since I was a young teenager, I had wanted to build a hovercraft. Back in around 1980 I sent off to the Hover Club of Great Britain, as it was then, for an information pack. But it all looked very complicated and was clearly out of my league at that point. I knew nothing about engineering then (some would say I still don’t…), but I suppose it stayed at the back of my mind. One day I would do it.
Join the HCGB
Then in Summer 2002, having finished building a double garage at the end of the garden, I decided that I wanted to do something a bit more interesting. My first thought was to take up paragliding, but a colleague at work who had done quite a bit of it put me off the idea, with stories of serious injury and death. Then I came across the Hovercraft Club of Great Britain on the internet, and joined the club. I sent off for the Constructors Guide, and the "Flying Without Wings" club video.
The video looked great - a mix of commercial and home made machines racing around the lakes in the grounds of Britain’s stately homes and parks through the Summer months. It sounded safer than paragliding – apart from the bit in the video that said "hovercrafting has a higher incident rate than other motor sports…". Racing certainly looked exciting, but what I had always really wanted to do was to build one myself.
Hovercraft racing is categorised depending mainly on the (two-stroke) engine size. Formula 3 (and Formula junior) are up to 250cc, Formula 2 is up to 500cc, and Formula 1 is anything over 500cc (wow!). There is also Formula 25 for four-stroke engines limited to 3,600 rpm.
As this was my first hovercraft, Formula 3 seemed like a good place to start.
The Constructor’s Guide by Jeremy Kemp (available from the HCGB) is fantastic! It doesn’t provide detailed plans, but it does give a description of every stage, explaining how and why it is done that way, and showing various approaches and explaining the pros and cons of each.
This is the book to get if you are even thinking of building a hovercraft.
Jon Curtis built his own hovercraft using just this book and his engineering skills (See the July 2003 section for details)
An Eagle 1 ?
One of the main hovercraft parts suppliers/builders in the UK seemed to be K&M Products, who produced plans for the "Eagle 1".
The Eagle 1 looked very similar to the hovercraft described in the constructor’s guide, but with the Eagle 1 plans you get 5 sheets of drawings, with every dimension detailed, plus an instruction book. This was excellent value for money.
The Eagle 1 certainly looked like a good place to start, but I wasn’t convinced that the construction was right for me – it was built largely from plywood - a plywood hovercraft? That sounded a bit…wooden, a bit…low-tech. I thought the sectional approach to the bow planing area and the rear corners looked rather ugly, but then that probably made it easier to build. Could I adapt the plans?
The Excellent Eagle 1 plans pack:
5 x A0 sheets plus a manual.
My dad had spent a lot of time in the glass fibre industry and was now working at a company which built glass fibre sheds (actually enclosures for utility companies), so I could build it from GRP and probably get the materials cheap, and plenty of advice.
The Constructor’s Guide also mentioned aluminium, but almost as an afterthought. I liked working with aluminium, and an aluminium Eagle 1 might also have much smoother lines. Surely I could produce a hull with a proper curved bow and smooth corners? The "Sheet Metal Handbook" (available from Amazon) gave me some ideas.
Aluminium construction looked very promising.
I phoned Ken Rigley, the top man at K&M. He had never heard of anyone building an aluminium Eagle 1. Yes, people had built aluminium hovercrafts before, but to other designs. He thought it might work, but he recommended that I use a GRP duct rather than trying to bend aluminium to such a complicated shape.
I worked out the approximate amount of sheet aluminium I would need and then rang my local friendly metal supplier Les, at Outlook Metals in Coulsdon, Surrey. As I was looking for a reasonable amount – maybe 20 square metres of 1.5mm thick sheet, he could get me a good price - good enough that it made the aluminium hull a feasible idea.
How much would this thing weigh? I put together a comprehensive Excel spreadsheet, based around the dimensions of the Eagle 1, and the fact that the density of aluminium is 2.7g/cm3 (or 2.7 tons per cubic metre). I could use 1.5mm thick aluminium sheet for all of the planing surfaces and the decking, 3mm sheet for the floor, 25mm diameter tubing with a 3mm wall thickness for the space frame, and bracing triangles made from 32mm x 3mm T-section.
The design comes together
So the hull design was gradually coming together. Clearly, with the redesign required to make it from aluminium, it wasn't going to be a true Eagle 1, but it would have the same dimensions, and the same basic hull shape - which meant that I could use the Eagle 1 skirt segments.
I would use the aluminium tubing around both the outer edge of the hull, and around the edge of the cockpit, and the deck could then sit between the two tubular frames. Clearly it would be difficult to attach the deck sheets to the round tubing so it would also need a 3/16" x 5/8" support strip added to the inside of the outer frame and the outside of the inner (cockpit) frame. This support strip would of course add to the weight, and according to the spread sheet the bare hull looked like being around 75kg.
On top of this I would need to add the weight of an engine, engine frame and fan frame, fan, transmission components, fuel tank, rudders etc. I had a look around at other hovercrafts. Mine was clearly not going to be a lightweight machine, but like everything else I build, it wasn’t going to break easily. And let’s face it, when you‘re 6’4" tall and weigh 95kg you’re not going to get a job as a jockey. And remember what I said at the beginning – for me it’s the build that’s important, and not the winning.
So the hovercraft hull was going to be based on a space frame design, which would need to be welded together. Hmmm… can you weld aluminium easily?
A search on the internet revealed two things:1) Ordinary mortals (mainly Americans with their 110V mains system) said you would need your own power station to generate enough power to heat the metal sufficiently, and the result would probably be poor quality anyway.There was virtually nothing about welding aluminium at home using ordinary (single phase 240V mains powered) MIGs. That looked like bad news – really bad news.
2) Professional welding sites took it all in their stride, but then they used TIG and MIG sets costing thousands of pounds, and my budget was more modest.
I asked around in work and found a couple of people who knew others who could do the welding work if I couldn’t do it myself, but it would obviously cost money, and this wasn’t a route I wanted to take. If someone else was going to build my hovercraft then I might as well buy a complete ready-made hull.
A week later, my local branch of Machine Mart had a "VAT free day", so I went along to look at their MIG welders. I would need a MIG to build the fan frame and engine frame out of mild steel anyway.
I asked the man behind the counter whether I would be able to weld aluminium using one of their Clarke MIG machines.
"Have you done any steel MIG welding before", he asked – "Yes I have", I said.
"No problems then", he replied (seeing a £150 sale coming up). "This is what you need", putting a Clarke 150T onto my trolley, along with a reel of 0.8mm aluminium wire, some 1.0mm tips and a bottle of Argon. Well he had confidence in my abilities anyway…
I decided that I would have a serious go at aluminium welding, and if, after loads of practise, I still wasn’t getting anywhere then I would get it welded professionally
So how was I going to bend the outer frame into some sort of graceful curve? I had gone through the problems of bending 1" steel tubing before when I built a soft-top frame for my Landrover. This had been much more difficult than I had previously imagined, for two reasons:I eventually found a solution to this problem by buying a Clarke 6 ton hydraulic pipe bender (it’s the bend capacity which is 6 tons, it weighs a lot less!). This comes with a range of formers that give a fixed bend radius and also support the sides of the tube, preventing it from collapsing. The downside to this is that you can only bend tubing if it matches the diameter of the formers. So choose your tubing according to the formers you have available. Alternatively, you can make a filler "sleeve" to reduce the internal width of the former – that way you can use any width of tube.
Tubing is tough stuff (the good bit) and difficult to bend (bad bit) - even a long-lever roller-action pipe bender (a bigger version of a plumber’s pipe bender) was not man enough for the job. I tried this method and discovered an alternative way of moving a bench around the workshop. Once we had fixed the bench down firmly the pipe bender started to bend and at this point decide that we really weren't using the right tool – like the song says "it ain’t easy". To make things worse, tubing tends to collapse as it is bent, especially if the "side-walls" are not supported.
If I could bend the aluminium tubing like I bent the steel tubing then the space frame would be no problem.
So the aluminium hovercraft was on. I ordered the big components – the duct, expanded polystyrene buoyancy foam, duct guard and skirt from Ken Rigley (K&M products), and the aluminium from Outlooks.
Aluminium shopping list:
10 sheets of 2m x 1m x 1.5mm aluminium for the main decks and planing surfaces.
1 sheet of 2m x 1m x 3mm for the floor.
4 lengths of 4m x 25mm x 3mm tubing for the frame
1 length of 4m x 32mm x 4mm flat strip for various strengthening pieces.
plus various lengths of 5mm x 50mm strip, U channel, 12mm angle and 25mm angle
My garage was ill equipped to build a hovercraft, so the first thing was to build a large fold-away bench to cut the sheet metal on, and storage brackets hanging from the roof trusses for the lengths of T-section, angle and tubing. It also needed improvements to its electrical supply, as at that point the whole garage was running off a very long 13amp extension lead. The MIG welder needed a 16Amp industrial socket, and an electric heater would certainly help on those chilly Sunday mornings.
I work at an engineering research company. A group of engineers from work once started building a hovercraft (in their own time), but as far as I can work out they only got as far as dismantling a Citroen engine. On hearing that I had started building a hovercraft, Dave Darlington, (one of the old timers who remembered the previous attempt) bet me 50p that it would never leave my garage under its own power. I am seriously looking forward to the day when I can call in that bet!
You’ll notice that I use imperial and metric measurements almost interchangeably. That’s because I was brought up on both. But even today, aluminium tube can be sold in imperial widths and metric lengths.
Just remember: 30cm is about 1ft, 1 metre is 3ft 3" (or roughly a yard) and 1/16" = 1.5mm