January 2003

The aluminium deck support strips run all of the way around the inside of the outer frame, and around the outside of the inner frame. The same technique is used wherever a panel is joined onto the frame Ė such as where the planing surfaces on the underside of the hull join to the outer frame, and where the top of the cockpit sides join onto the inner frame.

I fixed the support strips to the frame using stainless steel M3 screws. This meant first drilling the 3/16" strip (4.5mm) with a series of 3mm holes at around 200mm spacing, then spotting through the holes and drilling a 2.5m hole  into the tubing, which is then tapped to M3, and a screw fitted. January saw me doing a lot of drilling and tapping to fit these small screws, along the entire length of each deck support strip and hull . It might be tedious but itís a great way of fixing the metal in the right place prior to welding it.

I cut out side deck panels using the jigsaw and guillotine. The side deck panels are just rectangles, size 1617mm x 524mm. They are screwed onto the deck supports using stainless steel M6 screws. The screw holes in the deck support strip are first drilled to 5mm diameter, then tapped using a reversible low-speed cordless drill with a variable-torque collar. This is a very fast way of tapping a lot of holes. As long as you set the drill to "screwdriver" speed, you can give the trigger a quick burst to cut the thread, then switch it into reverse with the other hand to clear the swarf, then switch it to forward again and cut more thread again. It takes just a few seconds to tap a neat hole in 3/8" aluminium, which is just as well when you have hundreds of holes to tap. I used plenty of thread cutting lubricant here Ė but it eventually got everywhere.

Where the tubular frame has been bent to shape, the deck support strip also needs to be bent, to almost the same curvature. Bending the deck support strip is done in much the same way as the frame tubing, except that the support strip has a rectangular cross-section of 3/16" x 5/8", and needs to be bent the "hard" way Ė i.e. with its short width facing the inside of the bend. This means supporting it very carefully in the pipe bender, so that it doesnít suddenly flip-over sideways. Once it has flipped over it tends to get distorted, and then itís likely to happen again.

Joining the frame tubes

Sometimes, the easiest way of ensuring that a component will fit is to make it much longer than needed, then trim it back to the required size. So for the rear corners I put the corner bend into a relatively long length of tubing then trimmed it, so that when it was joined to the front half of the outer frame the overall length of the hovercraft was then correct.

Initially I joined the rear of the inner frame to the rear of the outer frame using short lengths of aluminium rod inside the inner frame ends, which can then be drilled and tapped up their centres. Screws running across the inner frame ends hold the two rods in place, and screws through the outer frame run through the centre of the rods. I fish-mouthed the inner frame ends so that they would join in a neat "T". These joints will eventually be underneath the rear of the duct, and will be welded rather than just bolted. This should give a strong but temporary joint, until the frame can be welded, which means that I can now assemble the outer frame for the first time!

Propshaft options

Looking ahead at the options for connecting the engine to the bottom pulley, I really wanted to use some sort of flexible coupling, rather than just fit the bottom pulley directly onto the end of the crankshaft. The K&M solution uses a crankshaft adaptor, plus a flexible coupling, then an intermediate shaft, another flexible coupling, and finally the shaft which carries the lower pulley. These coupling shaft is supported by two pillow bearing blocks. The total cost for all these would be around £170 including the bearings so I checked out a few alternatives. A normal car-type propshaft with UJs looked just as expensive as the rubber coupling type, so no real advantage there. In fact, getting a UJ propshaft custom made would be a pain. Either way, spinning it at 10,000 rpm seems a bit hairy. So in the end I went with the K&M method after all (picture above).


You know what itís like when you get back from your holidays in the Winter and the house is cold? Even with the central heating flat out it takes a couple of days to warm the house up again, right? Well as the garage is normally unheated thatís what itís like in there most Winter weekends. An electric heater makes a difference in the area where I stand, but I canít bring the whole garage up to temperature quickly. At one point the temperature inside the garage reached 1C below freezing!

A real bind

I decided to join the frame pieces together using 5" lengths of 19mm diameter tubing, which is roughly the same diameter as the inside of the frame tubing. I tried fitting the outer frame sections together, but the joining tubes jammed with an inch still left to go. I couldnít bang them together any further, and I couldnít pull them apart. I got really mad at this point, but luckily the wife appeared before I could attack the frame with a large hammer. This calmed me down. But I still couldnít get the tubes apart.

During the week, I asked Roger from workshops about the problem "Does aluminium bind like that then?" I ask him. "Course it does" says Roger, "You want to watch that". Right. Thanks Roger. Iíll remember that...

So what should I do next? I had two lengths of tubing that make up the side frame, but theyíre about an inch away from being joined together properly, and couldnít be separated either. So I cut through the joining tube, hoping to be able to drill the remaining pieces out, but then I needed a 19mm drill bit... and of course a chuck that will take such a wide bit.

I borrowed a 19mm drill bit (with a half inch shank) from work and tried drilling out the remains of the tube that was binding and stopping the outer frame from going together properly. It started OK, but then the handheld mains drill got so hot that the fan melted! The drill was on its way to Silicon Heaven anyway, so I dismantled it, removed the remains of the fan and reassembled the drill. I continued drilling carefully, and things seemed OK. Well it got the job done anyway. I drilled out most of the tube then knocked the rest of it deeper into the frame, and inserted a new joining tube.

Looking back, I was getting really pissed off with this jammed tubing, but it turned out OK in the end.

After 50¼ hours the deck and top frame is now assembled !!!!Ė although it's not finished as it has still got to be welded (gulp) together.

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