Vulnerable Plywood

I'm not sure when this happened but after I flipped the hull over I noticed a small pebble (~ 1/2") embedded in the plywood. It must have been on the ground when the hovercraft was placed on the ground.  More evidence that I need to get some skid plates on the bottom asap.

The pebble embedded

The pebble popped out.

Plywood sides for lift fan cutout

I've added some plywood sides for the lift fan cutout. Reinforcing the strength of the hull for the extra stressed sections, and protecting the foam some more.

Round 1:

Round 2:

Due to a lack of clamps I had to do this in two shots. And each piece only got 3 clamps. I had to go back and do a little rework to reglue a few corners where the plywood warped or the underlying surface was slightly recessed and I needed more localized pressure.  Another improvement would have been to use 6" backing boards for applying the pressure to avoid the warping. However I didn't have any on hand so I just went ahead with the direct clamping.

Here's a photo of the touch up I did, after flipping it back over.

Gluing down a few extra corners.

Sanding down the corners

My gluing has left several rough edges where the plywood stuck out so I spent a while today taking those off with the belt sander. Here's my timelapse:


This took longer than I expected. One of the challenges was that I discovered that I was not getting the suction from vacuum, nor the impeller built into the sander. I discovered that although I had previously cleared the foam from the intake. There was still foam in the narrow passage between the back of the belt on it's way to the impeller. The passage is about as deep as the sander is wide. But it's only about half a cm tall. And the foam pieces clog that up pretty well. To discover and clear that clog I had to take apart the belted gearbox which drives the impeller and sander.

The sander partially dissassembled to clear the clogged exhaust vent. 

Sanding without the suction left a lot of dust:

At the end you can see me getting ready to add plywood to the sides of the lift fan cutout.

Ends attached

With the sides done, I moved onto the ends. This was a little later than usual.

This required extending the pipe clams with the couplers which worked relatively well. The half inch ones on the bottom drooped noticeably. And in the morning I did find one of the bottom clamps lying on the ground.

Also painting the epoxy is notably harder in poor light for confirming coverage. 

The ends in place, late at night. 

Due to the broken boards, I spliced a few extra ones together to spread the load vertically. It seemed to work fine.

One half of the spliced boards

Sides Part 2

The results of the first round of sides turned out well. The masking tape came off cleanly. And the sides stayed attached. Amazingly even the tape on the corners basically came right off the epoxy.

First side piece with tape removed

The second round of sides were put on using the same techniques. The long clamps on the bottom are harder single handed. But I worked out a way with some string.



You'll note that I didn't use as much tape on the top and bottom. I don't think it helped too much, glue still seeped out, and most of what was needed was just the pressure.

I stuck some extra bits of cardboard in to help in a few places at the edges

 After I got things working, I added a few more turns to the clamps....

One too many cranks on the clamps. 

This would have worked better with 1x6" boards instead of 1x8" boards. This also shows how strong the hull itself is, as it was cantilevered off the edge of the hull.

The whole board cracked.

I didn't have anything to replace the broken board so I left it. From the first picture of the broken board you can see that the bottom edge is still well pressed so at least it should be firmly glued at the corners so that's the most important.

The First Sides

The first two sides have been attached. The epoxy is setting now.

However what took the most time this afternoon was the prep work. When gluing the foam together some cuts weren't quite straight and so the sides didn't all quite line up. Sometimes there was not quite enough space. To smooth it off well enough for gluing we used a belt sander. Here's the timelapse.  Unfortunately it cut off before we did any of the gluing. Hopefully we'll get a bigger battery for future attempts. Here's the timelapse.

Technical note: We mostly used 120 grit paper which went through the foam quickly.  When there was some large wood overhangs (~ 0.5") we upped it to 80 grit to go a little faster.  Also the foam coming off in chunks actually clogged the suction mechanism in the sander, but once cleared the suction of the sander plus the shop vac was quite effective.


Jumping to the end you can see the sides held on by the pipe clamps.

Completed application of two sides

As you can see we're using four Pony pipe clamps to hold the plywood to the sides of the craft. The trick we used was to first paint the epoxy on. Then hold the plywood in place with masking tape. (Both for ease of placement and to control the seeping epoxy.

Then there's a layer of cardboard and then solid wood planks on the outside. The goal is that the elasticity of the compressed cardboard will distribute the loading across the plywood and let it conform to the imperfect surface while we crank tight on the wood outside.

Close up of  pipe clamp, with cardboard and outer layer of wood. 

The other risk of this design is that none of it comes off due to epoxy everywhere, but we'll find out another day...

The final results from the back, showing our fancy saw horses.

Thanks again to Michael for all the help. While I was sanding he also did some fixup work on the edge where the plywood had not been clamped down well enough and was delaminating from the foam.

Fixup work for plywood delaminating. 

Plywood for the sides

For the sides of the hull I need to lay up some plywood. To do this I need strips about 6.25". I thought about finding a table saw, but decided just to go ahead with the circular saw.

I made myself a convenient cutting area using a pallet and some loose wood.

My cutting setup

With this I was able to rip using the circular saw relatively conveniently. 

Initially I planned to use 8' and 4' lengths along the sides and 6' lengths spanning the sides. But I realized that would have the same joint locations as the flat sheets so instead I made six 6' sections to have the joints offset from the foam and surface joints. 

The strips of plywood for the sides of the hull.

Plywood Touch Up

The last gluing left some small voids where we didn't get enough weight on top of the plywood and it warped. Here you can see the problem.

Plywood peeling back

I used a small brush and mixing stick to get epoxy underneath and then used a higher density of weight to hold it down. 

Fixing a few locations of glued plywood on the surface.

Hull topped with plywood

With my newly restocked epoxy supply Michael again helped me lay up another layer. Here's the results:


With the plywood precut it went quite quickly. We were able to cover the whole surface. Which left one unintended consequence which was that we ran a little bit short on weights. So anything heavy in the garage ended up holding down the plywood. The thin plywood is not exactly flat and it bows quite a bit requiring weights at close to 6 to 8 inches separation when it's biased upward. Here's a picture of the result.

The other upgrade to note is that I picked up a furniture dolly which made the movement of the hull much easier.

We moved it in the middle to keep the epoxy cool while we spread it. But the garage floor is not flat enough for adding the weights. (When the hovercraft hull is flat on the concrete. One corner by the door is almost 1.5" in the air. So we moved it outside to use the flat driveway concrete once we had already spread the epoxy and placed the plywood.

Second, thrid plywood gluing and second side cutting.

I'm making good progress on the hovercraft.  With the help of Michael I filled in the center of the plywood on the first side. The biggest update to the process was to do it not in full sunlight and use more boards for holding down the plywood. The 12-18" gaps between weights was enough for the 1/8th inch plywood to flex noteably.



The following night I had some time to finish up the side. I tried sawing on the saw horses since the hovercraft was in the way of the pallet. I used the angle iron to make a fence to run the saw along instead of drawing the line. However the flexible board bowed enough that the saw couldn't follow the fence(it drifted under the fence). So the cut was not exactly straight. You'll see the door close. As it was late at night I closed the door when sawing so that I wouldn't be too loud for the neighbors.



The next round of work encompassed cutting all the plywood for the second side. Again I was cutting with the door closed, but I moved back to cutting on the pallet. The plywood still twisted enough to jam the saw once I was 6' into the 8' cut so I finished the cut coming from the other side. I need to prop up the outside edge too. I was going to glue it all down too, but on inspection I noticed that I was getting close to the bottom of my epoxy supply so i decided not to continue and find out that I was half way through gluing and didn't have enough epoxy. More's on order and should be here soon.


I'm going to need to rip plywood edges to go around the perimeter which will be pretty narrow. This will definitely require a proper fence as they will be narrow strips. Or a table saw would be optimal.

I also have ordered a furniture dolly to help me manuver the hovercraft around the garage. It's now heavy enough i had to get help moving it by myself. I could lift it, but it's so awkwardly big that it's impossible to move without dragging or skuffing. And the foam is still quite exposed.