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.

First plywood gluing

So now that we have the core of the hull formed out of foam. The next step is to attach plywood to the exterior. It has two major functions. The first is to be a tensile element making the whole hull an effective box beam.  Secondly it also protects the foam from impacts and denting.

Here's another timelapse of the process.



Thanks to Michael G for the help. And Wim provided some good comments at the end.

To cut the plywood the cordless Makita circular saw was

Weighted plywood for curing epoxy. 

You'll note that the weighting is much denser. This is due to the flexibility of the plywood. I will need to inspect more closely, however even with this density I believe there are still pockets where the plywood is separated from the foam.

Also an issue we identifed was that working in the full sun the epoxy seemed to almost dry out on the foam before we flipped the wood on top. Even though we were well within the expected 30 minute setup time. We rewetted the area but I'll need to do some inspection, and I will avoid full sun in the future.

In the timelapse you can see the fan ducts for the lift fan was placed on the hull as we discussed mounting options for the front motor. This needs to be addressed at this point so that we make sure to cut the plywood for the front end correctly.

Here it is parked inside:

The craft back inside the garage

Even with 3 layers and the plywood. It's still trivially liftable by two people.

Foam layer 3 of 3

I've completed layer 3 of the foam for the hull. Here's a view of it curing.

Foam layer 3 completed

Here's another timelapse of the process.

This required cutting out one last sheet of foam. One whole sheet of foam. And you'll see that on the far side I used several small left over pieces of foam to fill in the rest of the area since I cannot easily find more large foam boards.  (See previous post)

The other update you'll see here is me filling in my new weight objects. I felt that the available heavy objects was risking having pockets of uneven attachment so I invested in 12 Home Depot buckets with tops and filled them about half way with water to provide a good uniform weighting, but be easily moveable.  I looked for 3 gallon buckets but they cost noteably more than the 5 gallon ones.

Second Gluing

After the first gluing. I'm much more liberal with the epoxy and am trying the flatter concrete in the driveway. Luckily I live in sunny California so the conditions in the driveway are almost exactly the same as in the garage. Here's a nice high-res timelapse. I've figured out that I can do the high-res videos quite well however it was pretty dark so the quality is not super great.

You'll see that I made a small mistake in cutting the holes which required a slight adjustment but otherwise I think it went well and I expect to find it solidly glued in the morning. The flatter surface helped a lot and the weights seemed much less necessary to keep gaps from forming.

Here's the result in the morning.

With weights still on it

Weights removed

There's one slight caviat that with the extra epoxy some slipped down one of the cracks and I appear to have glued it to the ground....

I couldn't force it manually without risking breaking the whole piece. As I could only get leverage from the edges.  So I slid a string underneath and sawed away, until the string broke. Then I slide some cardboard underneath both sides of the glue by bending upward and when I let it back down the weight of the rest of the foam pried upward such that a manual pull at the edge was enough.

Here's the results:

Remaining dead string and chunks of foam left. Note the sawed foam at the bottom left vs the chunks at the top right from prying.

There was a little cosmetic damage and one hole I'll try to patch. 

But the resultant hull is pretty awesome.

The resultant hull. 

The bike gives a good sense of scale.  And for reference I can lift the whole thing with a single hand assuming I can balance it. I moved it into the garage single handedly despite it being 12 feet long and 6 feet wide.

First Gluing

I've taken the first big step toward putting the hovercraft together. I've cut up almost all the foam I have so I guess it's time to glue it together. I got the glue out and laid the bigest flat pieces out in the garage and go to gluing.  Here's a timelapse of the process.

I only painted one side of the foam using i think about two sets of pumpings. I just wetted the surface like we did in the test case. But the test case was so small that the surfaces mated together very well. In this case I suddenly noticed quite how non flat the floor of my garage is. There are several cracks. I tried to do the active section not over the cracks. 

I also ended up using practically everything heavy in the garage. Unfortunately I don't have a couple extra sets of wheels lying around that would be a nice addition.  

First set of weights

Adding more weight 

A view from the other side with even more

You'll note as time went on I kept adding more weights since I could feel the foam not bonding. 

Finding Owens Corning Foamular 150

So with my changes to the design I'm going to need some more foam. So I started out with a simple trip to Home Depot to pick up this very standard construction material. From Home Depot, I called Lowes, and they didn't have it either.

The kit contains Owen's Corning Foamular 150 with scored 4'x8' sheets 2" thick. This is easily findable on the home depot website. they say pick up in store only. "Pick up Today: Free" but you need to read the fine print. "Item not sold at Kiefer Rd #640" And if you "Check nearby Stores" the list is empty. I tried searching in New England too and they appear to not stock it anywhere.

I even tried walking into Home Depot and looking around. They had 1/2" 3/4" and 1" stock. Some of it tongue and groove, but nothing thick enough. There was a lot of the R-Tech Expanded Polystyrene (EPS) which is noteably different from Extruded Polystyrene (XPS). Finding extruded foam is apparently an issue for model railroaders too. See this discussion.

I found another Bay Area hovercrafter who said they found it at HD-Supply. In this thread]

# baseline2013-12-10 11:16
I live in the SF Bay Area and am having a similar experience finding foam. Finally found some $expensive$ sheets at HD supply in SF which caters to more commercial customers. According to the blogs, the RC community has had the same problem we have and have proposed to form a coop to buy minimum quantities and keep a strategic supply for members to purchase. It is an interesting idea and one that we might consider if enough of us are clustered together. I am about to start my project if there are others in the SF Bay Area, let me know. I believe that a minimum purchase would cover almost 2 crafts. It may be more cost effective to buy a pallet and have it shipped to us directly.

Unfortunately after calling HD Supply they didn't have it either. Nor does Orchard Supply Hardware.

I'm surprised at the amount of info I can get on the insulation from Owens Corning directly. homeowner's site or the commercial site.


They have a nice vendor search However it seems that in the 100 closest locations Home Depot is the only place that stocks Foam Insulation. All the others only stock the fiberglass rolls.

It's quite frustrating, especially since I drive past their factory on Central Expressway in Santa Clara several times a week.

One layer of the hull cut

Today I've cut out the first layer of the hull. I'm planning a simplified rectangular base for ease of construction. As i hope the bow will never be punching through a wave and don't expect to be going fast enough for air drag to be an issue this should be fine.

For a sense of scale take a look at the boards laid out together.

First layer of the hull layed out side by side. 

Another view

A slightly closer look

And I was able to cut out a pass through for the lift fan. 

The cutout for the lift fan

With the knife next to it.

Cutting out the hole took much longer than the other two cuts due to needing to not overrun the corners and penetration cutting.

Fresh Grease

One more update on the trailer. It's got grease nipples for the hinged doors and latches.  After a few years the grease has become pretty dry so I picked up a grease gun to take care of them.

Freshly greased latch hinge

With a nice hanger.

Doubled Solar Power

Last weekend with the help of Michael I successfully installed 2 more solar panels on the roof of the trailer.

This gives the trailer 960W rated power.

Having completed installing the solar panels it has finished most of my work focused on the trailer.  And the inside is now looking pretty empty.  I think it's time to start getting back to the hovercraft.

A clean empty trailer