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

Power System Remounted

After quite a while of hacking I'm approaching feature completeness. So I spent most of the day yesterday cleaning up the installation technique. There's now a cabinet for the computer, and the electronics are mounted away from the floor for cleanlyness. And the batteries all all solidly mounted to the floor with proper battery mounts.

The much cleaned up installation.

There's still some cable management to be dealt with. And my Arduino based voltage measuring circuit is not fully hooked up yet.

Inverter hooked up and outlets installed

The second part of the inverter charger is the inverter. I now have nice outlets where I can plug in most any power tool.  One is up front and the other is in the back. 

The 110V power socket at the rear of the trailer. (The wood wall is the back ramp, closed.)  

The forward outlet next to the window. 

The losses in the inverter are a little bit higher than I had hoped. When in charge only mode it appears to draw 195mA. And when inverting it consumes 1.2A. Under load I think the losses drop to a little under 1.0A. At least there is a DC-OFF switch position which does not load the batteries.