Hybrid Foamie Trailer With Plywood Skeleton

[Chefsonora]

Finally figured out that i need to actually make a build thread! I've been making good progress and learning a lot. Got the trailer pretty well set (actually took it out for a spin to the lumber yard today to get foam and more plywood) seems to track well and doesn't bounce around. Decided to extend the nose 1', so I tried my hand at scarf joints for the floor. Seemed to work pretty good, made a jig to use with the skilsaw. Keeping the design sort of fluid so as to incorporate ideas and make things more challenging for myself, lol. I'm building the framework out of plywood laminated together with TB3. They are sort of a flat bottomed hoop that will be encapsulated into the foam/plywood sandwich floor, then joined together with laminated stringers. This will also provide structure and strength to add in supports for a bunk and roof rack and vent. Gaps will all be filled with 1-1/2" foam and glued. Mostly relying on glue and spray foam for connection and structure rather than fasteners (besides finish nailer staples). All wood will be sealed with epoxy before skinning the whole thing with PMF, Inside & out. Going for a rounded edge profile front and top to avoid sharp edge joints to (hopefully) shed water better. Building it as a pod that can be removed from the trailer. I guess that's good for now. Started work on it the end of May.

 

[Chefsonora]

It's taking shape! Started on the cabin today, first 3 sets of ribs set, blocked, glued and nailed in place. That top front panel is where the 24" round dome skylight will be mounted. ( found a relatively local plastic fabricator that will do the 24" diameter and 12" tall polycarbonate skylight for $250 and I can save on shipping (I was quoted about $500 shipped from the east coast). Im excited to see some structure coming together after noodling it for so long!

 

[Chefsonora]

Getting the ribs up and starting to feel the shape. Threw some canvas over it and it looks like a Conestoga wagon!

 

[Pmullen503]

How will you do the curved sections? Kerfed foam? V grooved foam?

 

[Chefsonora]

How will you do the curved sections? Kerfed foam? V grooved foam?

Im going to tie the ribs together with mortised stringers, then from those I will add quarter circle 1/4" plywood to shape the contour, fill with blocks of foam, then Im going to use aluminum flashing lined with wax paper and fill the voids with expanding foam. Then (hopefully) easily remove the flashing and shave down the excess and smooth it all out. If all goes as envisioned in my head of course!

 

[Pmullen503]

Im going to tie the ribs together with mortised stringers, then from those I will add quarter circle 1/4" plywood to shape the contour, fill with blocks of foam, then Im going to use aluminum flashing lined with wax paper and fill the voids with expanding foam. Then (hopefully) easily remove the flashing and shave down the excess and smooth it all out. If all goes as envisioned in my head of course!

Test a piece of 1/4" to see if you can bend it. I think you'll need to kerf it to get it to bend to that radius. Even 1/8" will be tough to bend.

The flashing on the inside is interesting. Be aware that expanding can foam cures with moisture from the atmosphere. You may want to mist that surface with water before you apply the flashing. The flashing will block any moisture from the atmosphere.

There are 2 part expanding foams that don't require moisture but they are much more expensive.

Either way, I suggest you build a test section to try the technique out before you commit to the actual trailer.

 

[Chefsonora]

Thanks for replying, it's nice to put another brain on something that has been stuck in mine for quite a while! What i meant about the 1/4" plywood was that they are hoops cut flat at the same radius as the coved corners. They will go in edge on into mortises cut into the stringers to present the contour profile. The areas in between them will be largely filled with fitted blocks of the foam panel. Then the flashing to conform to the contour before filling with the Great Stuff (I got the Pro with the applicator to control the foam better) I have messed around with the foam some (with wetting the area for bonding and cure) , but doing a real mock up is probably a prudent measure. I have definitely found that how it seems in my head conceptually is often much different in practice and reality bites me in the ass!

 

[Pmullen503]

So I understand, you plan to put 1/4" hoops with foam between them? How are the hoops spaced - every foot, every two feet?

You should also be aware that when you have wood and foam at the surface under PMF, those joints will eventually telegraph through the canvas due to (presumably) differential thermal expansion.

My first trailer was similarly built with 2x2" supports for roof racks and foam in between. After a year you could tell exactly where the wood was vs. the foam under the canvas covering.

 

[Chefsonora]

So I understand, you plan to put 1/4" hoops with foam between them? How are the hoops spaced - every foot, every two feet?

You should also be aware that when you have wood and foam at the surface under PMF, those joints will eventually telegraph through the canvas due to (presumably) differential thermal expansion.

My first trailer was similarly built with 2x2" supports for roof racks and foam in between. After a year you could tell exactly where the wood was vs. the foam under the canvas covering.

So the hoops will be 8-10" apart in the areas between the ribs (which are mostly between 14-20").

I am aware of the eventual telegraphing of the framework through the skin. In your experience did you find it to cause undue wear or particular failure points? Because my build is not incorporating any plywood sheathing (besides the floor) im expecting a fair amount of flex and hope to be able to accommodate that (my contention is that rigid breaks and that designing for flex will allow for movement without undue damage)

 

[Pmullen503]

There has been *almost* no tearing/cracks in the PMF covering in the interface between foam and wood and that was only near the hatch which is a high stress area. I built this one over a decade ago and has been stored outside all that time. I did repaint once.

The telegraphing is primarily an aesthetic issue. But for some, that's a big deal.

 

[Chefsonora]

There has been *almost* no tearing/cracks in the PMF covering in the interface between foam and wood and that was only near the hatch which is a high stress area. I built this one over a decade ago and has been stored outside all that time. I did repaint once.

The telegraphing is primarily an aesthetic issue. But for some, that's a big deal.

I'm kind of thinking of leaning in to it for the interior. Since my ribs are about 2" thick and I'm using 1-1/2" foam, if I go for smooth exterior, then on the inside I can lay in the PMF to have the ribs stand proud and decorate them with some sort of trim (or even veneer strips).

Got my last ribs up and blocked in last night. Gave the stern one a 15° forward tilt (I figure the hatch drainage will be a bit easier on a slope). Now I need to block out my mounting points to be able to go through the foam sandwich floor. I figure I'll slather it all up with thickened epoxy to seal it all up (so that holes through will be isolated) got any tips for mounting to the trailer?

 

[Pmullen503]

Only when you get ready to bolt it to the trailer, go through the trouble of shimming over the cross members to get them the same height as the side rails.

On my last build (also on an HF trailer) I bolted the cross members from the bottom so I wouldn't have to deal with the bolt heads. Some of the floor bolts use the empty top holes (PIA to line those up!)

 

[KCStudly]

On the expanding foam, wax paper and shaping issue, I have a little experience.

What I found was that waxed paper doesn't release very well. The wax seems to melt and attach the paper to the foam. Packing tape or cellophane work much better.

But then I also found that expanding foam makes terrible filler. In my climate, at least, it tended to skin over and then expand from within with large bubbles. Is soon as I sanded through the outer layer there would be an unstable matrix of large voids.

I would recommend using as large a block of "solid" foam as you can and carving/shaping back as close to you desired shape from there. Only use fillers to do the final fairing.

 

[Chefsonora]

On the expanding foam, wax paper and shaping issue, I have a little experience.

What I found was that waxed paper doesn't release very well. The wax seems to melt and attach the paper to the foam. Packing tape or cellophane work much better.

But then I also found that expanding foam makes terrible filler. In my climate, at least, it tended to skin over and then expand from within with large bubbles. Is soon as I sanded through the outer layer there would be an unstable matrix of large voids.

I would recommend using as large a block of "solid" foam as you can and carving/shaping back as close to you desired shape from there. Only use fillers to do the final fairing.

Another thing I was considering is using some heavy Kraft paper to help form the shape that would remain and be bonded to the PU foam and could be spritzed with water to assist with curing the foam. Fairing would be pretty straightforward as well. Have you ever tried the Durham's water putty to fair areas?

 

[Pmullen503]

Another thing I was considering is using some heavy Kraft paper to help form the shape that would remain and be bonded to the PU foam and could be spritzed with water to assist with curing the foam. Fairing would be pretty straightforward as well. Have you ever tried the Durham's water putty to fair areas?

A lot of novel techniques, best to test first.

Durhams is not very sandable by itself. I have actually used Durhams/lite spackling in a 1 to 3 ratio to make lite spackle more sandable (several years ago they started adding latex to lite spackle which ruined it's sandability on foam). This was on RC planes. I since discovered Sherwin Williams Shrink-Free spackling which sands well.

 

[Chefsonora]

A lot of novel techniques, best to test first.

Durhams is not very sandable by itself. I have actually used Durhams/lite spackling in a 1 to 3 ratio to make lite spackle more sandable (several years ago they started adding latex to lite spackle which ruined it's sandability on foam). This was on RC planes. I since discovered Sherwin Williams Shrink-Free spackling which sands well.

I'm going to mock up a coved section to experiment with my concepts (with the stringers, 1/4 hoops and block it in with chunks of the polyiso foam and give the Kraft paper a try to see if it'll hold the contour. Doing some research on the PU foam (and Gorilla glue) the addition of moisture to those surfaces should really assist in the curing (largely because here in the NorCal foothills it's been bone dry and in the 90's every day) and the moisture is essential for the chemical reaction to cure. Using one of those personal misters to slightly dampen the surfaces seems to work pretty well without flooding the area. I'll see how it goes and report back!

Got the mounting points blocked in on the floor and foam cut. Pulled it down off the tables so I can get that all glued in before laying in my sandwich plywood (using 2 sheets of 3/8" plywood with the 1-1/2" polyiso foam between). Now that it's stable on flat ground I can start notching the stringers and ribs to tie that all together. I'm anticipating a huge increase in rigidity with that and ill be able to tip it on its side and hit the bottom edges with the roundover on the router before sealing it up with penetrating epoxy (on the bottom, as well as all of the exposed edges of the ribs/stringers). Please forgive my rambling, sort of free associating my next steps, lol.

 

[Chefsonora]

Got the upper stringers installed today! It is exponentially more rigid and very strong (easily supports my fat ass)! I'm glad that i went with the laminated plywood stringers, much less worry about splitting or getting too week where they are mortised out. I did my mock up section for the coved corners,I gorilla glued the chinks of foam in and drilled/ skewered them in place. Faced the outside with Kraft paper stapled down. Jabbed the foil surface with a pointy bitand went to town withe foam. Ive used great stuff quite a bit, but the Pro with the dispenser is a game changer! Easy to contro, adjustable spray volume, firm metal tip( i was able to actually pierce the chunks of foam to get behind it). Definitely room for improvement on the mock up(should have scuffed or scored the foil on the foam chunks) and my deposition shou have been more careful to fill all the voids. I am pleased and confident overall that it will work, especially if i back up the Kraft paper with the aluminum flashing to control expansion)

Good, productive day overall and i am very encouraged by the strength and rigidity of this framework. (I can grab the stern most rib and pull hard enough to lift the front off the ground without any creaking or cracking)

 

[Chefsonora]

Had to drape it to feel the shape with the stringers in. Got the base smoothed and ready for the floor. Ready to glue and screw it down (had to make a few adjustments trimming my notches for the ribs, I swear I measured it 3 times.) I like that it is light enough to lift and move around.

 

[Chefsonora]

flipped on its top, sanded and ready for the penetrating epoxy! Going to fully coat all wood to (hopefully) slow down any rotting from moisture. Any tips would be appreciated as far as how many coats is appropriate and good application techniques (I've got 4" foam rollers

 

[Chefsonora]

Gotten majority of my framing/ stringers in(including roof vent and door, hole cut in the front panel for the stargazer dome. Ready for another round of penetrating epoxy. Going tomorrow to pillage parts off of the salvaged truck bed camper(mostly for windows, but I'm gonna be on the lookout for useful goodies). Grabbing the old radial arm saw too in order to make a pin router that may eventually become cnc controlled (at least 2 axis). I hope to be able to get more controlled results from a pin router, because my results in wood with a handheld router are dodgy at best. I figure that once I get windows framed out im ready to fget the foam in! Feeling like decent progress and its shaping up. Im anxious to get it out camping, so I may forgo some of the finer details to be able to get up to the high country and escape the scorching temps!