1/2” vs 3/4” Bulkheads?

[eporter123]

I just got back from the plywood store empty handed. Too many good options!

I want to replace the missing bulkheads with an appropriate plywood. I’m thinking a shop grade Baltic birch type. I’ve read they were 1/2” from the factory? The wood tabs on the shell have a 3/4” gap.

I’m thinking of building two bulkheads using a 3/4” Baltic Birch type plywood ripped 24” wide. I’m not keeping the factory bathroom or kitchen setup, so I’m thinking that bulkhead width will be sufficient with a thicker plywood? I’ll likely horizontally tie the bulkheads into the back wall or add a thinner vertical strip of ply there. Basically build two closet boxes in each corner. 3/4” main bulkhead, 1/2” other walls.

Is it worth the extra weight/expense in this area? I’m thinking yes for the 3/4” narrower bulkheads. Then cabinets and such will be 1/2” and 1/4” plywood as necessary.

[Blue~Go]

I just got back from the plywood store empty handed. Too many good options!

Well you obviously weren't at the typical big-box home store: "Hmm, should I get the two-ply 1/2" or the three ply with a crappy core and two 1/128" veneers?" Sheesh.

I want to replace the missing bulkheads with an appropriate plywood. I’m thinking a shop grade Baltic birch type. I’ve read they were 1/2” from the factory? The wood tabs on the shell have a 3/4” gap.

I wonder if you might have a 5/8" "gap"? (Actually I'm not sure about tabs and gap - my Chinook didn't really have tabs like you'd have on a boat, but did have a couple of caulking "tabs" (i.e. fillet like things made of caulk) around the refrigerator area where they had siliconed it because an absorption refrigerator needs to be airtight to the living space.) Actually I wonder if that's where you are seeing 3/4"? If so, here is what could explain it: At least in my Concourse, those two main bulkheads are 5/8" thick. This consists of 1/2" plywood, with a layer of 1/8" "wallpaper board" (common in RV's) attached to it for a total of 5/8". BUT, in the refrigerator compartment, because of the aforementioned need to seal it from the living space, they also had scrap pieces of that same 1/8" board caulked in place on the back side of that bulkhead as part of the "seal the gaps" scheme. So that area would have been 3/4" in total, but only 1/2" of anything meaningful (the plywood).

In the original design, I don't think 3/4" is necessary, because the bulkheads are "stiffened" by furniture that is attached to them. i.e. the kitchen counter and the stove counter, etc. If you are going to have free-floating bulkheads (that is, no counters butting up to them), then I could see going 3/4". There's really no downside except weight, that I can think of. You might also consider some type of edging on the aisle side to stiffen that edge (if no counters etc.). It also depends on what else you have either hanging on the ceiling (trying to pull it down), or making things sturdy (other walls, etc.).

I’m thinking of building two bulkheads using a 3/4” Baltic Birch type plywood ripped 24” wide. I’m not keeping the factory bathroom or kitchen setup, so I’m thinking that bulkhead width will be sufficient with a thicker plywood? I’ll likely horizontally tie the bulkheads into the back wall or add a thinner vertical strip of ply there. Basically build two closet boxes in each corner. 3/4” main bulkhead, 1/2” other walls.

That sounds like it would probably be fine. The stock shower bulkhead does pick up a bit more of the roof since it's about 36" wide, but you could accomplish that some other way maybe. Are you too tall to have something running across the ceiling there?

There are also other ways (usually more work, which is likely why they weren't done) to stiffen things up. Real fiberglass tabbing at the walls (if the walls can't bow outward, the overhead is less likely to sag), overhead beams inside (think boat), or even a mini truss type thing on the roof. How far do you need to go? Good question. I can see where my Air-conditioner was on the roof. There is not a large sag (water won't pool there), but just a wee tiny bit. Not that the main bulkheads would change that, but just saying it's not un-saggable, depending on what you have on top. I would imagine even the stock side cabinets help a little bit because they are basically boxes that are attached to both roof and walls (that said, it may be negated since they are nearly 100# apiece - geesh, I could not believe how heavy mine were.

Is it worth the extra weight/expense in this area? I’m thinking yes for the 3/4” narrower bulkheads. Then cabinets and such will be 1/2” and 1/4” plywood as necessary.

There are so many better ways in which one could both lighten and stiffen our Chinooks. That said, they work pretty well as-built and certainly didn't need to be MORE expensive when they were new. Yeah, kind of a non-answer. But it's also true. Even a 3/4" bulkhead isn't necessarily better or even needed. Think about how and what you are going to tie into the bulkheads into and how that might actually help more (even if they are thinner). Like an egg carton or cardboard wine box vs. making that same carton out of thick wood but with no interior supports. Does that help? (Or make things more cloudy? ) I don't know all the details of your layout/counters/cabinets so it's hard to say for sure just based on the bulkheads alone.

Fiberglass is inherently floppy and it's good to make it as stiff as possible (without introducing stress risers).

[SMan]

I don't really have an opinion concerning 3/4 v 1/2 but doubt given the amount of material weight it would be that much of an issue. I would use what "fits" better for you.


I build acoustic guitars as a hobby and love Baltic birch for its structural rigidity and stability. It doesn't bow like traditional Birch, A/C, or CDX plywood can and can be finished easily if exposed. I use it for neck blocks, heels, and jigs I use to build with. Great stuff. I've also used it to build furniture projects.

[eporter123]

I went to Mr. Plywood, a local lumber (& plywood) store with a great selection. Lots of construction & cabinet grade plywoods and some decent paneling and wood trim and lots more.

The “tabs” and “slot” I was talking about are in this pic, in front of the generator spot. It measures ~3/4” wide. But maybe that’s not what the bulkhead slotted into? There’s a similar spot on the drivers side, but it’s missing one of the vertical wood strips that was attached to the shell.

Or am I missing how these are really built?

[Blue~Go]

Okay, I see what you mean by slot. On mine, the forward piece of plywood doesn't exist, and the bulkhead lands in the center of the rear piece. The plywood piece is fiberglassed to the wall, but the bulkhead wasn't tabbed to the plywood piece or the wall. If I remember correctly, there might have been a cleat screwed to the plywood piece and the bulkhead screwed to the cleat? Guess I'll have to go back and look at some of my pics.

Looking at your photo closer, I wonder if the bulkhead did "slot in" after all? Reason I say that is remember how I described the fridge side of that wall being caulked in place with paneling scraps? Well I think I see that same familiar caulk line on the rear strip, toward the back of the strip. Maybe the forward strip was added just because it gives the stove/outside paneling something extra to land on.

Have you had a look at the 1999 roof and wall blueprints in the reference section? When I took things apart mine matched that pretty much exactly (well, it is a '99). Just the one wood strip. Here is just the driver's side wall drawing:

[Blue~Go]

Here is an annotated photo showing what I mean. I know you may not be putting things back as they were, but figured it might be interesting. You can ignore the cardboard stuff at the bottom; I was mocking up different ideas for how to re-do this area. This was the "HoosierB" idea

So I'm guessing your second ply strip was just extra support for the stove outside wall paneling or something like that. The caulk lines make it look like the bulkheads and etc. were like mine, I think.

[Blue~Go]

I just want to clarify a minor mistake I made in my annotated photo above. Just so any future Chinook archeologists aren't led astray

So, I mentioned that that thin, forward most caulk line was on a piece of oak trim that was forward of the bulkhead. The trim IS about 1" wide, but in looking at something else I realized that it was placed 90º to how I thought, so in terms of the caulk line, that was only about 1/4" ahead of the bulkhead (which itself is 5/8", consisting of 1/2" plywood and 1/8" wallboard). It "sealed" the trim piece to the outside stove wall on the forward edge.

[Blue~Go]

I also just realized something else. You (eporter) have a Premier. Somehow I forgot that (and I can't see that info when I'm posting). That may explain "extra" strip? Reason I say that is that I believe the window cutouts are more "stick framed" in the Premier (which you can just see on the right of your shower wall photo). On the Concourse, the windows have huge plywood surrounds, which are then cutout for the parallelogram shape.

You can see on your shower wall photo how the window framing on the rear side of the sink window is kind of "extended" beyond where it would need to be. I realize that the extra "stick" on the other side is far from any window, but I wonder if it's there due to somewhat of an "extension" of that method there?

Anyway, sorry for forgetting you had a Premier.

[eporter123]

Wow, thanks for all the info. Just getting back from a weekend car camping with our tiny Honda Fit. A buddy had a brand new 4x4 crew van Sprinter. Jealous...

So maybe I'll cut some 3/4" ply to the profile of the outer shell, then attach to the existing vertical "caulked" plywood with pocket screws and/or some angle brackets.

In looking at your fridge area, I see added horizontal plywood strips that I'm missing. Circled in red below. I think those would help rigidity? Not sure if my shower side is missing a vertical strip, there's not much wood to attach to there. Maybe I should add a new strip?

I'm planning on going around the shell and adding some kind of caulk-tube adhesive (polyurethane sika/something?) to all the exposed strips of wood that are "mostly" glassed to the shell, but still wiggle/move a bit. I may add some additional strips for reinforcement/mounting/rigidity. Thoughts on best adhesive that's not too spendy?

[Blue~Go]

If I were going to add strength or attachment strength, I think I'd just use epoxy resin and fiberglass cloth. After all, it's a molded fiberglass shell, and that's how boats are done. Immensely stronger than just toenailed fasteners, and no holes. However, that's not to say the original didn't work, but since you have to do something (because yours is a shell).

The basic idea is that you want everything rigidly connected such that nothing can wiggle or flop. There are many possible ways (including construction adhesives), but fiberglass/epoxy would be like the native way.

On a boat one would typically set in the bulkheads basically against the outer shell/deck (roof), but with a thin piece of foam between bulkhead and wall (like say 1/2"). This is to keep from having a point of contact that makes a visible line. Then you take thickened epoxy and make a fillet (this is an angled bit of epoxy in the corner so it's gradual not a 90º angle). Then take fiberglass "tape" (strips that are about 4" wide) wetted with epoxy resin and run them along both the wall and the bulkhead. Kind of like "taping" an inside corner of sheetrock, if you've done that.

Another way (slightly less strong but still plenty, plenty strong for our Chinooks) is to put some clear packing tape on the bulkhead so that the fiberglass tape doesn't stick to it. Then lay up the tape on the wall and "on" the bulkhead as per normal. After it cures, move the bulkhead aside (resin won't stick to it cause of the smooth tape). Now you have a rigid "flange." After it cures you can bolt or otherwise fasten the flange to the bulkhead. This is still quite strong but the bulkhead would be more easily removable, changed, repaired later, etc.

Those extra "wood" pieces on my refrigerator wall may be wood or foam. They are glassed in to the shell by Chinook. Similar to a cored deck or roof, they add strength (much more by being encapsulated between two skins vs. just stuck on).

There are books and pamphlets (and I'm sure videos) showing more about bulkheads. If you are interested, I could probably dig some up. Since you may or may not go that way, I won't blather on more now.

When I put my new main bulkhead in on the stove side (shower side isn't being removed), I'll probably do some variation of the tabbed/flange/fastening way with the packing tape. A second advantage to that way is that you aren't relying on a fiberglass bond to the bulkhead surface. Mine are varnished ply which I'd have to sand to a perfect line while not going past the line. So instead fastening to the flange doesn't rely on that bonding (only bonded to the outside wall).

But again, you can take this as far or as "not far" as you like. Obviously the original is more relying on the number of joints and etc. because many of the individual ones aren't all that hot (but together it works).