The interior shell

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Conditions to metal boat interior construction

Building the interior of a wood boat, fiberglass boat or metal boat takes different approaches. The particularities of building the accommodations of a steel boat are that not only you can’t screw into the metal but its also best to not attach directly to the metal.

As it was said on the insulation page, condensation is the battle of a metal boat; therefore we need to avoid any “thermo-bridge” (thermal conductor going from cooler to warmer environment)
A bolt going through a rib and sticking up in the ambient air would be colder then the ambient air and cause condensation.
Fasteners withing the insulation layers is not as much of an issue but I wanted to avoid having to drill and bolt through steel; its a hassle.
T frames are also a little more complicated to deal with then flat frames as you cant just a attache a piece of would on the side.

I also wanted to limit my need of forward thinking regarding the construction of the interior accommodations, hence not relying of strategically positioned structures to attache the interior elements.

So, I didn’t want to use fasteners (or as little as possible) and I didn’t want to rely on the position of my wood members.

It’s from all those parameters that I came up with a solution, I had never seen used but that I am very confident will work really well.

The solution was to have an interior plywood shell to which I could fasten the interior accommodations and to rely on adhesives to secure that “interior shell”
The interior shell is a layer of plywood over all of the frames and insulation. It is the equivalent of the interior wall surface in our houses. 
With all the seems sealed, my “interior shell” could also act create a vapor barrier between the interior ambient air and the cold hull. 

I  set myself one other condition to the construction of my interior. Like the rest of the boat, and even more so then on deck, I want it to feel like a wooden boat.

Building the interior shell

The concept is there but then the question was how to attach the plywood shell to the steel structure? (while respecting the no fasteners through the steel frame constraint)

I used wood blocks which I glued under the rib flange to screw the shell too. The beauty with this is that ultimately we dont rely on the adhesive, it was only there to maintain the blocks while I screwed into them. The assembly end up self clamped to the rib flange. 
I covered the top of the flanges with 1/4 armacell for a bit of added insulation. 

The shell is made of 3/8 plywood. For most it will be on the inside of cupboards or bunk structures. The rest will be finished accordingly but all will be painted. The seams are taped with epoxy and fiberglass; this whole “shell” makes a second vapor barrier, adding to the insulation. The vertical seems are covered with a strip of 1/4 plywood, adding to the wood boat look with apparent wood ribs.
The upper corner, at the joint of hull sides and ceiling, I am keeping an opening running behind the knees for electrical wiring. 

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