Just came across this forum and thread. I'm looking at doing a full sized caravan using these techniques.
One question I have is about the plywood stringers (apologies if it's been discussed - it's a very long and interesting thread). Are they mostly for protection, or for structural reasons? I'd imagined that the foam sandwich would have been stiff enough for most parts? For thin structures like the cantilever sides, I can see how the foam keeps the stiff outer ply separated, forming a thick skinned sandwich, but was wondering how much stronger that is than a straight foam layup? My knowledge of composite structures is a but thin, but I'd read that the skin stiffness in something like that is not that important, it's the strength in tension/compression of the outer ply that's important. On the flip side, I guess with very thick/stiff skins, you might change the nature of the structure completely, and if you have point loadings, you need to distribute that evenly into the foam, so it doesn't shear or crush (don't bend the foam under load).
I read the bit about the floor flexing. Did that pan out OK in the end? I'm lucky with the caravan, in that I have a closed box, with small openings, and the internal furniture structure makes an almost continuous box beam up each side of the body, so there are no really nasty situations like that.
Anyway, thanks for the inspiration and giving me the confidence that I'm on track. For a while, I thought I was going to need to use some fancy structural cores that cost 5-10x what these foams do, but with a bit of lateral thinking, and working within the limits of the material, you can do a lot with these lower strength foams.
Cheers
Darren