Ocean, i wonder if you have considered materials other than tradiional sand cement on your houseboat design. I was just thinking, again since your design and mine share some qualities, since your interior framework is carrying the load and is somewhat flexable, if a thinner coat of traditional material, in slabs, affixed to the frame, and then coated over with flexable water proofing material, might produce a less heavy and less crack-prone shell. For instance the type of construction used in bath and shower stalls, that type of fiberglass cement panel, or that type of watertight cement plaster/stucco? Or even no panels, and just the interior vinyl plaster/cement? What about the plaster/cement used to coat the exterior of buildings in Bahamas or Bermuda, or various Mediteranean areas? Or portland added to vinyl paint? How thin is reasonable when aiming for flexable?
Today i ordered up some pretty thin steel for the boat, which is going to be a lot more flexable than i am accustomed to building with (altho i have a 4×8 sheet of the stuff already), so i was daydreaming of how flexable i could make the cement coating. I am reducing the structural requirements of the cement coating down to “adhere and exist in sea water for dirt cheap, in any ocean state, just short of delicate”, and no grounding or contact with other structures. Just how cheap can i maintain “lasts forever and keeps out salt water”, and under what conditions. Even considering applying 1 psi of dry nitrogen inside, because it’s cheap if it never leaks out, nothing grows in it, it suppresses fire, and it doesn’t convert to anything corrosive. Just brainstorming.
No, just the ferrocement over plywood. (as you said, similar to Bahama or Bermuda buildings coating). My floats are not design flexible but rigid, heavy and maintenance free. (well, other than scraping the bottom once in a while if you’ll have to motor for a longer distance and in order to save on fuel). If you use anything else other than ferrocement you will have to haul out once a year to repaint the bottom and check for blisters (if fiberglass) or rust (if steel). That can get expensive.
I would advise against any flexibility in the hull of the float or pontoon. If too flexible, it can break under a serious pounding or if running aground on a rocky bottom.