Hello J,
It’s a hard one to visualise. I can offer some breadcrumbs.
To me, my first thought would be to continue with the inflatable material on the outside, PVC or even better Hypalon, chlorosulfonated polyethylene (CSPE) synthetic rubber. They’re impressively solid materials when you bring air pressure into the mixture and they offer chemical, heat and ultraviolet light resistance. Obviously, they can cope with expansion, flex etc. If the inner hull was GRP fibreglass, it would mean you could have chambers on top of chambers, even if they got damaged the grp hull would still hold the water.
If that’s not what you’re after,
Some questions come to mind, is the boat designed to be hauled out? or do we have foul build-up to contend with? does it need to be rigid enough to land on a stoney beach etc? Will it be designed for longevity, replaceable? repairable.
I feel like perhaps you’re looking more into a solid rubber, UV resistance, salt resistant, flex. Off the top of my head, I can’t think of a rubber suitable, but I know where I’d look.
High end – Diving gear, flippers, fins they would need all of the above and they would need to hold up flex, without wear.
As a side note, you can actually buy a flexible additive for polyester resins which make the resin flexible once set. I’ve used it and works great. I’ve never tried add a flexible additive to the fibreglass layer itself.
I bet your guys at Eastcoast fibreglass would have some suggestions for you.
Feel free to send over any other question, I hope that’s of help.
