thanks for the help so far guys
i have attached a cut away of the full size and this is my mates idea too
My guess is that the pockets have baffles running from back to front hence the ribbed shape when inflated. For reliability there are two options, 1) make up bags as per the original using 4 oz. Polyurethane coated polyester, which is air tight material but needs a glue sourcing or heat welding at the seams. 2) make up the deck in laminated wood as I did for the RNLI model. We have since found out they were blown up with an onboard compressor and then topped up due to the failing of air tight material available at the time we think the model had one too