hi Goliat thanks for your kind comments. the bridgedeck and topsides is made from h80 pvc foam and vinylester, the hulls are infused in solid glass 3600 gm2 below the waterline and 1800 gm2 above the waterline below the bridgedeck. I think your idea would work if you built the boat in pvc foam and glass. the cheaper method would be extruded polystyrene foam with epoxy and glass. if you made the hulls from solid polystyrene blocks and completely wrapped them in glass then the sheer properties are not as critical as the foam is structurally supported to an extent by the glass. your main problem would be delamination of the glass and the lack of compression strength to withstand impacts. I know of several guys with polystyrene foam and glass boats who have had good longevity from there craft that are used as tenders. the big plus side is you can get polystyrene very cheaply. one guy collected foam packaging and glued it all together but I think he spent as much on glue as what the single block of foam would have cost. the bridgedeck would need to be more substantial than polystyrene foam by itself. you could laminate some plywood stringers through the foam and then laminate it like a surfboard. it really depends on the span between the hulls. another option is to use some light scaffolding aluminium tube underneath the bridgedeck in several places. these can go through the hulls and form structural beams to hold it all together. all these options are relatively cheap compared to aluminium and easy to build with so you can afford to experiment. a boat with an outboard will need to take a lot more punishment and will need to be tougher it really depends on your usage. check out expandacraft on the net they might give you some inspiration.