Victoria Foam Building InfoBy VictoriaPDRClub1 #901 "Victoria2" (3ar)
In comparison with wooden construction, I have less concern with plastics getting wet, and much less construction/finishing time. Raw material costs are higher here in BC, but considering fasteners and finishing costs for plywood, probably quite comparable. I'm at about $200 for this Duck. Polystyrene foam is cheap, strong and completely waterproof, but it is incompatible with the usual glassing resin - polyester. Epoxy works with styrofoam, but it is very expensive. But there's a new generation of waterproof PVA glues that are compatible with styrofoam and about the same price as polyester, so I'm giving them a try.
The adhesive to glue the styrofoam monocoque together was another problem, since PVA requires moisture loss to set, and a 3" deep joint won't breathe - i.e. it would never set. However, after much experimentation, Ordinary Portland Cement and PVA glue can be mixed. OPC needs to absorb moisture to set, while PVA needs to lose it - the two work together great. It's not a very strong adhesive, unfortunately, but it is very cheap and has a long working time - so it's fairly easy to liberally coat all the joints and arrange the styrofoam in a single batch.
The last difficulty encountered so far was one of measurements. While the thickness of plywood isn't really important in keeping the regulation hull profile, 3" of styrofoam extends well beyond the profile cut into the side panels. I had to extend the side panels forward to meet the bow angle in order to account for the thickness of the hull panel. Plus, styrofoam comes in a semi-metric size, not quite 4'x8'. I'll send some plans once I've checked the finished hull for compliance.
The weight should be extremely low, and self-rescue buoyancy will be taken care of by having drain plugs at the lowest point of the side panels - the styrofoam should float the entire hull dry when there's no-one onboard. On the downside, I expect the low weight will make stability an issue.
More as work progresses. I attach a couple of photos of hand-trimming the bow angle after the cement had set. The styrofoam loved the orbital sander - it formed quickly
but cleanly, without beading out or tearing. It only took about 15 mins to round the glassing curves in all the hull edges.