I weighed my GIS hull today. It is only 45kg!! My guess is that even though it has 3 coats of epoxy, sanding has removed 1.5 coats. I also think the very low humidity over the past few weeks has resulted in some substantial loss of weight. I really can't see it going over 50kg now, so I’m pretty sure this is the lightest GIS ever built, and by a long shot, maybe 10kg at least. Not bad for a 15.5 footer with a 5 foot beam!
It really is the result of timber choice. Also, I have not deviated from the plans, and I've faithfully followed the designer's construction methods. When I've analysed the hull, I can't see that any strength has been given away at all by my choice of timber, as the Paulownia frames really only take the place of epoxy fillets and glass in other stitch and glue type constructions. Every bit of the hull is just as stiff as it should be. Certainly the Joubert Gaboon/Okoume ply has also made a difference.
Originally I was concerned that the Paulownia frames would dent easily, but the epoxy coating fixed that. It toughens it up enormously with coating.
My tip? Build your boat in winter, but do not epoxy coat until summer when the humidity is lowest. If not possible, get the hull into an air conditioned space for a week or two prior to coating with epoxy, which will seal it up. I really do believe now that moisture has a substantial impact on weight.
I’m really happy about this low weight as it means that I have a boat that will be easy for me to handle and move around, on and off its trailer, all by myself. It will also perform so much better on the water too!