I did not have much luck with Bunnings and xps. Maybe in NSW is different market with building standards at.al.
Great effort Matt. Joke is that I have bought the same foam and am in the process of doing a similar build . . . well . . . the core foam part at this stage. I had not considered using stringers but it seems a good idea . . . helps set the rocker and should provide a bit of structural integrity. I was going to try the kerning method for the rocker so maybe the foam will not be under stress before glassing.
So did you use the glass between the ply and the foam and none on the exterior?
Will be keen to see how it holds up.
Downunder: this is the stuff I bought. It comes in 50mm thick also I think.
Knauf Insulation 1200 x 600 x 30mm XPS Multi-Use Foam Board.
Thanks Max. That is uncanny !Yes, it Knauf multipurpose board , the 30mm thick. I think the 50mm would be good for making a board with the rocker carved in. I wanted a lot of rocker for the waves and exactly as you said one of the roles of the stringers is to bend the rocker into it rather than curing the laminate on a rocker table. I was able to bend about 80mm rocker over c. 500mm. I felt like this was about the limit as in one test piece I did the foam did split when I tried to bend it around a tighter radius than that.
I read quite a few posts on swaylocks about using XPS. It was mixed in terms of success people had but those people that did get it to work ( and work really well) seemed to all say that it was important to really rough the surface of the foam up ( a solid sanding with 40 grit after shaping it) because it is genuinely closed cell and so resin can't penetrate like it can into the pores in EPS foam.
Yep, I did use a layer of glass between the core and the ply. 4oz, reasonably resin rich and with a few small holes drilled in the ply to let excess flow out. I also put glass over the exterior. I used 10oz stitched glass because I happen to have it on hand. I did the top and bottom laminates in separate passes so the rails have 2 x 10z glass. Because I was thinking of it as an XPS foam assisted hollow board i figured having solid rails was important to creating the torsion box 'effect' that wooden rails do on hollow boards.
One other thing I found at Bunnings is they now sell Sika polyurethane glue which I used for bonding the foam to the stringers. It came highly recommended by some boat builders who used the ply/XPS sandwich to make quite large structural panels. It requires a lot of clamping pressure to get a strong bond so I only used it on the stringer/core bonding where I could clamp it like a mofo.
Good luck with yours and I'm keen to see some pics as it progressed.
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