Hey Fossil,

The stiffener or strengthening strut was merely trial and error for my plywood board. I copied the board outline from a Flyboards Flydoor (M) as I wanted a light wind board.

Board was a simple construct, 2 layers of 4mm marine ply for most of the board, tapering off to 1 - 4mm layer around the ends. I put an additional layer (4mm) underneath the footpads, mainly to enable insert of threaded sleeves for the footpad screws but this also adds a little bit of strength here I suppose also. It's only coated in a timber floor decking hardener (didn't know what else to use and it sounded ok at the time), which is waterproof but probably wears off from rubbing in the back of the ute during transport (only in small areas).

The typical 8mm thickness of the ply board seemed OK for strength but was really flexy and basically "stuck" to the water if you tried to produce any pop or when jumping. I thought about fibreglassing but with no experience in this area I thought I'd glue on some stiffeners to the toe and heelside edge to stiffen up the board without adding too much weight.

I started out with 40mm high stiffeners made from two pieces of 4mm ply glued together, just because that's what I had lying around. I was skeptical about whether the glue would hold under load but have had no issues with it so far (mind you the board only gets used in lightish wind). The stiffeners added significantly more strength and since then I have planed/sanded them down a bit (to between 20-30mm high). The good thing about this is you can control the flex along the length of the board by taking off more or less stiffener height where you want it (e.g. to give more flex at the tips).

I am happy with the board but there are a couple of cons: (1) The heelside (or toeside if you're riding toeside) stiffener tends to affect the water flowing over the board at the front tip - not dramatically but enough to be noticeable. This could lead to the board diving (scooping under) a little in choppy conditions and also when jumping (water gets on top of the board adding to weight and suction to the surface when trying to lift off); (2) A skinnier board might not have enough space to fit the footpads in between the toeside and heelside stiffeners.

If I was to make another one I'd probably do it a bit differently, but it was an interesting experiment nonetheless. I think and ply/foam sandwich construction would have merit with vertical ply stiffeners along the heel and toeside edge forming the boundary of the inner foam sandich section. These would both stiffen the board as well as provide formwork for the foam inner section (provding enough thickness for steel thread inserts for footpads).

Chaps.