My original calculations were as follows:
* 1 marine ply sheet of dimensions 2440x1220x4 = 5.49kg
* 1 board sheet of dimensions 1400x440x4 = 1.14kg
* 2 layer board = 2.28kg
Anyway, a board that was 8mm (2x 4mm) thick proved a tad too flexible for a longish (more than 140cm) board, so I decided to switch to 6mm ply, even though the website http://www.teethgrinder.co.uk said that a 12mm board wasn't very flexible.
I had to use about 200kg of weight to put the rocker in my 12mm board, but it held, with just normal wood glue used in the sandwich. I've probably got about 30-40mm of rocker, with a nice 65cm central flat spot.
The new weight calculations aren't great though:
* 1 marine ply sheet of dimensions 2500x1220x6 = 9.80kg
* 1 board sheet of dimensions 1400x440x4 = 1.96kg
* 2 layer board = 3.92kg
The board is not very flexible at all, which is what I wanted for a lightwind board, but I was thinking that for my next project - a stubbie - I might use 1 sheet of 6mm and one of 4mm, which should give me the right amount of flex, and bring the weight down.
Also, the 6mm board I used is a 5-ply board that is a helluva lot heavier than some 3-ply 5mm board that's available. I reckon that you can get a 10mm board (140x44) down to about 2.75kg.
Anyway, I cut the outline of the board (a copy of a North Robo) with a jigsaw, shaped the rails with a plane, sanded it all off with a rotary sander. Then I drilled holes for fins, footstraps and handle, put in the T-Nuts (countersunk) filled the holes. I'm now ready to varnish.
I'll post some photos up here as soon as I get my camera working on my work PC.
Glassfibre seems to be completely OTT - when testing my board, I put the ends of my board on blocks and bounced up and down on the middle. Apart from acting like a very good trampoline, it stood the punishment with no problems at all.
What about drilling holes in the middle layer? I reckon if I do a 3-layer (3x3mm would be ideal) board, I'll make the sandwich layer into total swiss cheese. Might loose a lot of the layer bonding strength though
This geezer used a balsawood/hollow core: http://students.washington.edu/~rainer/ ... oard4.html
Do your plyboards float?