A couple of comments on the above.

1. F18 boards are really heavy due to their length, the rules and the boat's huge righting moment compared to an F16.

2. I have to dissagree with Rolf, for projects like this wood is not good it is great. Built properly a carbon skinned timber cored foil will be about the same weight as the foam carbon alternative and if you're not paying for the labour they're much cheaper (this was demonstrated to me by a very good i14 builder).

3. I agree with Macca with regards to a bit of extra thickness. 2mm (19->21mm) will add over 20% more strength