Kiml,

Just considering only the wood, not any skins that you may apply, and leaving all dimensions the same except the thickness:

   a. 4 mm is 33% heavier than 3 mm.

   b. 4 mm is over twice as stiff as 3 mm when flexed across the thickness.

   c. The edges on 4mm will have 33% more surface area than on 3 mm.

Without knowing more about your design and construction techniques I can't really say how all these will affect you.

As far as using 4 mm on longer boats instead of 3 mm, that also depends on the boat design. Is it high or low speed, what kind of internal support structure is there, what kind of outer skins, if any are there and how thick are they, etc.?

Laszlo

Hi Kimi, 

it would be great if you could post your proposed design. 

that said, a single ply of 4mm okoume would not be stiff enough for what i understand to be an 8 foot long (9 inch wide) ski.

i would start with a look at regular water skis and focus on what is the anticipated thickness of the ski.  most skis  are going to be somewhere close to 2 cm thick....so while you could consider using the plywood as skins, you will need some kind of core material to get the ski thick enough that it will have the stiffness required.   high density marine foams, balsa wood, etc, are common cores.....and help to keep the skis light (vs a solid wood ski).  i am almost certain you would also need to glass the skis to get the required strength..so apologies if i am stating the obvious...its just not clear from your note how much you have studied this or thought about how you are engineering your design.

h