Jo, you make an interesting point, even though the directional is a bigger board and I had the straps set all the way forward, I see there is a fair amount more "tail" behind my back foot on the ply twintip. I have ridden directionals before and always try to keep more weight on my front foot when I use a directional board, but apparently it was not enough to make up the difference in this case.
I laminate the boards from 3mm birch plywood using a vacuum bag on a rocker table with the concave built in. The table has more rocker and more concave than the finished board because for some reason there is quite a bit more springback with the compound curves in the wood (wood sheets don't like to do that). Another benefit of the compound curvature in the board is that it makes it stiffer than a similar plywood board with a flat bottom so although it flexes for a smooth easy ride it does not have so much of that "wet rag" feeling that some people hate in plywood boards. The boards also have a glass skin which further increases stiffness and strength and adds a lot of durability.
I took some pics of one of the boards with a digital camera, but i don't have a server to store them on right now so I can't post them here.
These are the boards I was talking about when I made my post titled "a $200 kiteboard?" I will be building and selling these boards complete with straps, pads and fins, but it takes more in time and material than I had thought initally so the final price will be $250 USD for a brand new board complete.
I am going to build some that are even wider for "the ultimate lightwind board" I'd say the ones I have right now are good all-around boards with excellent lightwind ability, but because they are so thin they do not get overpowered easily at all so are probably not as good as they could be for lightwinds.
If you are interested in photos or maybe even buying one, you can e-mail me at firstname.lastname@example.org
I will try to post some photos here soon.