Is this a kiteboard or wall art?

[sflinux]

Try it. I have broken many alaias while kiting in the surf. But they are so fun, it was worth it. Mine usually broke from being pounded by waves. The Naish alaia is glass reinforced.

[alpaia]

Yes mine broke also this way, still i agree they are fun.
Glassed wooden boards are heavy unless very light wood like paulownia is used, and dents leave ugly white marks.
Varnished board with cross grain reinforcement works fine also and easy to repair

[Benson]

No rule limiting you indeed, go all the way with your creativity.
The curve in tt fins is easy to sand flat to fit your board.
I reshaped a broken windsurfing fin and threaded it,
Works but not worth the hassle, it was to add a greenwashing recycling touch to the project.

Glueing 2 layers of plywood like 10mm would be nice to build in the rocker, but heavy as you said.

I have built boards with 2 layers of 3mm poplar plywood and pu glue sandwiching xps core.
No scratch resistance so i ended up glassing them, very light but not simple nor green.

I am guessing 2 skins of 4mm birch plywood with a light skeleton would work with varnish only.
You could have the skins glued together at the tips to have flex, and just add an approx 10mm frame at the center where the board needs stiffness, tapering down the thickness gradually towards the tips. Using strips of lighter wood like spurce, one for each rail and others across, many small strips evenly spaced. Would still be possible to put in the rocker when glueing it together.
The tiki would be about 6kg I think, so average.
(Approx 1m2 x 2 skins, birch 700kg/m3, 4mm so 2.8kg/m2, light frame, varnish... about 6kg, decent)

What do you think ?

Thanks for the idea. I haven't found 4mm Baltic birch in my area. I can get 3mm or 6mm in 152cm x 152cm sheets. I can buy 6mm fir in longer lengths, but it would add about 0.75kg

I found 2.7mm meranti for a much lower price than 3mm birch. Did the 3mm poplar scratch enough to let water in?

For the frame, I'm thinking of using 10mm square dowels. I could put them parallel to the rails and shape them all at once with a hand plane, but the plywood would have less support in its weaker direction.

I found some 12cm ABS two-screw surfboard fins with flat top surfaces. Is 12cm too big for an alaia?

Are there any rules of thumb for length and width? I'd like to ride in 11 knots with a 9.5m kite (I'm 60kg).

[alpaia]

Hi meranti 2.7mm would bd stronger than 3mm poplar i guess, with a harder surface.
Yes the issue with poplar is that is soaks up water if scratched, and it is not rot resistant.
Planing down the frame to taper it for the nose sounds like a good idea.
At the nose i guess the frame could be down to zero to have flex and max glueing area.
You could have rail strips lenthways and strips across the board in the middle,
As you are concerned about plywood max strength direction.
I guess the board will be rigid enough lengthways but heel dents could be an issue.

Make some trials with scrap maybe ? To figure out the spacing of strips, by jumping on it etc..

The fin could nearly be omitted, the upwind ability comes from the long straight rail.
If you have a 12cm fin then maybe put one center fin only.
Anything from a 5cm tt fin up to your 12cm will work.

If you are light a 200 x 45 cm board would be enough.
If you can bend 5 to 10cm nose rocker on the last 50cm when glueing the board it will fly through chop.
I made a 200 x 50 cm board, I am 90kg and go upwind at 10-11knots with a 13m kite.

Again, whatever you do will work somehow, just try ....