Going off how wide the board looks there heaps big. There not going to make it easy to learn though as the foil thickness is huge.

While learning it won't make a huge difference.
But even at 17mm thick plywood may be a bit week once you start getting powered up. Most plywood has multidirectional grain, so only about half the timber fibres are running in the direction you need.
Remember on a windsurfer planing at speed, about 95% of lateral resistance is taken by the fin, don't know what the lateral load on your yacht wheels is but think that when making fins.

Most fins are less than 10% chord to thickness ratio, you can go thicker at low speeds, but at higher speeds, thicker fins create too much low pressure, and will suck air down (ventilate) at medium-high speeds and start cavitating at high/very speeds.

What I've done, with glass/carbon covered timber fins, is have a glass bead around the leading edge and tip, that way you have a chance of repairing it before water gets to the timber.

Just felt my cup of knowledge filling up a bit
The second fin I made has a little hollow and a small tip high up
. . when we made hobie fins for these lakes the cavitation wa horrendous and a ex board maker suggested adding these little bits to stop the air getting all the way to the tip. bugger me it worked to.

the original blades would make a howling noise and the tube of air under water would be quite visible. with the new blades it was just a few bubbles .
check list now reads
boards rough but ok
fins -crude and slow , but ok
sails 2.5-6m cool
water levels OK
blowfly levels-way too high for the rest of the family for at least 2 months.

landyacht said...

[>>>>
If I get to the stage of making a glass covered wood fin, would Ibe better off using straight timber rather than ply.
I do have some really nice Oregon that is 30+rings to the inch

In my opinion definitely, unless it's a special ply with all the grain running the same way.
Oregon should be ideal, light strong and stable. I've got some off cuts of western red cedar that I've used so far, but my supply is getting low. Oregon will be on the list of timbers I'll be looking out for.

But once you cover the timber with glass, the timber is taking less tensional load, it's more in compression.

In fact balsa in end grain is used as a sandwich material, it doesn't contribute to the tension of the sandwich only the compressive forces applied by the skin in tension.

But it does feel nicer having all those timber fibres aligned along the fin.