Got a bit of a question for someknowledgable windsurfers.
I'm making some fins for some old boards that we ( kids and I ) can learn to sail on when the weather cools and the flies go away.
the lake is only 1.5m in the middle, About 0.5-07m near the access area, but with a fairly clean mud bottom.
the 2 fins Ive made are 300 and 350 in depth and Im wondering if they will be large enough to steer the boards(85-100l)
the fins are in 20mm exterior ply. our experience is that rudders and hulls get chewed out in the bush , so simple ply/varnish is best for starters .
So , are these fins going to be big enough?
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.
the lake has a rocky edge and experience has been that rudders and fins get demolished. When we sailed a Hobie there we made custom ply blades and glssed them, but with damage the wter got under the glass.
This time I thought I might go with just the ply , so It as to be thicker. Make sense?
only 17mmthick at the top tho.
Its hard when all Ive seen is photos, so I guess its suck it and see.
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.
prior to buying some mfc fins i was breaking a lot of handmade glass fins.
rocks, pipes, beaches 
that sort of stuff.......
the guy making my fins built me an "industructable" fin.
it had a vaccum bagged timber veneer on the outside of a glass moulded fin. it was solid as. it was also fairly thick so it's low wind outcomes were truly impressive but it had no top end.
still it was fun trying out different things.
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.
