Plastic centreboard for a dinghy?

[Lakesailor]

An aluminium board would need only to be 5/8th" thick for the leverage to raise a capsized boat

Wow! That's a load thicker than my aluminium centre plate was. The boat it sold now, but I would guess it was 8mm thick. Certainly no more than 1 cm.
You can see the pivot bolt just below the thwart. I think it drew nearly 3 ft.

 

[fireball]

<snip>To be honest I doubt that many materials 20mm thick could withstand 100kg on the tip of a 4ft board... I always thought that when righting a capsize you stand on the board much closer to the hull and lean your weight out... plus I'm nowhere near 100kg anyway...

You may not stand on the edge of the board - but if you're in the water you've got to get there first ... oh and if you turtle - you could end up grabing the end of the board to lean against - when horizontal you've now got a significant proportion of your weight on the end of the board ....

Now - about this plywood ....

 

[ianat182]

Not to gild the lily, but I expect you already careen the Wayfarer when you clean off the crud underneath, barnacles etc in the casing are more likely to score or damage the centreboard.
I built both centreboard, and rudder blades, stocks and casings for my Wayfarer five years ago and enjoyed doing it. As I recall there are two shape templates for the rudder blade but the centreboard is a standard shape and size.
10mm aluminium plate probably would do it.
I had an old National 12 that had a galvanized iron one that needed a block and tackle to raise it, and that was about 8-10mm if I recall, for a dinghy that was around 150lbs.
The Wayfarer is around the 400lb mark, plus a lot of water, and crew, inside if you use the RYA recommended drill/method albeit immersed.
How do you cope with keeping the water (rain) out of her on the mooring - is she the MK3 Self Draining version ?

 

[snowleopard]

I designed and built my own International 14 in GRP foam sandwich. I was aiming for a light but strong construction. When it came to the plate, I made it by bonding 2 layers of 12mm PVC foam together with a thin laminate of grp in the centre then carved it to an aerofoil section using an angle grinder with 24 grit sanding disc. Once I had the shape I sheathed it with glass rovings, mostly running along the length for stiffness. The overall weight is about the same as 1/2" ply but much more efficient and stronger.