Is 25mm ply feasible for a 14 rudder?
25 mm is the correct thickness, but there's plywood and there's plywood
Marine ply is what you want - waterproof glue and lots of layers:
Has anyone done this before or could they please point me to where I might find some information.
The Brazilians used to make wooden rudders - the Hobie 16 World Championship perpetual trophies are made from two of them.
I don't want to spend $100's of dollars so a quick knockup of the blades with a jigsaw / router, round off the edges to give it the right profile and slap some sort of coating on it.
Now that's where you go off track. You can't just "round off the edges" and expect them to behave nicely - they'll stall and ventilate at anything but very low speeds. They will be very twitchy at higher speeds and the boat will be difficult to control.
Instead of reading up on the finish paint, read up on this:http://books.google.com/books/about/The ... ZYUGNyuboC