Plywood (newbie)

I have the hull for a Snark Sunchaser that I am trying to turn back into a working sailboat, with minimal expense (I just got a screaming deal on a sail and spars; still need a mast). Among other things, I need to make a dagger-board and rudder. To buy those parts would cost $100 and $175, respectively. Marine plywood is expensive, too. Is it possible to use ordinary lumber-yard plywood if it's sealed appropriately (say, with a good, thick coat of polyurethane)?

 

RKnack.
i just tried to post a rather long post and it got lost in the cyber-vacuum somewhere.
Short answer is yes.
If you have to, you can use a non-marine grade ply by sealing it up appropriatly.
Long answer may start something like,
no, you cannot just use "ordinary lumberyard plywood" which could mean anything. You need to make sure of a range of things that others who know more than me have explained in a lot of places here…
you want a structural qualitiy ply with no voids etc. One that won't warp etc. Some non-marine ply's are rubbish, some great, some use marine wood with rubbish glue, some use marine glue with rubbish wood, etc.....
You will have to have a search around, and just read through a lot of the threads that deal with these issues.
I tried to do a quick search to find the threads I am thinking of but can't remember at the moment.

Once you have found out what ply you are looking for,
you need to seal it.
I would use epoxy, (which is not cheap).
first a CPES (clear penetrating epoxy sealent), especially on all of your endgrains, let it take as much as it can, then dry, then maybe a light sand, then repeat, and repeat. (see thread about possibly making your own http://boatdesign.net/forums/showthread.php?t=12061)
then I would apply anormal thicker layer of epoxy, before painting- Polyurethane or enamel, or what ever else you choose.

If you do manage to completely seal the wood, which epoxy is great for, and then protect the epoxy with a good paint, then the non-marine wood you have used, that would normally rot when wet, will not.
At least that is the plan. It will have to be strong enough to bear any loads that you will be giving it.

If you are looking to do a cheap CHEAP job, and epoxy is too expensive, then read through this thread for some good dodgy shortcuts.
http://boatdesign.net/forums/showthread.php?t=11610

Good luck.
and read your way around, you will find more than you need on this subject, and explained better than I just have.