Reinforcing aluminum transom (which wood?)

[moloch16]

The 14' used Jon I bought a while back had some wood bolted to the aluminum transom for (I assume) reinforcement purposes.  Wood was doing fine but I needed to remove it for another reason and since it's old I figured I might as well replace it.

Looks like the wood I removed was plywood and didn't seem warped or rotting.  This is surprising because I thought regular plywood left in the elements would not hold up very well.  I seriously doubt the wood is marine grade because I can tell the fella I bought the boat from didn't not splurge on boat parts.

I'm thinking I would replace the wood with some fresh plywood with a few coats of exterior paint.  Will this do the trick?  Or do I need marine grade?

I hear treated wood is a no-no on aluminum, is this true?

Any recommendations are much appreciated!

[Micro]

Treated wood is corrosive to aluminum.  It contains a copper salt that gives it it's greenish color.  I think marine grade is corrosive to aluminum as well.  I think that it is used by itself, or in fiberglass applications.

If I were going to use something, I'd get a sheet of plastic plywood.  They sell it at BPS.  It's sort of expensive, but its super-strong, lighter than plywood, rot-PROOF, and can be worked with wood-working tools.  

Otherwise, plain plywood with several coats of water-proofer should do the trick.

[cart7t]

Plywood with several coats of epoxy resin will work fine. Do not use fiberglass resin as it's too brittle for that sort of application.

[Catt]

Wolmanized® pressure-treated wood is a mixture of the oxides of copper, chromium, and arsenate known as CCA.  If attached to aluminum a barrier must be applied between the plywood and the aluminum, such barriers would be a zinc chromate primer with an polyurethante topcoat on the aluminum. Another barrier would be a thin layer of silicone sealant between the plywood and the aluminum.

[Olebiker]

Wolmanized® pressure-treated wood is a mixture of the oxides of copper, chromium, and arsenate known as CCA.

I think that CCA treated wood has been off the market for a couple of years now.  Here is something about fasteners from the Wolmanized web site:

*Aluminum can be used when a protective barrier prevents direct contact between the aluminum and the wood. Direct contact, in the presence of moisture, will create a galvanic corrosion cell between the materials. The barrier may be heavy plastic sheeting, rubber, vinyl, asphalt roofing paper, or even a good industrial coating. The barrier should provide complete separation and remain intact for the period of service of the aluminum.

[Low_Budget_Hooker]

The stuff you are looking for is  "starboard".  It's a plastic based composite, used to replace wood in marine applications.  The stuff is great, we used it for all the backing plates on the rod holders on the tuna boat.

A piece that size will cost you about $15-$25.  Worth it.  you'll never deal with it again.