Re: marine plywood... necessity or not?
I kinda agree on the PT thing. I mean, it's mostly for reassurance of the owner. If the wood gets wet it won't rot, but it will swell up hard enough to tear thin fiberglass, and the weight will still be there. Who wants a soaked boat even if it's not going to rot? Just take it out and fix it.
On the ply question, marine vs. exterior grade is kind of a religious issue for many. But, here is some info on the subject.
First, use marine grade if you're engaging in a project where the designer specifies it... marine is stronger and stiffer than exterior because of the number of plies, so if a designer calculated in that strength you'll want to make sure you use the materials he expected.
Marine is easy to work with... it's usually very smooth sanded, very flat, holds dimensions well, and looks good. Certified for marine use, it may be important to use this for insurance purposes.
Marine ply is stronger than exterior. This is because of two things. It has a larger number of plies, usually twice as many as exterior, which improves its strength tremendously. Second, it has no voids... all the plies are continuous with no air gaps inside the wood. This means you get uniform strength across the piece and no little reservoirs for water to collect.
Marine ply is NOT treated against rot. Common myth. Sometimes the wood it's made from is high quality tropical hardwood (so marine ply can be less eco-friendly than exterior) which has some rot resistance, but there's nothing special about the marine name as far as rot goes. You can get marine PT plywood, which is both marine and rot treated, but the same comment as above applies for it... who wants to have a non rotting boat that's soaked with water?
In reality most production boats are not made with marine ply. They're built using exterior of some sort, and more and more new boats have no wood at all. Using exterior on your repair or modification is a pretty safe thing to do, especially if you follow the rules for encapsulating it in glass or resin.
I use Arauco ply (from a factory in Chile) and plyfloor underlayment (baltic birch ply) for stuff right now. I can get both at Menards, both have almost no voids, the Arauco is AC (not officially X yet, but it has the same waterproofing) and usually has both sides almost AA and is renewable, being made from fast growing plantation trees instead of wild grown hardwood. Both are $20-$30 a sheet. More expensive than CDX, but much nicer to work with.
Erik
). i do work with wood a lot so i know high quality wood. the thing with marine ply seems to be less voids? is there much of a difference between a high quality piece of non-marine ply and marine ply? (besides the cost.. 
