Re: Marine ply info.
Well, first let's define our terms.<br /><br />what is marine ply and what is normal plywood?<br /><br />"marine plywood" is a term used by the American Plywood Association to describe a certain grade of exterior fir plywood. you can refer to the APA site for the specific details, but the ONLY differences in "marine" ply and regular exterior grade fir is the width of the gaps allowed in the internal plies, and the finish.<br /><br />The difference in the gaps in the internal plies is between exterior and marine 1/8". contrary to what is often posted marine plywood can and does have voids in it.<br /><br />contrary to what many will post, "marine" fir plywood is not "stronger" than non-marine fir plywood. Marine plywood is made to the same strength specifications as exterior.<br /><br />Marine ply was developed primarily to have a nice face on both sides for ease of finishing and fairing. In the marine environment the patches ("dutchmen") will print through and so it was important to minimize their number and effect. So unless you're going for a yatch finish the use of marine plywod probably is not called for. In fact most of the transoms in pleasure boats in the United States have as their core some pretty rough exterior plywood.<br /><br />Then in the category of "normal" plywood, you have southern yellow pine, which is what you get in a large part of the country for plwood. SYP is more dense than fir, and so for an equal thickness of fir, (1/4 " say) SYP will be heavier AND stronger. <br /><br />Again depending on how important ease of finishing is SYP is perfectly suitable for building a boat. There are still thousands of plywood skiffs across the south that have survived for yers with nothing but a coat of porch paint to portiect them. Right now there is some very good B/C SYP that will compare with the best fir, and some outstanding A/C SYP. Basically all SYP is exterior grade.<br /><br />the you have pressure treated SYP, which is underrated as a boat building material, and which is being used more and more by commerical boat builders. Biggest rap on PT SYP is that it is ugly as homemmade sin, but for parts that will be fiberglassed or out of sight, its the ticket. It the material that isbeing used in more and more transoms.<br /><br />Then you hve the exotics, meranti and okume being the two prime ones. While many people refer to these generically as "marine", technically they are not. These plywoods can be any thing from asiatic trash, to being manufactured to certain standards which make them suitable for marine use. Occume is not at all durable --see the Glen-L site for a good discussion. Meranti is about as durable as pine or fir. Regardless of what anyone says, the primary advantage of these outrageously expensive exotics is that they have nice, patch free faces because the manufactures are still able to use old grwoth logs, and in the case of ocume, its very light (and correspondingly weak).
Where was this info when I started my Renov project. A year ago there were a lot of questions asked about plywood substitutes but no good answers.