is marine grade ply needed?

I'm redoing the floor and transom in an old fiberglass fishing boat and having a hard time finding marine grade ply that is the thickness i need and cost effective. I'm thinking that if i fully glass the floor and transom any wood should do but I may be off base. I want the boat to last a good 8 years or so but my budget is limited. I was origionally going to use marine grade ply and go cheap on the polyester resin but i think mabye regular ply with epoxy might be best. I've heard that resin will not bond weel to PT ply. Is the a next best to marine grade? And about the stringers, the old ones are 2bys. Could i use treated boards or is there something better? This is my first major boat project and I just dont want any major screw ups. lots of good info on this site thank you all. Any help would be much appreciated.

 

Ply--Wood

Nathan, take a look at:
http://www.boatdesign.net/wiki/MaterialsForBoatbuilding#Marine_Grade_Plywood
and maybe other stuff on the Wiki there (It's just started...)

Bottom line: Marine plywood has guaranteed solid internal plies, but the glue and wood itself is the same as other plywood. If it's not the hull, other grades may well be suitable..

Transom: I think you might think about PT or your own treatment with Ethylene Glycol. I redid a transom with 3/4 PT but scrubbed it, and dried it for quite a while, which is what the plywood MFG's say is needed... See:
http://www.boatdesign.net/wiki/MaterialsForBoatbuilding#Pressure-Treated_Wood

 

thanks Terry,I Would have never thought anti freeze. Have You used this method? If so how many coats and what kind of drying time did you need? About the PT what do you mean by scrubbing and how long to dry it? Thanks, Nathan

 

PT or etc. Plywood

Nathan, best to read Dave Carnell's article and the West System article about Epoxy adhesion to treated wood,in the Wiki.. It's Here:
---------
Preservative-Treated wood>General Discussion>Paint and Epoxy Adhesion VS Preservative Treatments

Tests by West System on adhesion of Epoxy on wood treated with Ethylene Glycol and Ethylene Glycol - Borate: http://www.maritime.org/conf/conf-reynolds-mat2.htm

In my case, I scrubbed the PT 3/4" "better grade" plywood with a commercial detergent like "409" or "Fantastic" (in the US) and a stiff brush, flooded it with water and scrubbed again, set it on edge for a hour until it had no liquid water on it. Then I supported it against a picnic table, angled towards the sun, for (I think) 2 days, turning it a couple of times. Then I put it on my covered porch for about a week or 10 days. It seemed bone dry. I cut it to size, and cut the exhaust holes and tiller opening, and it soaked up West Epoxy immediately into the edge grains, for 3 or so coats. I did 3 or 4 coats on the side that went in against the old transom (from outside, I added it aft of the original, in my case). Thickened epoxy and then screwed/clamped in place. Um.. Then another 2 coats? on the outside, followed by glass on the edge join, and then glass over the entire transom. Recoat, sand, fair, and Marine white finish (2 coats).. Seems really solid now..after 5 years about..

 

Using antifreeze or a home made solution on plywood, will only serve to protect the outer veneers of the plywood, as the glue lines between the veneers will prevent the solution, (whatever it is) from getting through to the inner layers (where likely problems will cause the most damage). This is the point of using water proof adhesives in panel construction. Preventive solutions on solid lumber can be effective, but on ply you need to encapsulate with plastic.

Nathan, you can use an exterior grade of plywood, but you should look for a panel that has specific ratings on it. Usually stamped, printed on the surface or a label, you'll see "Rated Continuous Ground Contact" or "WBP" (Water, Boil Proof). Many panels will say something like "Exterior Sheathing" which means it isn't likely to hold up. These panels are designed to be used on a house, with Tyvek house wrap and additional siding materials covering the plywood and aren't suitable for marine use in most cases.

The basic problems with most exterior ply grades are; poor panel construction, which includes lots of voids (gaps in the veneers), surface defects, repairs, minimum number of veneers, overlapping veneers and less then quality species used within the panel veneers, and non-water proof glues (truly water proof).

PT plywood suffers from similar issues, if using exterior grades, with the additional problems of removing the wax and worse, having to wait several months for the stuff to dry. In the chemical treatment of PT products, the material is placed in a big autoclave and liquid is introduced under much pressure, which drive the chemicals up, into the end grain and through the surface. The wood (solid) or wood product (plywood) is then shipped, literally dripping wet. Just stop by the local lumber store and flip through a stack of PT and you'll see how truly wet these panels are. Now they can be dried, but it takes quite while for all the interior veneers to dry, unless you happen to have a kiln sitting in the corner of your barn.

If you find some good looking exterior ply that has WBP glue, then use epoxy to embalm it. There are several threads covering this subject, use the search tool above and take a look. I'd recommend an outer sheathing of cloth (6 to 8 ounce will be fine) set in epoxy to provide additional water proofing and especially abrasion resistance. Decks and soles take a fair amount of abuse, so this is a wise course.

Stringers can be solid PT, but the drying issue and wax thing has to be dealt with. I'd just use a good looking (straight grain, knot free) piece(s) of Douglas fur or SYP (Southern Yellow Pine) for these, well epoxied and tabbed into the structure. Make sure you install weep holes so moisture can drain aft.

Don't use polyester on your transom or stringers. As you have found, it doesn't stick well and didn't do much of a job water proofing. Use epoxy or vinylester resin (in that order of preference) which will provide much better performance.

The real issues are: how things get attached to the sole and care. This is how most problems start. Fastener holes must be sealed, standing water must not be permitted and equipment must have bedding kept in good shape. Let any of these things get away from you and you'll start the cycle all over again.

 

Thanks so much Terry and Par I feel like im on the right track again. Love this thread!! Nathan