Marine Plywood (revised)

[Wade Lippman]

I am building a boat using plywood. The plywood will get wet frequently, but will never be submerged. Nor will the boat be in the water for more than a few hours at a time. There will be many fasteners going into it. What do I have to do to protect it? Is paint enough? Epoxy? Epoxy/glass? What kind of plywood?

My wood dealer has "Marine Fir" at a reasonable price, and "Marine Okoume-Joubert" at a somewhat less than reasonable price (more than twice the price of marine fir). Are either of these useful.

And I just found "mahogany marine plywood" at another dealer at the about the same price as the okoume.
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I have been doing some reading and it seems that the best choice is meranti, even though no one around here sells it. (sadly, it is cheaper to order 4 sheets of okoume for $60 each and pay $150 shipping as to buy the 4 sheets locally for $120 each; so not having the meranti locally isn't a problem.)
Apparently meranti is the only marine ply considered durable, and since I will have a lot of screw holes that I can't really epoxy (well, I could; but I don't want to...) durability is good.

However I have also been reading up on meranti, and the wood is not durable! How can the plywood be durable when the wood isn't?

I am confused.

[Jim Creech]

Regular house type plywood does not use a waterproof glue and the occasional getting wet will cause it to delaminate very quickly. There are also voids in the interior plies which will cause problems with fasteners. Marine grade plywood does use waterproof glue and all internal voids are filled. (note the football shaped patch commonly found on the exterior ply). The really good marine plywood will have at least 5 equal thickness layers (no thin veneer outer ply), no internal voids and no outer layer patches, hence the higher price and, in my opinion, worth every penny. The Okume would be my first choice followed by the mahogany then the fir.

[Bill Bauer]

I have built two boats and for all of your time and trouble, settle for nothing less then marine grade ply. At twice the price you do not want to have to worry about the rot or de-lamination that will occur if you use something less. Check these folks for the best prices I have found.

http://plywood.boatbuildercentral.com/

Bill Bauer

[Wade Lippman]

I have built two boats and for all of your time and trouble, settle for nothing less then marine grade ply. At twice the price you do not want to have to worry about the rot or de-lamination that will occur if you use something less. Check these folks for the best prices I have found.

http://plywood.boatbuildercentral.com/

Bill Bauer

They have the best shipping rates and good explanations of the differences between the materials. Thanks.

[Bob Smalser]

First of all....all exterior ply uses the same marine-grade glue, which is phenol or phenol-resorcinol. These days even interior plywoods use it, as it's no longer cost effective for the mill to have a separate setup for interior glues when the price of phenol is similar.

What causes unprotected lams in the cheaper plywood grades to crack and delam isn't the glue, it's the wood movement in the outer lamination. Various luans, okoumes, khayas, etc of light weight also having interlocked grain don't move much seasonally, and won't check as much. In turn, the outer lam of Douglas Fir and Western Larch moves considerably when unprotected, and benefits from even a thin, 3oz layer of glass and epoxy to slow down the moisture ingress that's the cause of the checking.

Factors to consider:

1) Okoume isn't a durable wood and DF, WL and Red Meranti generally are. It also needs to be protected with glass and epoxy if you expect long life.

2) Meranti is much heavier than Okoume.

3) Doug Fir CDX exterior sheathing plywood will work OK for utility projects if you fill any edge voids with thickened epoxy, sound for and inject epoxy into any other interior voids after the hull is assembled, and cover all surfaces with epoxy and glass. It can be had for 13 bucks a sheet where marine-grade DF, Okoume and Meranti run five or more times that.

4) The old marketing groups Red Meranti, Yellow Meranti (luan), and Tangile (commercially extinct) consist of hundreds of species of wood from the Philippines. There is a lot of variance in durability, but generally those woods sold as Red Meranti have a moderate level of durability. But like Okoume or Doug Fir, I wouldn't use it without at least a minimal coating of 3-4oz glass and epoxy, which is easy enough to do, and doesn't have to be ugly.

This laminated tiller yoke has a layer of 4oz glass and epoxy to protect it:

[raul segura]

Bob,
I have to say, I due enjoy your writing and the wealth of knowledge you leave on this forum. I have followed several of your threads and am always left with new in site. Ive read many of the same things you have mentioned here, but have scoured hundreds of pages for it and her yoiu have it in a tight package. Thanks for that.

[John Powers]

If it's on a trailer most of the time why not squeegee two coats of epoxy on marine fir? I like okume but the price is astounding and near Philly no one stocks it.

[raul segura]

Ive run some funky tests soaking and heat exterior ply and found that it dose stay together rather well but, the grains due swell and checker,as it is not a fine grain . If painted Id say this texture would radiate to surface and eliminate a good seal .If coated with epoxy, no glass Id bet it wouldn't take much of a bump to create a leaching point and then carry the grain across. Ether way seems to me its about wood then,( high quality or not ), How you go about protecting it and your time.The second point would become more important as the quality of the first gets less than perfect. I would glass at least to the water line.
A question I would have is how long should I expect a boat made of marine or cdx ply to last if only in water short periods of time and protected from the elements.