Marine plywood or Pressure-treated?

[Nick Talmo]

Well, the boat is 10 years old and had some soft spots on the deck (19'
Bayliner). I've pulled up the section of old decking and am drying everything
out (stringers still look real good). Now I need to put in a new section
about 5' by 5', from the transom area to just behind the front seats.
Question: Should I use marine plywood or pressure treated? I'm leaning
toward pressure-treated for obvious reasons of being in a moist environment.
But I'm also concerned about delamination and thus don't know which would be
better. (Of course I'll coat it with West epoxy and use mat on all seams).
Anyone with suggestions? Appreciate a post or email.

[Ed Welp]

I would lean towards the plywood, because of the weight.

[g. mellen]

Between the two choices go with Marine plywood. It will outlast pressure
treated.

Another choice that will be much better than either:

I've been told that there is a new composite plastic replacement for
marine use. Supposed to handle like wood (will take staples, screws,
nails), weighs less, and as it is a plastic, will never rot or crack.

It comes in 4'x8' sheets in the same thicknesses as plywood.

Unfortunately, I don't know the name of this product, but could find out
if you need.

The main drawback that I have heard of is the expense. Understand that
it's $140.00 for a 4' x 8' x 1/2" sheet.

Glenn
Southlake, Texas

[VfastPat]

Your not talking about Klegecell are you. I think that this is what high
end boats are using to cores of deck laminates these days.

vfastpat

[Jim Swist]

Marine plywood is laminated with resorcinol glue which is extremely
waterproof. Pressure treated plywood (or pressure treated any other kind of
wood) is generally not used in a marine environment. For one thing, the
pressure treatment not only resists penetration of water, but it
unfortunately also resists adhesion of many other things (paint, etc). PT
wood needs to cure in the open air for at least 6 months before you try to
apply anything to it. It's just not suited to marine construction.

[scott morris]

Jim Swist <sw...@nexen.com> writes:

I have used foundation-grade pressure treated plywood to construct hidden
bulkheads for repair without a problem. This stuff is rated to be buried
in wet ground and have a house sit on it, so I figured that the odd soaking
won't hurt it. It is **UGLY** though (note the "hidden" part). So far I have
had good luck with it.

Some test scraps that I used as garden stakes have survived several seasons
in the weather and in the ground without signs of delamination or
decomposition. I realize that this isn't exactly an ASTM salt-water immersion
test, but I'm a long way from the ocean.

In order to get epoxy and `glass to adhere, I run one of those wire-rod
paint strippers over the bond area, then wash it thoroughly with lacquer
thinner to provide a rough, clean adhesion area. Again, so far so good. To
paint it I use primer-sealer that is normally used to hide smoke stains
and other bleed-through marks. Cosmetically this stuff is the pits, it's rough
and green and ugly, but it's tough. Also, the sawdust is full of all kinds
of toxic nasties, so wear a good dust mask and cut outside if possible.

Your mileage may vary, but I think that these materials have some use in
marine construction.

Good Luck,

Scott

[Dave Shanks]

In article <4kgka2$f...@vixen.cso.uiuc.edu>,

smo...@ux1.cso.uiuc.edu (scott morris) wrote:
>I have used foundation-grade pressure treated plywood to construct hidden
>bulkheads for repair without a problem.

<snip!>

>Cosmetically this stuff is the pits, it's rough
>and green and ugly, but it's tough. Also, the sawdust is full of all kinds
>of toxic nasties, so wear a good dust mask and cut outside if possible.

Don't you worry that the "toxic nasties" will get into your bilge and from
there into the water you travel on? Or are you doing something to eliminate
the problem?

-Dave Shanks dsh...@vcd.hp.com

[Dave Anker]

I rebuilt the transom on my boat using pressure treated plywood. My
boat is a 17 foot fiberglass center console model. It has been 5
years and all is like new. I researched the idea of using marine
plywood versus pressure treated at the time of the rebuild. Here is
what I found:
- Marine plywood has no hidden voids. All layers have been
patched so that there are no
hollows in the inner layers.
- Marine plywood may have had more layers. I'm not sure of this
point, but I think this is
correct. If it is correct, it indicates that Marine plywood
is stronger than a plywood with
less layers.
- Marine plywood rots or else we wouldn't be having this
conversation.
- Pressure treated plywood may have voids. No guarantees that
the inner plys are solid.
This means there could only be one thin layer of wood between
you and an unfilled knot
hole in the second layer.

I used a scrap of the treated plywood to cover the kids sandbox for
the past 5 years. We get some nasty winters and plenty of rain in
Rochester. This piece of wood is still in good condition.

I did not have any trouble with epoxy adhering to the treated wood.
However, I did dry the plywood in the sun for a few days before
working with it. Looking under the deck back at the transom, it
looks the same today as the day that I finished the project.

Good luck on your project.

[1329C1000-Garry Heon(MT4084)NOBIN]

Are you serious?? With the miles and miles and miles of bulkheads made out
of this stuff (treated to 2.5 CCA not 0.4 CCA that you buy a the local home
center) I really doubt that a little piece in someone's boat is really going
to make a difference.

Garry Heon
ga...@holly.att.com
My Opinions....Lucent's Hardware.

[Doyle Alley]

Nope. Marine plywood will not outlast pressure treated in a moist
environment. The difference in marine plywood and regular plywood is that
marine plywood doesn't have any voids in it - making it useable for
hulls. There is nothing special in it that would keep it from rotting
due to moisture. Pressure treated, on the other hand, is very resistant
to rot.

I've just gone thru my deck and transom, replacing all the rotted plywood
with pressure treated. I had the same questions myself until I talked to
several boat builders here in south Florida. Just make sure the pressure
treated stuff is completely dry. It comes from the lumberyard with an
extremely high moisture content. I had to let mine sit in the garage for
several months before it was useable. Good luck.

[George and Brenda Fennell]

Pressure treated is a misnomer. It relates to the pressurized
infusion of toxic chemicals to retard infestations of insects. For
marine applications only use marine grade plywood. Marine grade is
specifically laid up for greater linear integrity and is glued with
waterproof glues necessary in moist boating areas.

[fas...@surf.com]

How do you deal with the fact that it is unpredictably weaker since it
has little or no void control and thus can't or shouldn't be used
structurally?