Marine Plywood
R. Jeff Gish, E.I.T.
MacMillan (automating their assembly-line machinery for construction of
laminated beams). Their chief engineer was talking to me about "marine
plywood" vs. structural plywood like sheathing and flooring. He told me
marine plywood had no special properties as far as resistance to water goes,
that it was just a structurally sound wood, making it very strong (i.e. no
knots and voids as in normal plywood). Basically he said marine plywood was
manufactured using the equivalent of grade 1 woods.
Any thoughts???
Just Some Guy
To my knowledge this is true. The only difference is in the manufacturing
process. With standard sheathing there can be knots and voids in the wood and
gaps in the glue and filler where marine plywood is always solid. I have worked
with both and have noticed this to be true. I owned a plywood owens boat and
found it to be very strong for the thickness of the wood. It was also finished
(painted) inside and out. This boat held up very well and was much easier to
maintain than the single plank pacemaker I owned later.
--
Regards,
John G.
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NJ Coastal and ICW - Somers Point / Ocean City
RBStern
soluable glue.
Rich Stern
Douglas King
>
> I was under the impression that marine plywood is manufactured with non-water
> soluable glue.
All plywoods (and pressed board, particle board, OSB, etc etc) are made
with waterproof glue.
"Marine plywood" starts with better wood, has more laminate layers, has
all the knots and splits filled, and uses a glue that is not only
waterproof but resistant to high temperatures as well.
I have built a few small boats using A/C exterior grade ply, at a
considerable price savings; but it was more work and wasted some wood to
take out the knots and fill the gaps. More work to put a good finish on
it, too.
Fresh Breezes- Doug King
--
This is what we look like when we're at our best:
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Paul Kruse
>I was under the impression that marine plywood is manufactured with non-water
>soluble glue.
That used to be a concern. These days, however; exterior plywood and
marine plywood use the same glue. If in doubt, however; it certainly
does not hurt to boil a sample of whatever you are using, to see if it
comes apart.
I had an occasion to read the spec's on plywood some time ago in my
job, when marine plywood became very difficult to come by, mostly
because of the concerns about protecting the Spotted Owl at the time.
Two things stood out. One was that the marine plywood has tighter
control over which species of wood to use. This does not mean that
other species might not be just as good, only that fewer of them
managed to get into the spec. The only other main difference between
marine and BB exterior is that the marine has no internal voids.
You can buy various "special" grades of plywood, which meet all the
requirements of marine grade, except that they are of a different
species of wood. Of course, any time you order a special grade of
plywood, you normally have a fairly large minimum quantity to order.
(That normally works out to a "unit" or a stack about four feet high.)
As long as I'm not putting serious compound bends into the plywood, I
find that the exterior grade works just fine. If I am bending it, I
find that quarter inch exterior grade will have very few voids. I've
been known to laminate up multiple thickness of that when thicker
sheets are required.
Boatless, but building M/V Doulos I and Doulos II
http://www.trawlerworld.com/abuilding/doulos001.html
Paul Kruse
plk...@iu.net
Port Canaveral, FL, USA
Michael J Porter
R. Jeff Gish, E.I.T. <jg...@kwbes.com> wrote:
=>When I was in college I worked on an automation system for Truss Joist
=>MacMillan (automating their assembly-line machinery for construction of
=>laminated beams). Their chief engineer was talking to me about "marine
=>plywood" vs. structural plywood like sheathing and flooring. He told me
=>marine plywood had no special properties as far as resistance to water goes,
=>that it was just a structurally sound wood, making it very strong (i.e. no
=>knots and voids as in normal plywood). Basically he said marine plywood was
=>manufactured using the equivalent of grade 1 woods.
=>
=>Any thoughts???
There are seems to be more plys. Perhaps it is possible to buy
high grade non-marine plywood with as many plys. Also, really
cheap plywood seems to have overlaps within the plys internal to
the sheet. The marine plywood I bought doesn't seem to have that.
Also, with regard to to water resistance, I would say it is better
than some grades of plywood. For instance, the plywood on my shed
is rotting away for no apparent reason other than a little rain
water (the plywood is mostly painted and not in contact with the
ground), but the plywood transom on my crabbing boat is just fine.
The marine plywood also seems to be much heavier.
I also paid $75 for a 3/4" 4'x8' sheet. Cut it in half long ways
and glued it together with Resorcinal glue (spelling?). Now I have
some really good 1 1/2" plywood.
Mike
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Mike Porter <mi...@udel.edu>
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