marine ply grain

[David Koschmann]

I am building a plywood catamaran (40'). The hulls are assembled instant
fashion from payson jointed full length panels. The outside is glass and
epoxy and the inside just epoxy. My question is can panels be assembled
without regard to face grain orientation using the most convienient
combination of available full size sheets and smaller partial sheets?
Does the orientation of the face grain have anything to do with the
strength of the panel?

[Ron Brown]

Face grain orientation does mater, for strength and for flex. The plans
should state. If not, check with the designer.

Ron Brown

David Koschmann <frit...@webtv.net> wrote in article
<12010-35...@newsd-113.bryant.webtv.net>...

[Foster Price]

To ignore the face orientation of ply for "hull planking" is in my opinion
asking for trouble. There are some instances when you may get away with it
IF the ply is of "balanced" construction. By this I mean the thickness of
plies in one orientation is matched by the thickness on the other
orientation. The number of plies isn't so critical, except when dealing
with 3 ply types. I am assisting in the building a Wharram Tiki 38 and it
has been much easier using a "balanced" ply where there is any degree of
compound curve in the panels. Ofcourse face orientation doesn't usually
matter for bulkheads etc. as long as you won't have your senses "offended"
by the finished product.

Foster Price, Southland, NZ

[Bill]

David Koschmann wrote:
>
> ...snip...

> Does the orientation of the face grain have anything to do with the
> strength of the panel?

On the marine ply that I am accustomed to (5ply), the outer 2 and middle
ply (lengthwise-grain) are thicker than the intermediate 2 ply's
(cross-wise grain). The outer 2 and middle ply comprise maybe 80% of the
cross-sectional area. So, about 80% of the board has grain length-wise
and on about 20% is grain cross-wise. This definitely affects strength
and flex. NA's use this to their advantage.

Although the plys are equal thickness in common plywood, there is still
an odd number of ply's, skewing the grain lengthwise. That is why it
matters which direction you nail common plywood to joists/rafters/studs.

--
-----------------------------------------------------------
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[macnaughton.com]

No, grain orientation is very important. The plywood will be strongest when
stressed in the direction of the grain in the face plys.

Tom MacNaughton
http://www.macnaughtongroup.com

David Koschmann wrote in message

<12010-35...@newsd-113.bryant.webtv.net>...
I am building a plywood catamaran (40'). The hulls are assembled instant
fashion from payson jointed full length panels. The outside is glass and
epoxy and the inside just epoxy. My question is can panels be assembled
without regard to face grain orientation using the most convienient
combination of available full size sheets and smaller partial sheets?

[Rene Dalmeyer]

As bending stresses the outer fibers the most, the outer fibers have to have
the correct direction to cope with the major bending stress.

The stiffness of a member is proportionate to the thickness ^4 This means
that five ply multiplex with equal thickness veneers is 40 times less stiff
with incorrectly oriented grain. For bending strength its ^3 meaning about
15 times weaker. Quite amazing. I once had a multiplex center board snap due
to mis-aligning the grain.

The rough calculation discounts the capability of the cross-grain veneer to
take up any stress. Depending on the wood used it will be capable of
contributing. Where ocume will contribute very little and birch will do
quite well.

--
--rened
--To reply remove spam from address. Too much spam

macnaughton.com wrote in message <35bdc...@newsfeed1.cybertours.com>...

[Pierre Mitham]

I would have to disagree here. It's been my experience that the facts below
are reversed, take a walk into any common lumber yard, There are an odd
number of plys but the exterior grade ply has different thicknesses. It is
the marine grade ply that has their ply's being of equal thickness, there
are an odd number of plys but they are all the same. It's usually the
exterior grade stuff that has ply's of different thicknesses. Take a look
at a sheet of 1/4 exterior ply, you'll see that 80% of the thickness is the
center ply with the face ply makeing up the other 20%. Marine ply on the
other hand(good stuff anyway) has 5 plys all of equal thickness. Just
create a scarf in one end, you'll see each layer, taking up an equal amount
of the scarf's length.

Pierre.

Bill wrote in message <35BCA4...@alaska.net>...
>David Koschmann wrote:
>>
>> ...snip...

>> Does the orientation of the face grain have anything to do with the
>> strength of the panel?
>