plywood warping

[CK 17]

I did a search and didn't find much on the subject.

I finshed building the main hatch for my NIS 23 last spring and it's been in the basement since. I noticed the other day it has warped some. It is 1/4 plywood with 1/4 mahogany strips laminated to resemble planking. I've clamped some structure to it in order to hold it flat until I'm ready for it.

I am now ready to start cutting the bulkheads and they are made of 3/8 or 1/2 plywood. I'm concerned that the same could happen to these if they remain unsupported for a long period of time.

I'd like to cut the bulkheads now so that I have time to epoxy coat them while their still flat in my basement rather than after the hull it built. I would be setting them up on the strong back in the spring.

Anybody out there have experience with preventing plywood from warping over the long term.

Joe Schena

[RonW]

It is all due to quality. You get what you pay for if you are lucky. Would I be correct in GUESSING that it is exterior plywood instead of marine?

[Bob Smalser]

The only preventative is to keep the humidity equal on both sides. Stack a big pile of it laying flat out in an open shed and then have the rainy season arrive, and the top layers are prone to warp.

Damp basement and carport floors are culprits, too.

If none of that applies, then it's the quality of the plywood.

[ 10-15-2004, 04:27 PM: Message edited by: Bob Smalser ]

[CK 17]

Originally posted by RonW:
Would I be correct in GUESSING that it is exterior plywood instead of marine?You would be wrong, Ron. It was marine grade mahogany.

Joe Schena

[kc8pql]

Originally posted by joe schena:
It is 1/4 plywood with 1/4 mahogany strips laminated to resemble planking. Laminating the strips turned a balanced construction (the ply) into an unbalanced panel. The solid and the ply react differently to changes in moisture content. This kind of construction can work ok, but it needs to be held by a structure strong enough to overpower the panel movement. Making the panel and then letting it lay around for a long time is asking for trouble.

[RonW]

Yep i agree with KC8 totally, was getting ready to type when his message appeared. I have some 3/8 five ply douglas fir marine ply outside for over a year, and am totally amazed at what perfect condition it is in, no warping , delaminating at all.Good marine plywood is amazing, so you added stress to the panel without securing it for reinforcement.

[CK 17]

Originally posted by kc8pql:
"Laminating the strips turned a balanced construction (the ply) into an unbalanced panel. "
I was wondering if that had anything to do with it. Thanks for the info

Joe Schena

[JimD]

Don't know what went wrong with yours, Joe, but I made the hatch for my Glen-L Minuet exactly the same way only used cedar for the planking, epoxy glued and encapsulated. It is outside year after year with only a tarp over it and has not warped at all. Still slides perfectly.

[kc8pql]

Originally posted by JimD:
I made the hatch for my Glen-L Minuet exactly the same way only used cedar for the planking, epoxy glued and encapsulated. .That's the difference. Your hatch is finished and sealed. Joe's was just an unfinished panel. My hatches and cockpit seats are made the same way. It works just fine. Letteing unfinished parts sit around for months is what can cause problems.

[ 10-15-2004, 06:28 PM: Message edited by: kc8pql ]

[Beowolf]

If I remember correctly from the Gougeon book, you both used strips that are too thick for the plywood as well. It seems that they recomend strips be no thicker than 1/8 in, to help control the warping and the movement caused by the strips expanding and contracting, but I may be thinking of something else.

Jeff

[Bruce Hooke]

Originally posted by Beowolf:
If I remember correctly from the Gougeon book, you both used strips that are too thick for the plywood as well. It seems that they recomend strips be no thicker than 1/8 in, to help control the warping and the movement caused by the strips expanding and contracting, but I may be thinking of something else.

JeffI believe the 1/8" figure comes up in the Gougeon Brother's literature for when you are applying deck strips to a heavier plywood substrate to make a "teak" deck, and that the logic is that thicker strips can overpower the epoxy joint that holds the teak to the substrate. The situation here is a little different but I think the logic does still probably apply -- ideally, if I were doing this I would probably use thinner strips, say 1/8". HOWEVER, since in this case the piece is already made, I would probably just stick with it because the chances are quite good that once it's finished and in place that it will hold up just fine.

I don't think you will have any problem with cutting the bulkheads in advance as long as you store them reasonably carefully (i.e., not on a damp surface and not leaning up with weight pushing against the middle to cause them to bow -- really obvious stuff -- nothing that special should be required). Coating them in advance is an excellent idea. The "flow-coating" method described in the Gougeon Brother's book has worked very well for me -- it basically amounts to putting on all three coats of epoxy in one layer that largely self-levels. If you will be using most of a sheet of plywood you can even coat the whole sheet and then cut it up into the individual parts.

[kc8pql]

I guess I should have noted that my hatches are 1/8" solid laminated over three layers of 1/4" ply.

hatch detail (http://tinypic.com/ctkkj)

[mdevour]

Balanced and unbalanced; finished and not... makes me think:

I built the solid mast and spars for our little boat from well seasoned Sitka spruce. But how I treated them along the way is a testimony for balanced structures.

Each part was laminated from two pieces. I cut the halves from the same blank. Then I flipped one piece so the cut face was out but the ends were still matched up. Glued up this way, all the warp, cup & twist -- and there were some of each -- cancel each other out.

After I roughly shaped them, they sat around for over a year in the garage, a tent in the back yard, or in my basement. I fully expected them to warp, but when I finally got around to finishing them, they were still perfectly straight. :cool:

Before that I've never had wood sit around unfinished that didn't warp, so there ya go.

That's my $.02 (US).

Mike D.