Marine grade plywood

plowjeep · Post by **plowjeep** » Mon Mar 04, 2013 1:55 pm

Just joined the forum, even though I've had my Shopsmith for close to twenty years. I'm not a woodworker. I use my SS to support my other hobbies. Right now I am restoring a Starcraft aluminum boat.
I have used 1/2" marine grade fir plywood for everything so far. I need to make two pieces 4"x56"x1/4" and was wondering if I could take some 1/2" plywood re-saw it on the bandsaw or run it through the planner. I hate to buy a full sheet of 1/4" just for these two pieces. Thanks, Brian

shipwright · Post by **shipwright** » Mon Mar 04, 2013 2:24 pm

What parts are you replacing? Plywood is a structural sandwich and would be compromised if re-sawn and could go very wrong in a planer when you come to the ply that's parallel to the knives. On top of that sawing all that glue is never good. If it's not a piece that's in the water, just get some good two sides 1/4" and get it done. There's very little difference these days between that and marine PW anyway.

Paul M

markap · Post by **markap** » Mon Mar 04, 2013 2:48 pm

shipwright wrote:What parts are you replacing? Plywood is a structural sandwich and would be compromised if re-sawn and could go very wrong in a planer when you come to the ply that's parallel to the knives. On top of that sawing all that glue is never good. If it's not a piece that's in the water, just get some good two sides 1/4" and get it done. There's very little difference these days between that and marine PW anyway.

Paul M

I had some non-marine grade high quality plywood completely delaminate in 6 months (maybe less) I'd stick with marine grade. Resawing "might" work but I don't think running it through the planer or jointer is a good idea due to the cross grain as mentioned above

forrestb · Post by **forrestb** » Mon Mar 04, 2013 3:04 pm

If you decide to re-saw or use planer (I wouldn't) be prepared for dulled blades afterwards. The glue between the plys will be hard on sharp edges.

I would try to find some small quantity of 1/4 from a boat yard.

Forrest

frank81 · Post by **frank81** » Mon Mar 04, 2013 3:05 pm

I work for an aluminum boat manufacturer. Don't do it. Half of a 1/2" and 1/4" are not equivalent in plywood, and if you could get a decent piece out of a planer or band saw it wouldn't hold up. You don't want to purposely intorduce delamination.

plowjeep · Post by **plowjeep** » Mon Mar 04, 2013 5:37 pm

Thanks, for the fast responses. The pieces are made out 1/4" because once covered with foam and vinyl they are bent in a slight curve around the inside of the open bow. I already cut them out of 1/2" but it is too stiff. Do you think that I could make some relief cuts on the back of the plywood. It's not that I don't want to buy a sheet of 1/4" It's a 45 mile round trip to the plywood supplier. They need to be made out of marine plywood since the ones I removed where completely rotting. Thanks, Brian

plowjeep · Post by **plowjeep** » Mon Mar 04, 2013 5:54 pm

Here is a picture.

shipwright · Post by **shipwright** » Mon Mar 04, 2013 7:02 pm

They weren't rotting because they weren't marine plywood. They were rotting because they were allowed to be alternately wet and dry with fresh water. Go ahead and use any decent 1/4" plywood. It would be a waste of money to use marine grade. The key to improving your chances against rot is sealing out the moisture. I'd seal with S1 epoxy sealer (System Three) although any real (not 5 minute) epoxy would do fine.

You can kerf out the back a little to make it bend if you want. It's basically a trim piece. Just fill the kerfs with epoxy if you do.

( I've done that on hulls that were only 3/8" thick to get a round stern. The kerfs were on the inside, filled with epoxy and then light glass cloth was applied on the outside. Some of these boats are 30 years old and have seen heavy daily use and to the best of my knowledge, never had a problem with this. You can see those boats here.)

Edit: I just checked that link and there's even a photo in it that shows the kerfs.
Paul M

plowjeep · Post by **plowjeep** » Mon Mar 04, 2013 8:42 pm

Paul,

Thanks for the advice. I am going to saw cut the plywood and see if I can bend it. I don't have anything to lose. If that doesn't work I'll buy some 1/4". I think most of the rot was from neglect.
The previous owner must have left it out in the weather. I have sealed everything with marine spar varnish. I'm sure epoxy would have been better, but I think I'll be ok. The original interior lasted almost 30 years and it wasn't sealed with anything.
This boat will get limited use during the summer and be stored undercover. I am envious your skills. Someday I want to build a wooden boat. Do you know of any kits or plans for a beginner. I am pushing to get this done next month. Thanks again, Brian

ps Did post the photo upside down? Its upside down on my Ipad, but right side up on my desktop

pinebeetle · Post by **pinebeetle** » Mon Mar 04, 2013 11:38 pm

Plywood gets it strength via the lay up process, with the strands running alternate directions. To rip or plane down a piece of 1/2 to 1/4 would give you basically a worth less piece of wood. Cutting groves in it will greatly reduce it strength, but if you have enough bow that it can't reverse, you might get away with it. Realize that the top an bottom cord run one way and the middle cords run the the other unless you have 5 ply or greater plywood. Once you cut all the way through the bottom face, the middle will break as you are with the grain.
My understanding of Marine grade is a little more resistant GLUE, and tighter restrictions on the voids in the middle layers, but the wood itself is not treated nor is it rot resistance.
and your picture came through fine on my computer!!

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