Bending Plywood

B

Bryant B...and Peso

New member
I'm currently getting into building layout / marsh boats and I am experimenting with water and plywood bending.

What procedures do y'all use for bending plywood or making it more pliable - steam, molds ?

Please share your thoughts in detail. Thanks!

Regards,
Bryant "Hammertime" Bagley
 
Marine plywood (because it has more plys than normal) bends well - especially the Okume plywood. To really really bend it you need to score it with a table saw, say 3/4 of the depth with the scores about 1/2" apart - then fill the void with thickened epoxy after the bending occurs.

I have never heard of steaming plywood, but then again I'm pretending I know what I'm talking about in the first place.

Good luck
 
Bryant, I have had really good luck bending strips of marine plywood with just dry heat from a heat gun. I don't know if it would work on big pieces though. Check out duckboat specs, sneak boats, Armstrong Broadbill and you can see what I did for my coaming. Heat, bend and glue up laminations. Got to get to bed now but feel free to ask more if that doesn't make sense. Good luck.
 
The thickness of the ply will determine how much you can bend it with out scoring or heating. If you noticed ply will bend side to side better than length wise. Compound bending is very limited. Question what are you trying to bend it for and how much?
 
I like to make 45 degrees kerfs (v shaped) about 1/2 the thickness of the plywood. Obviously the more kerfs and the deeper the more you can bend. Being a boat I suspect you are after gentle curves that are just out of the reach of what the plywood can do on it's own. You can make them with a table saw and the blade tilted to 45 degrees or by using a hand router with a pointed cutter and a straight edge. It's repititious by straightforward. You can torture the plywood to get it to form compound curves but one again I suspect if the boat was designed to be built in plywood and not strips it should be doable without going to extremes.
 
Typically applying heat and moisture to plywood causes the glues to release their bond, which is not something you want to do in a boat.

I believe WEST Systems Epoxy in their online Epoxy Works "magazine" has done testing on wetting/heating various plywoods and then testing their strength. OR it was in Wooden Boat Magazine. Anyway the result of the testing was don't do it if you want the material to last for any length of time.

If you are trying to build a boat with compound curves you can use thin plywood and build up layers with epoxy between the layers in a process called cold molding. Cold molding with veneers is a common and old method for making sail boat hulls. The veneers can be solid wood or thin plywood. The mould is typically a frame over which you sandwich the veneers in opposing diagonal layers.

If you want to make radius bends then scarfing the inner face like others have mentioned works very well. For outdoors applications I fill the scarfs with slightly thickend epoxy and then quickly set the piece into the curve I am making. Then clean up the squeeze out.
 
Bending plywood is available. It's plies are all oriented in one direction and it really doesn't have any strength. You can get it so it beds the 4' way or the 8' way. It is usually very thin and used to wrap posts and sweeping corners. Can't think of an application on a boat that requires strength and a sharp bend..what are you making?
 
My questions to you is what type of construction are you using? Is it stitch and glue or bending plywood over a frame and what thickness of plywood is required for the boat. Is the boat you are building your own design or is it from a purchased plan?


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Thanks for the responses gents.....I am currently building some layout / marsh boats of my own "eyeball it" design, refined along the way, of course......no plans......no help.....just me and Peso fetching up some birds and building a boat = the Hammertime way !!

I am bending 5.2mm lauan over a frame on it, but I am wanting to experiment with other methods / plywood thicknesses / laminates & wacky wood / etc.....your reponses have been very helpful indeed. I will be making some serious mods on the next one - this one is kind of plain. But then again, I am not as well outfitted as a true woodworker should be.......all I have is a drill, a circular saw, and a rotozip!! whoa!

This week my timeline has been slowed down, as I've been busy with some tedious fillets - wow that's time consuming! I am using cabosil on this one....I think I will try wood flour next time. The cabosil is too light - it has a mind of it's own and flies out of the tub. It also takes too much trouble to get enough mixed up....eats up the resin.

If anyone is down my way, hit me up and come by....we'll look at the boat and go work some dogs!

Best regards,
Bryant
 
You should make sure the plywood you have selected passes the boil test. Cut square inch piece and place it in a pot of boiling water for five minutes and see if the lamination does not separate. This should be and an indication for that a good exterior type glue was used.

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You can bend 1/4 or 3/8 plywood in one direction fairly easily.

Attach your plywood to part of your frame that is perpendicular to the direction of the bend you want to make in the piece. Cover all of the the outside of the plywood with a piece of cotton rag or terrycloth and pour hot water from a kettle over it slowly until all the cloth is wet and clings to the surface of the plywood. After a minute or two start to bend the plywood down to meet the frame - do this very slowly. You can bend by hand or use 1 or 2 wratchet type straps to add pressure slowly over 15-20 minutes. Rewet the cloth every 3-5 minutes until the plywood meets the frame. This works well and we have not had any problems with the plywood delaminating.

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Using this method you can make some pretty tight bends in light plywood.
 
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