ZIP : Bending plywood question

Outboard designs up to 14'

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Bill W
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ZIP : Bending plywood question

Post by Bill W » Wed May 31, 2017 12:35 pm

Is it okay to steam heat plywood to aid in bending ? I worry about delaminating. I'm using Okoume marine plywood. Having trouble bending the side panel around the transom.

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Roberta
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Re: ZIP : Bending plywood question

Post by Roberta » Wed May 31, 2017 12:43 pm

Steam will raise the grain in the plywood and will need sanding later. Try using rachet straps and pull it in slowly.

Roberta
Roberta "Queen of the Boat Builders"
Built Zip "Oliver IV", Super Spartan "Jimmy 70", and Torpedo "The Glen L".

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Bill Edmundson
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Re: ZIP : Bending plywood question

Post by Bill Edmundson » Wed May 31, 2017 12:47 pm

Steam/boiling water will help. Roberta is right. To test cut a small test coupon and boil it about an hour. It should not delaminate. It may be marked WBP (water and boil proof).

Bill
Mini -Tug, KH Tahoe 19 & Bartender 24 - There can be no miracle recoveries without first screwing up.
Tahoe 19 Build

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vupilot
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Re: ZIP : Bending plywood question

Post by vupilot » Wed May 31, 2017 1:06 pm

I used hot towels and soaked them with boiling water for an hour. Then used a ratchet strap around the keel. Tighten slowly, let it sit several hours and tighten some more, it may take overnight. Get the screws in and leave it strapped and screwed overnight. Then after a while take it apart and glue it and it should go pretty easy now.

Take your time.

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BayouBengal
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Re: ZIP : Bending plywood question

Post by BayouBengal » Wed May 31, 2017 1:45 pm

I steamed and bent, then bent wood laminated (cold molded) a lot of plywood and then adhered veneer to it in shaping the interior panels of my latest build. I had concerns about both, delamination of the plywood glues and delamination of the veneer; so, I destructive tested quite a few various pieces of plywood with veneer attached. I steamed them and then threw them in our landscaping fountain and observed until my wife started bugging me to take them out (a little over a week). I noticed no delamination on the various samples that I used. Most were just external grade plywood, not marine, as the purpose of my testing was to verify that I could use external grade PW instead of marine for this purpose. The glues tested for the veneer was Ultra-Cat veneer glue, Titebond III, and GelMagic.

So steam and water did not degrade the glue and cause delamination of neither the marine ply or external grade; but, I'll also state this. Under enough cyclic stress, marine grade will definitely delaminate. The hull plans I worked with (Clarkcraft Mariner) was poorly designed. I made modifications such as adding additional longitudinals to the hull and I actually don't think the hull could be built without the modifications. The plans called for plywood planking the hull, but the front third of the boat really should have been cold-molded. I planked the hull in accordance with the plans, but in the midst of repeatedly installing and removing the planking during the fitting process, I began to notice weakness in the plywood caused by cracking of the inner-plys. Once I determined the dimension of the planking, I recut fresh sheets instead of using the fatigued sheets I'd been using for fitting. However, this haunted me over the course of time as I kept thinking about the PW delaminating and having no recourse to patch or fix it if it happened. The shortened version of this long story is that I went back in and more or less cold molded the front of my hull in reverse by adding plywood between the framing on the inside of the boat. While this will not necessarily stop the delamination, I now would at least be able to inject glue and have something to screw any delaminated sheets back in place in event it happens (some of the extreme curvature parts of the front of my boat are 1.25" thick cold molded PW).

The point is, I don't think the PW will delaminate because of the glue dissolving, but I do think it could delaminate due to cyclic stress that it will incur while in operation. I know he's not a favorite amongst Glen-L builders because in large part he "poo-poos" on the use of epoxy as a boatbuilding adhesive, but Don Danenberg has documented the stress related marine ply delamination. Mr. Danenberg's stance is essentially, the epoxy will not give way; the delamination will occur because the inner-plys of the wood itself will crack under countinuous cyclic stress related to minimal flexing of the PW during trailering and use. It's beyond my comprehension as to precisely why, but unlike hardwood, PW will never reach a de-stressed state after steam bending.

My plan if I ever build another boat is to cold mold or bent wood laminate everything with any serious stress. Many will say this is going overboard and it probably is, but the flip side is that It would only be slightly more trouble to cold mold the tough bending sections and it results in a much more stable hull. Yes, I know, Mr Witt designed these boats to be easily planked with PW and last a lifetime; and you can and they will. But cold-molding adds quite a degree of safety and stress relief on the wood. In my case some stress relief on the wood because of the thinner plys, but moreso, stress relief on my brain to not cringe so much every time I hit someone's wake at a bad angle.

Bill W
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Re: ZIP : Bending plywood question

Post by Bill W » Thu Jun 01, 2017 6:07 am

Okay, thanks all. Used Roberta's suggestion of the ratchet strap and it worked great. I was a little surprised at how the side sort of bulges out to get around the transom, but I guess that's how it's designed. Never noticed that on a boat before.

Bill W
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Location: Dallas, Texas

Re: ZIP : Bending plywood question

Post by Bill W » Thu Jun 01, 2017 6:10 am

Hey, BayouBengal : great treatise on the bending of plywood. You've obviously have a lot of experience and thought about it a lot. I'm thinking my next build will be cold molded also.

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Roberta
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Re: ZIP : Bending plywood question

Post by Roberta » Thu Jun 01, 2017 6:31 am

After the Zip, the Torpedo is a great cold molded project.

Roberta :) :D
Roberta "Queen of the Boat Builders"
Built Zip "Oliver IV", Super Spartan "Jimmy 70", and Torpedo "The Glen L".

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BayouBengal
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Re: ZIP : Bending plywood question

Post by BayouBengal » Thu Jun 01, 2017 10:56 am

"treatise" on the bending of plywood.
"Treatise???" Yes, I guess I do get carried away thinking/writing/talking about this stuff. But honestly, I would darn near cold mold anything and everything. I'd cold mold my Squirt if I were building it again. :lol:

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