3/8 plywood to 1/4 plywood

Questions about modifying a design

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NomAndy
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3/8 plywood to 1/4 plywood

Post by NomAndy » Wed Aug 15, 2018 5:30 pm

Any problems with putting 1/4 ply on a boat designed for 3/8 if you put 2-3 more layers of fiberglass on the hull? First time builder and I’m worried about bending 3/8 plywood. Never bent plywood before so I have no idea if 3/8 will bend okay. Building the 18 foot Nomad starting this fall. Using oak for the frames and plan on covering the entire hull with fiberglass and painting. I’ve got way more fiberglass experience than wood hull building so I’m clueless

NomAndy
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Re: 3/8 plywood to 1/4 plywood

Post by NomAndy » Wed Aug 15, 2018 5:38 pm

Also on a side note...plans say hull weight is 800. When she’s all finished what’s going to be a good size engine to hang on the back. I want to be able to accommodate 4-6 people on board, no tubing or skiers, just a river boat. Plans say max of 100hp.

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hoodman
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Re: 3/8 plywood to 1/4 plywood

Post by hoodman » Wed Aug 15, 2018 5:50 pm

3/8 will bend. If the design says 3/8 then I would stick with that. The fiberglass isn't for added strength. It's basically just hull armor. It protects and waterproofs the hull. The trick for bending in the forward sections is to lay old towels on the plywood and pour boiling water on the towels. Let that soak for about 15 minutes and bending the ply will be much easier. Clamp or screw in place until dry. The panel will somewhat retain that shape. After it's dry you can glue in place.
Matt

Building a Geronimo......!
viewtopic.php?f=2&t=25139

dmac
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Re: 3/8 plywood to 1/4 plywood

Post by dmac » Wed Aug 15, 2018 6:18 pm

Agree with Matt. I would not use plywood thinner than that stipulated by the design. If you are genuinely concerned about bending 3/8" plywood, you might consider using 2 layers of 1/4". A slightly over-built boat is no big deal but an under-built boat could be a concern. Whatever you decide, Nomad is just a great design and it will be very exciting to see it come into being! Good luck with it.

NomAndy
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Re: 3/8 plywood to 1/4 plywood

Post by NomAndy » Wed Aug 15, 2018 6:21 pm

hoodman wrote:3/8 will bend. If the design says 3/8 then I would stick with that. The fiberglass isn't for added strength. It's basically just hull armor. It protects and waterproofs the hull. The trick for bending in the forward sections is to lay old towels on the plywood and pour boiling water on the towels. Let that soak for about 15 minutes and bending the ply will be much easier. Clamp or screw in place until dry. The panel will somewhat retain that shape. After it's dry you can glue in place.
Gotcha, I’ll stick with the 3/8ths. How many layers of fiberglass over the 3/8 then? Is one layer adequate? I’ve built a few speakers boxes out of fiberglass so strength wasn’t a huge issue. Thanks for dealing with my dumb questions!

NomAndy
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Re: 3/8 plywood to 1/4 plywood

Post by NomAndy » Wed Aug 15, 2018 6:25 pm

dmac wrote:Agree with Matt. I would not use plywood thinner than that stipulated by the design. If you are genuinely concerned about bending 3/8" plywood, you might consider using 2 layers of 1/4". A slightly over-built boat is no big deal but an under-built boat could be a concern. Whatever you decide, Nomad is just a great design and it will be very exciting to see it come into being! Good luck with it.
The only thing that’s genuinely concerning is my ignorance! This will be my first time shaping plywood like this. It will probably be better than what’s going on in my head, but just the thought of bending wood that’s 3/8 thick boggles my mind. Thank you for your advice

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hoodman
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Re: 3/8 plywood to 1/4 plywood

Post by hoodman » Wed Aug 15, 2018 6:53 pm

One layer of 6 or 7 oz cloth is what most folks go with. Much more than that and you're just adding unneeded weight.
Matt

Building a Geronimo......!
viewtopic.php?f=2&t=25139

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hoodman
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Re: 3/8 plywood to 1/4 plywood

Post by hoodman » Wed Aug 15, 2018 6:54 pm

I suggest getting Glen's book Boatbuilding with Plywood. It will answer a lot of your questions and a lot of the things in the plans will make much more sense.
Matt

Building a Geronimo......!
viewtopic.php?f=2&t=25139

PeterG
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Re: 3/8 plywood to 1/4 plywood

Post by PeterG » Wed Aug 15, 2018 8:09 pm

Some marine plywoods are more flexible than others. The Hydrotek Meranti plywoods are stiffer compared to Douglas Fir plywoods and Okoume plywoods, but it works well for flat pieces and very strong. I am using Okoume for the planking on my Malahini for it's better flexibility on the hull shape. Definitely use the plan thicknesses for your plywood. It is possible to make plan planking thickness by laminating two or more thinner layers but that makes for much more expense and work and probably a little more weight.
Murphy's Law: Anything that can go wrong, will go wrong.
Griffin's Law: Murphy was an optimist.

NomAndy
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Re: 3/8 plywood to 1/4 plywood

Post by NomAndy » Thu Aug 16, 2018 5:27 am

hoodman wrote:I suggest getting Glen's book Boatbuilding with Plywood. It will answer a lot of your questions and a lot of the things in the plans will make much more sense.
Ordered it this morning, THANKS!

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watkibe
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Re: 3/8 plywood to 1/4 plywood

Post by watkibe » Sat Aug 25, 2018 9:41 pm

If Glen says use 3/8, then 3/8 will work, and will work the best. The choice of size of wood is always a balance between strength and weight.
Glen's work is outstanding because of the amount of detail that is included in the plan package. He had so much experience, not just as a naval architect, but as a builder, that he knew what was vital information. And I suspect he was cheap, because he tried to fit all the information on as few sheets as possible ! Think of the costs over many years for unnecessarily high postage, paper, and printing costs...
I bring this up because Glen was also very concise yet detailed. The plans and drawings have always contained the info I wanted if I looked long enough. I started out keeping a notebook with questions about things I thought were missing ; the longer I looked, I found all the answers.
Have fun, enjoy your project.
From the Great Pacific Northwest !

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