Plywood frames?

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Grampa Steve
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Plywood frames?

Post by Grampa Steve » Sun Jan 02, 2011 2:42 pm

Hi Guys,
New old guy here thinkin' of building the Bell Isle 23.
I have two first questions;

1) The plans call for the frames to be glued up and gusseted from multiple pieces of 1" lumber. Can that be substituted with marine plywood cut from a single sheet?

2) If so, I can laminate two 1/2 sheets together to get the 1" thickness, and I think it would make the hull much stronger. If so could I use a single sheet of 3/4 marine plywood?

Sorry if these are stupid questions but I know there is no such thing....just stupid answers lol. Thanks for goin' easy on me.

Grampa Steve

upspirate

Re: Plywood frames?

Post by upspirate » Sun Jan 02, 2011 3:02 pm

Welcome Grampa

This has been discussed here before and the consensus is that on these designs,the screws for chines,shears,battens,keel, etc would be going into end grain of the plywood and won't provide as much holding power as solid limber.When you are building the frame there is considerable stress put on pieces as they are bent around the frames

Also if you were to construct a frame from plywood and a comparable one from solid with gussets,the ply wood one is much flimsier torsionaly ,which as the hull is built is not such a problem as all things start to brace and form a structure as more parts are added,but initially you need the proper shape

Plywood bulkheads and frames are used in stitch and glue construction,but these are epoxied in place and the other longitudinal pieces aren't screwed to the edges of these

I know there are other builders (some very high dollar boats) that use this method, but we as a rule say to build the frames as designed.

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thudpucker
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Re: Plywood frames?

Post by thudpucker » Sun Jan 02, 2011 3:46 pm

I'm not wanting to argue with anybody on this board about Glu-lams in a boat.

All my life I've made "sisters" out of Ply for my wood boats.
I built a house with the Celing Rafter span of 25' over the living room made up of 10" of Glu-lam ply.
I used good Ply inside the house and Marine ply on the boats.

Any span where your not screwing or nailing into the exposed edges of the Glu-lam Ply is just about bullet proof if the Environment of the wood is secure. That's what I think :P

In all the boat forums and reading on Wood that I've done and been exposed to....I never ever ran across a way to seal the exposed edges of Ply in that Hostile environment of the Bilges.
The best effort I ever saw was a series of Wood, screwed through the two boards on either side of the Glu-lam and through bolts "Pinching" the exposed edges of the Glu-lam together.
I don't know if that worked or not.

Grampa Steve
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Re: Plywood frames?

Post by Grampa Steve » Sun Jan 02, 2011 7:14 pm

thudpucker wrote:
In all the boat forums and reading on Wood that I've done and been exposed to....I never ever ran across a way to seal the exposed edges of Ply in that Hostile environment of the Bilges.
The best effort I ever saw was a series of Wood, screwed through the two boards on either side of the Glu-lam and through bolts "Pinching" the exposed edges of the Glu-lam together.
I don't know if that worked or not.
Thanks so far guys,
This answer seems to make more sense to me for an argument against plywood frames. Intuitively, the stringers are placing a positive load on the majority of the notched frames, so I'd think securing mechanically is only a temporary fixturing. Don't they remove the stringer screws in some wooden racing sailboats to reduce weight? If it's still an issue I think longer screws would address it?
In the case of the exposed edges why wouldn't a good coat of epoxy thickened with micro balloons not properly seal this area?
Thanks for your continued combined wisdom.

Grampa Steve

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Bill Edmundson
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Re: Plywood frames?

Post by Bill Edmundson » Sun Jan 02, 2011 7:46 pm

We have discussed this many times. I've changed my mind a little. I think you can do your frames from ply with a CNC. But, if you use good marine ply it is much more expensive. Too much waste goes on the floor. Nails and screws need to go in at an angle to cross the grain of the ply.

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

Grampa Steve
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Re: Plywood frames?

Post by Grampa Steve » Sun Jan 02, 2011 8:03 pm

Bill Edmundson wrote:We have discussed this many times. I've changed my mind a little. I think you can do your frames from ply with a CNC. But, if you use good marine ply it is much more expensive. Too much waste goes on the floor. Nails and screws need to go in at an angle to cross the grain of the ply.

Bill
Thanks Bill, love your Tahoe 19. I have access to a CNC router, therefore the interest in this labor saving approach. Looks pretty certain 2 complete frames can be nested out of a 4X8 sheet.
Now, do you think it would be foolish to go with 3/4" and increase the web and inner radius dimensions that would have been waste anyway?

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thudpucker
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Re: Plywood frames?

Post by thudpucker » Sun Jan 02, 2011 9:34 pm

Bill's right about all that waste.
It reminds me of the first shop vac I ever saw. "WOW" That'd sure be handy!
When your overhauling an older wooden boat with oily bilges and old Cedar planking skinned down, you have some of the Best Campfire wood in creation. It makes for a Colorful evening in front of the fireplace with 'Her' too.

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Bill Edmundson
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Re: Plywood frames?

Post by Bill Edmundson » Mon Jan 03, 2011 7:26 am

Steve

You might contact Bill Childs at Bartender Boats. He has done plywood frame kits. He has stopped doing the kits. But, he will have real knowedge of working with ply frames.

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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billy c
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Re: Plywood frames?

Post by billy c » Mon Jan 03, 2011 7:47 am

Steve here is the thread that ran a few months back on the same exact subject.
http://glen-l.com/phpBB2/viewtopic.php? ... cnc#p82618
hope it helps out this current discussion for you
-Billy
(insert Witty phrase here)
Billy's Belle Isle website

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BruceDow
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Re: Plywood frames?

Post by BruceDow » Mon Jan 03, 2011 7:49 am

Here is another builder who designed his boats to be built with plywood frames...


Image

http://www.clarionboats.com/

I'll echo UPS Pirate's belief that "nobody ever regretted following the plans"...

But if you are confident that you know the structural properties of the plywood versus the solid timber, I would not want to get in your way. If you do choose to go this way, please keep a good photo record for us and let us know how it turns out.
Bruce.

~~ Do what you love, and love what you do. ~~
~~ To me - only my boat is not yet perfect. Everybody else's is to be admired for I know the path they have walked (Dave Lott, 2010) ~~
Dow's Monaco Project

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Mr Hot Rod
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Re: Plywood frames?

Post by Mr Hot Rod » Mon Jan 03, 2011 10:34 am

As Bill has stated, substituting ply for solid lumber has been discussed many times.

Here are quotes from two Naval Architects on the subject of substituting ply for lumber :
This doesn't mean that you can't use ply. You'll have to know your ply properties and ensure that the number of load-bearing plies can safely carry the intended load.

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Bill Edmundson
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Re: Plywood frames?

Post by Bill Edmundson » Mon Jan 03, 2011 1:50 pm

I'm not really a plywood frame believer. There are some very pretty boats being done that way. But, the quality of the material is not known until after the cuts.

For a one time build I don't think you can save that much time. The plans don't come ready for the computer. I would rather handle the wood than a keyboard.

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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Mr Hot Rod
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Re: Plywood frames?

Post by Mr Hot Rod » Mon Jan 03, 2011 3:32 pm

Another option would be to lay up a marine grade plywood-cored woven roving or unidirectional fiberglass composite. The tensile strength of fiberglass woven roving (31,000 PSI) and unidirectional fiberglass cloth (53,000 PSI) exceeds that of Douglas Fir plywood (5,570 PSI). The modulus of elasticity (stiffness) of the woven roving and fiberglass cloth is comparable to that of Douglas Fir and Oak. If we were to laminate mat and woven roving to both sides of a marine grade plywood sheet before cutting it with the CNC, we'd have adequate strength and total encapsulation. Fiberglass and epoxy supplies would quickly escalate costs.

Materials and molding techniques are discussed in great detail in Ken Hankinson's book Fiberglass Boatbuilding for Amateurs. The book is worth the cost of admission and a great source for ideas.

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Oyster
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Re: Plywood frames?

Post by Oyster » Mon Jan 03, 2011 3:53 pm

This is an issue that I see having some really negative affects here a few years down the road in exposed and neglected boats, which happens quite a bit.
Who will end up with your boat down the road and not have the insight to address the numerous issues of engrains in semi closed areas that we cannot see either. Unless you are building encapsulated and composite hulls using plywood as the grid material, you maybe saving now but causing some real issues down the road. For sure there are some boats that subscribe to cut to shape plywood parts. But for the complete build when solid timber is called for, there are reasons for this.

That said there are ways to use plywood successfully with the epoxy. Do you save time and money for the extra effort which will improve your overall build both now and down the road? Questionable at best, IMO. The way the plans are done and the standard building method for most of the builds here which in most cases are first time builders, you quite possibly are building in a huge flaw into your hard work.
I could write a book and probably irritate some when talking about this issue for the general builds on this board, but will forgo it.

When discussing CNC router cutting of parts, to be sure there are huge boats being done that way using plywood. But these boats incorporate a completely different building method than most boats here.

Grampa Steve
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Re: Plywood frames?

Post by Grampa Steve » Mon Jan 03, 2011 3:57 pm

Wow,
Great info guys. Thanks.
Billy, your beautiful Belle Isle 23 got me hooked.
I'm gettin' my "go for it" hat on.

Grampa Steve

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