Plywood Frames?

Designs for inboard or outboard power

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beermkr
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Plywood Frames?

Post by beermkr »

I have been doing a lot of looking and wondered about building the frames out of plywood instead of solid stock. Seems that if I buy a 4x8 sheet of 3/4 marine plywood for $100 and rip it into 8 6"x8' boards then it will be much cheaper @ $12.50 per board than buying solid stock.

Additionaly I would think that the Ply would (get it) be more dimensionally stable in the long run.

Any thoughts?

Mike
Mike Pensinger
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rasorinc
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plywood for frames

Post by rasorinc »

Glen-L does not recomend plywood for frames. Plywood is quite flexable
which is not a characteristic you want in frames or stringers. Also it usually does not hold screws as well as solid wood.
Wood lasts generations.

Matthew Edmondson
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Post by Matthew Edmondson »

NO ! NO! NO! USE SOLID!
I cut it, and I cut it, and it was still too short!

beermkr
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Post by beermkr »

Okay, no plywood frames it is.

Mike
Mike Pensinger
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basilkies
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Post by basilkies »

I'm not saying plywood frames are a good idea, but I haven't seen anything that supports the theory that they are bad for frames.

Let's start with the screw holding ability of plywood. It's been my understanding that screws are mainly for holding the pieces together till the glue dries. This makes sense to me. Also, I recall reading that it's not good to tighten clamps to much because you squeeze the glue out, so screws into plywood might be better. If that gets to be an issue I bet you could screw the pieces in to measure and cut then unscrew and lay down the glue and use ring nails that are slightly bigger than the holes to finish the job.

Getting back to screws holding, I got a book on the Tolman Skiff. The designer is a carpenter turned boat builder in Alaska who has been building boats for over 20 years. He uses sheet rock screws and those plywood beams (I can't think of what they're called but they have ply wood with a wider board glued along the narrow side top and bottom) for longitudinal stringers and he even cut them in half the long way because one side glues to the bottom and gets it reenforcement from that. So, if you did the same thing with the stringers using screws as tempary fasteners then used the stitch and glue method of attaching the bottom you'd be good to go. I bet you'd use enough epoxy to end up spending as much money and spend more time doing it, though.


As for the idea that plywood frames would be less flexible than solid wood, I wonder. I bet if you laid up plywood 1 1/2 inches thick it would be pretty stiff. If this really is an issue you could stiffen the stringers considerably by running a thin piece of plywood on the inside of the boat. A lot of boats are built that way anyway.

It would be fun to get together with a few guys and play around with some plywood and see what could be cooked up!

John K
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Post by John K »

If you do a search on plywood frames, you will find a ton of information about why not do them.

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AAARGH
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Post by AAARGH »

Dudley Dix uses plywood Frames in his designs and Glen-L uses plywood for the Stem, Frame gusset doublers and on some designs as bulkheads. Dont forget the entire boat is planked in plywood.

Plywood does have its place. I think if you do check you will find Pros and Cons for Plywood Frames you would find builders for both applications.

Paul

beermkr
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Post by beermkr »

That is kind of what I am thinking. I think most people would be hard pressed to bend a piece of 3/4 marine ply any more than they could bend the solid stock.

I realize that thin ply bends and that there is ply specifically made for bending by putting all the grain directions in the same orientation.

I think I am going to go ahead and do it. I think that once the boat is glues and fastened together then there should be no issues with strenght and rigidity.

Mike
Mike Pensinger
Radford, VA
Chief Brewer - The River Company Restaurant and Brewery
Beer, Life and Other Semi-Interesting Things - http://www.pensinger.net

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Rich Coey
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Post by Rich Coey »

Mike,

My advise would be not to use plywood for the frames on these designs. The problem is not once everything is glued and fastened, it's getting to that point. I built the Monaco and can tell you bending in the chine, shear, and even the battens towards the bow puts ALOT of pressure on the frames and after the frames are notched for the battens there is only about 2 inches of width left at those points. As an example take a piece of 3/4" x 2" ply you can easily break that over your knee, now try it with the same size using white oak. Even if you made the frames wider to gain strength there are other problem. The holding power of screws into the edge of plywood is not great and most of the fastening goes into the edge of the frames. Also with plywood the epoxy will tend to soak into the edge and that could leave you with a weak, starved joint.

Rich

DonBing
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Post by DonBing »

One of the cons I hear about is the inherent weakness of a screw holding in what is basically endgrain. If you put a dowel through the frame it will hold as good as any board. Well worth the extra effort IMO.

Don

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Bill Edmundson
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Post by Bill Edmundson »

:roll: Well :shock: I'm prone to sarcasm. But, if you don't want a traditional boat... don't build one!

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

upspirate

Post by upspirate »

Build it that way if you want....just let us know how it works out.

I'm inclined to believe that for a boat designed to use solid frames,I'd use solid frames.

Like Rich said,as you are building it,there are lots of stresses on the frames in lots of directions and I would be inclined to think ply frames wouldn't hold up well at this point.

As for doweling,let us know how that works too,but I think that the bigger hole to dowel would weaken the member till the glue dries & there is lots of problems breaking members(chine,shears etc) now & the directions say not to drill certain places til after the planking is on.

I can see that a plywood frame added & filleted later to a hull would be strong as in stitch and glue construction,but for conventional building,I just don't know.

Use it,but let us know

DonBing
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Post by DonBing »

When I was 12, me and a couple friends tried to build a hover craft out of plywood and a lawnmower engine. I think the plans came out of Popular Mechanics. It never hovered but that never stopped us from trying other dumb ideas; you win some, you lose some.

I'm all for the plywood frame idea but beermkr you hafta know this: If your intent is premised on economics, it's likely a false economy. I can understand 12.50/ bd ft is expensive. But you have a lot of suitable hardwood in your neighborhood (VA, TN, NC) that's probably more in the $7-8. And I think almost everybody on this forum will agree with me that the cost diff in wood will be a minor diff in the total cost of your boat.

But I also think almost everybody will help with advice. After all, some day we may want to make beer :lol: Good luck with whichever way you go.

Don

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AAARGH
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Post by AAARGH »

Over the years I attained many complete sets of plans: Hercules, Wild cat, CS 20, Bonanza, Rivera, Costal cruiser, A-Lure, Zip, Squirt and Eight Ball.
Someone said the fastening of screws won’t hold in the frames. Coastal Cruiser plans say you Do Not fasten the planking into the frames; fasteners only go into the Battens, sheer etc. Basally members running length wise. In fact it says the frames should not touch the planking.
Here’s a quote from the Coastal Cruiser instructions: For best performance, lightweight, strong wood such as Sitka Spruce, vertical grain Douglas fir and mahogany (either Philippine, Honduras, or African types ) are preferable. This same theme is in most of the other plans I have
Ok, my 2 cents; I think you can use ply for the frames but, should you use ply for the frames. If they are sealed especially on the edges and protected from splitting I think you could. If cost is an issue use Doug fir. The big thing to consider is cost of the wood is minimal to the overall cost of the entire boat. You might want to consider using solid frames (mahogany etc.)and build them one at a time and average the cost over time. This way you have good quality frames. Bottom line I think you could use ply frames but strongly feel you might regret it when the project is complete.

Paul

jb4lcm
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Plywood frames

Post by jb4lcm »

I would like to suggest that plywood frames would NOT be a good idea - many have already made some good reasons, but you may want to try a couple of tests first.

If you feel cutting a 4x8 sheet into 6" wide strips will be the same as solid wood, then take a piece of ply and cut it 6" wide and then see just how strong it is. Bend it a bit and you'll see that the solid wood has far more longitudinal strength - resisting the bend.

As for fasteners, you can really only put a fastener in plywood thru the face - NEVER thru the edge. Solid wood can be fastened from all 4 sides - not just the 2 face sides. So it is rather limited.

In my humble opinion, scrimping on materials in the beginning will only cause you concern later. Eliminate the concerns.

JB
My only fear in life, is after I die my wife will sell all my fishing gear for what I told her I paid for it!

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