Plywood: fasten to frames or stringers?

[troy2000]

Years ago I read a piece on building boats out of plywood. I don't remember who wrote it or where I found it, but I do remember the author insisting that plywood sides and bottoms on a boat should never be fastened directly to frames or bulkheads. According to him, the only proper method was to run stringers fore and aft, and fasten the plywood only to the stringers.

I wouldn't guarantee it, but it seems to me he was writing back around the early sixties. And I believe there were photographs of a fairly small motorboat under construction.

Does anyone agree...disagree...think it doesn't matter?

[gonzo]

There have been a few boats built like that. However, saying that it is the only method is rather fanatical.

[troy2000]

Originally Posted by gonzo

There have been a few boats built like that. However, saying that it is the only method is rather fanatical.

I wish I could remember who the author was. He was quite insistent about it. I think he believed fastening to a frame concentrated the stresses, or some such thing.

For some reason, I always thought I had read it in a Glen-L Marine how-to. But I just went and looked at their website, and that isn't what they say.

[boat fan]

Originally Posted by troy2000

I wish I could remember who the author was. He was quite insistent about it. I think he believed fastening to a frame concentrated the stresses, or some such thing.

For some reason, I always thought I had read it in a Glen-L Marine how-to. But I just went and looked at their website, and that isn't what they say.

They ( Glen - L ) say this ...

Q: Why shouldn't sheet plywood planking be fastened to transverse frames?
A: The properly designed plywood planked hull incorporates as much as possible the principles of monocoque or "stress-skin" construction. Stitch-and-glue boats that depend upon glue bonded junctions reinforced with fiberglass tape often show pure adherence to such principles. However, not everyone is willing to trust their fate only to glue bonds; many still favor the extra security that fastenings and some internal framework can provide. After all, such construction has been proven strong and durable for generations whereas stitch-and-glue is still the new kid on the block.
But even when transverse frames are part of the design, it's the plywood skin that takes the stresses which are ultimately transferred to glued and/or fastened joints at panel edges. And if such panels are fastened across the grain to frames, localized stresses that can lead to failure of the panel can be generated. How so?
Consider a plywood panel curved in place around a hull as being analogous to corrugated cardboard applied in similar fashion with the corrugations running lengthwise. We can pin or tack such a cardboard panel in place all around the edges to hold it in place. But what happens to the cardboard if we perforate it with a bunch of fastening holes crosswise? When bent in place, it would fold and break about the perforations.
The same thing can happen in a sheet of plywood. Placing a row of fastenings across the plywood panels grain can weaken a panel and create a similar condition, especially so when the panel is also under tension due to bending. Conditions are exacerbated on the higher-speed powerboat where panel loads are greatly magnified due to slamming loads while planing.
In short, while frames on the plywood boat may contact planking and be glued at such points, ordinarily such contact is not actually necessary and in fact, frames can usually be relieved from such contact without detriment. In reality, it's the longitudinal members doing most of the work and this is why planking panels are preferably fastened only to these and at the ends of panels and NOT to frames between.

There was a builder of high end Italian power boats that said he would never do it ...stress risers .
He also said not to notch stringers into frames for the same reason.

Don`t remember who he was either ...

[troy2000]

Then maybe it was a Glen-L publication I read it in, way back when. But if I remember right, the recommendation at the time was quite a bit stronger than "preferably."

I don't think I'll be building with plywood thin enough, or fastenings spaced closely enough, to worry about it.

[wardd]

build the skin then fit the frame to it

the basket approach appeals to me

[boat fan]

Troy ..these guys don`t like it either .....

Personally , I would build this way.

Nexus Marine Corporation

http://www.google.com.au/url?sa=t&so...rxztWeoCZt061w

The chine and all stringers have now been installed. All these members were precoated with epoxy resin before they were installed. The stringers have been glued and screwed to the outside of the frames. The stringers are not notched into the frames. This prevents "hard spots" in the planking. It's stronger this way. The aft cabin bulkhead and engine room bulkhead have to be watertight, so we have blocked between the stringers just on those bulkheads.


Here we are looking at the inside of the transom, portside. Again , with the bottom stringers passing over the aftmost frame and notching into the transom frame only.

[alan white]

Everything depends upon the design, the type of boat, size, etc.. Plywood is manufactured in several ways that vary in cross-grain stiffness. Imagine building a house floor using the plywood with the grain running parallel to the joists below. I've done this with plywood that had poor cross-grain strength and within a short time it sagged from normal loads.
A flattish hull shape with wide panels (such as a jon boat) could really use some cross-frames in contact with the plywood. On the other hand, a more curvacious hull like a nutshell pram is plenty strong enough with no cross-frames. It also matters, as said, if the plywood is stiff across the grain seen on the outside plys. It matters how robust the plywood relative to the boat's length. Nothing is as simple as to state all boats benefit from frames set in from the stringers.

[hoytedow]

One reason not to screw skin to frame is that quite often the frame is nothing more than plywood, which does not take a screw in the edge well.

[boat fan]

Nothing is as simple as to state all boats benefit from frames set in from the stringers.

Yes , quite true.
Stringer spacing has a lot to do with it also.( more stingers , thin skin ) ?
or opposite .

If you look at that power boat , the stringers are wider ( 2x ? ) and more closely spaced on the bottom.
Makes sense .


Troy ...your sharpie will have a relatively heavy bottom , no ?

As sharpies benefit by having a heavier bottom , ( 2 x layers ( or more ?) bottom planking ? ) I don`t believe there would be any issues here , regarding sufficient strength.

Water would collect easier in the lowest part of the " bilge " to mop up .....

I would just run the stringers over the frames.
Easier , faster build.
Better also for a trailer sharpie I think.

I noticed that they ( Nexus ) build their dory that way as well.
Should be good for you too.

What do you think ?

[jimm]

In boats I have designed, I am familiar with (30' to !30' range) longitudinal and athrotship frames comprise the structure below the waterline. bulkheads and soles constitute reinforcement to the hull above the waterline and are bonded wherever possible to the hull and deck to reinforce its strength. Ths includes seating structure and counter tops.

[alan white]

Dories are a good example of transverse framing in full contact with the hull. The bottom is very thick and there are no stringers. Not even a keelson. This is true whether of plywood or boards.
I believe very light structures do benefit from recessed transverse frames and many light stringers. When going very light, such as building a plywood skinned kayak, the web frames are usually recessed. This prevents hard spots and localized stresses. Hydrodynamically, very (1/8") thin plywood would act like a stiff fabric. Cross-framing ridges would not flex inward, whiile longintudinal dishing with recessed frames would have better flow characteristics.

[boat fan]

Dories are a good example of transverse framing in full contact with the hull. The bottom is very thick and there are no stringers.

Not on this one Alan :

No frame contact. Stringers on sides and bottom.

Like you say dories usually have trans. frames in contact with hull...no stringers ....

Unusual for a dory , but there it is.

I think the key here being planing dory.....

I would not build a sharpie without a keelson.



Custom wood 23' Planing Dory

[peter radclyffe]

it shows how weak plywood is

[boat fan]

Originally Posted by peter radclyffe

it shows how weak plywood is

Huh ?