Plywood: fasten to frames or stringers?

Discussion in 'Wooden Boat Building and Restoration' started by troy2000, Dec 8, 2009.

  1. peter radclyffe
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    peter radclyffe Senior Member

    this thread shows how weak ply is without longitudinal support, it is made in short lengths which is not so good for boats
    which require massive strength fore & aft
    locally ply is strong
    but
     
  2. boat fan
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    boat fan Senior Member

    Tell that to the guys that build aircraft out of it.

    Or the countless boat builders , both amateur and pro , that build quality boats with it every day.

    It`s not so " weak " when you cold mold , or laminate it either.

    Framing is needed when you build light.( with it )

    Using it as a core in composite construction makes the point moot.

    It certainly has it`s uses in boats.

    It has also put more amateur home builders on the water than any other material.

    Can`t be that bad I think.
     
  3. peter radclyffe
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    peter radclyffe Senior Member

    im not talking about laminating or cold molding,which is very strong
    perhaps the terminology is different where you are but here plywood is a sheet that comes in 8 x 4 FT, or 3 x 1.6 mt,
     
  4. boat fan
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    boat fan Senior Member

    So where are you going with this ?

    If you don`t like all the framing ....laminate.

    Or better still S & G ...still " weak " is it ?

    Add biax cloth ...whatever ....

    Cold molded plywood

    Lapstrake plywood

    Laminated plywood

    Plank on frame plywood

    S&G plywood

    Or a combination of all or any ....

    ...it`s still plywood , regardless.
     
  5. capt vimes
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    capt vimes Senior Member

    hmmm...
    it looks to me that virtually everything is possible with plywood in a way...

    it has been mentioned that stringers should not be notched in the frames...
    neither should the sheets be fastened/glued to the frames...
    screws shouldn't go in the edge of ply...

    everthing of these 'no-gos' have been done here:
    http://www.dixdesign.com/radply.htm

    what to think about that know... i'm confused... ;)
     
  6. Steve W
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    Steve W Senior Member

    Ok, ive just read the Glen L piece and its nonsense,while i have not built a large plywood boat i have built dozens of large cold molded boats over transverse frames and longitudinal stringers as well as small plywood boats.On the cold molded boats the stringers are not let into the frames,at least not until the frame contacts the skin so the skin is not fastened to the frame But i would argue that as an example if you have stringers on,say,6" spacings and you fasten into the stringers in a transverse line you are setting up the same stress riser situation as fastening into a frame,you would need to dilligently fasten in a staggered pattern to minimise this so they are 12" apart instead of 6". Many boats have the stringers notched all the way into the frames,pretty much all the early multihulls such as Cross and Horstman for example use deep plywood frames with the stringers let in all the way,it works,they probably dont fasten to the frame because the are thin but it is still a hard spot.Any time you put a fastener,particularly a screw thru the outer veneer which is under tension you weaken it somewhat but you dont have a choice,in practice it causes no problem as long as you dont bury it too deep.Also,he states that letting the stringers in is weaker,in what way?yes its going to weaken the frame but in most boats where they do it that way it is allowed for with frames that have more molded dimension and less sided dimension,ie,plywood frames.
    Steve.
     
  7. troy2000
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    troy2000 Senior Member

    For the sake of argument, let's say that if you're using the same amount of fasteners spread out over stringers that you would otherwise use at the frames, they'll be spread a lot farther apart. You probably won't be spacing them every six inches on each and every stringer.
     
  8. PAR
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    PAR Yacht Designer/Builder

    You have to talk apples to apples folks. The various engineering principles and application methods described above are all over the place. You can't compare one against the other evenly or with broad strokes of your mouths. Comparing thin skin closely spaced frame structures with thick skin monocoque is just crazy.

    The written pieces mentioned where penned well before the current composite and homogenous structures were developed. With the application of this new work, the previous statements of well intentioned authors are frankly moot, particularly in light of the nearly half a century of trials the modern methods have enjoyed since.

    But, if you want to buy into the comments of written works pushing 50 and 60 years old now, then go for it. In this same vain, you'd prefer to use cat gut for stitching wounds, no organ transplants, computers where the size of small utility sheds and there were only 3 TV channels, because these things were also all the rage back in 1960. I guess it's a good thing, most of the rest of us permitted some advancement of technology, engineering and concept in the last half a century.
     
  9. troy2000
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    troy2000 Senior Member

    Since I'm planning to build a type of boat that was developed well over a century ago (sharpie), using methods that were developed well over half a century ago (sheet plywood over frames or bulkheads), I'd say what was written fifty or sixty years ago may still be relevant to my particular project....:p
     
  10. boat fan
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    boat fan Senior Member



    I`m surprised by your comments here PAR.
    Reading them I get the feeling that you think running stringers without notching them into frames is somehow inferior,and at the very least , outdated.

    That`s not necessarily so.

    As far as " the method " being 50 or 60 years old , well , so is notching .Even older I`d say , with confidence.

    I don`t believe that anyone here was PAR.This is all relatively light weight construction we are talking about here.
    No mention of frame spacing per se either.

    Thick / thin ...skin ....as far as comparing them being " crazy " ...maybe not as much as anyone would think.
    In fact , they could be effectively applied on the one boat , and often are .
    Could very well be useful , particularly on a sharpie , as it can benefit from both .Thick bottom panel and lighter topsides.

    The only thing " crazy " is a blanket statement like " it shows how weak plywood is " ( post # 14 ) .....now that is " crazy ".
    And meaningless.

    In the end it really does not matter as much as what people claim.
    Either way can produce a good boat.There is nothing inferior about building this way that I can see.
     
  11. PAR
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    PAR Yacht Designer/Builder

    Without getting into engineering specifics, you still have to compare apples to apples.

    Notching, not notching who cares, if you apply the applicable engineering principles to the tasks. Relatively thin skin structures will have tight frame structure spacing or will be forced to consider monocoque techniques. Relatively thick skin structures (the bulk of plywood builds before taped seam became popular) will have far fewer frame landing locations in reference to the skin and also far fewer issues with stress risers as a result too (imagine that).

    You have to understand the differences before you can debate their respective merits or disadvantages. What I'm hearing in this thread is a mixing of methods and techniques, which just isn't reasonable.

    In other words, you can notch whatever you want without worry, if you understand and engineer the structure. The opposite is also true about not notching load bearers with longitudinals. Why in the hell any designer would want longitudinal stringers with load bearing planking, is beyond me, but that doesn't mean there aren't those that think it's necessary. It may not be sound engineering, but they think it's necessary and actually write articles about it.

    [​IMG]

    This is classic example of close spacing and a light skin will go over it. I guess they just want to keep their crews working longer, because a more "involved" skin would have decreased the need for all these pieces, lowering labor and material costs and likely lightening the boat too.

    This mentality is just like the ideas behind batten seam, which worked fine until engineers trusted plywood and modern adhesives enough to toss out the damn battens and scarf the panels over filleted stiffeners.

    I'm not saying the old techniques are terrible (most of them are), but I am saying you don't have to live with their flaws, excess weight and extra labor to produce, just by applying a little earned engineering knowledge.

    An example is glued lapstrake compared to traditional lapstrake. There's no contest between them. In every way you measure, the glued version is superior, except for one thing. It doesn't have dozens of bent ribs inside to harbor leaves, old rusting fish hooks and rot. I have to admit, I enjoy the look of the ribs, but I don't like keeping the gaps under them clean. Unless I was building a replica, I can see no good reason to not favor a glued lap build over a traditional lap build. This is just one example of modern engineering over the old stuff. You'll find in most cases, when it comes to structure, we've got the old timers whipped pretty good.

    If old Capt. Nat was alive today, he'd be all over epoxy and probably using polysulfide between the layers of a double planked hull. He was quick to see the benefits of most things as they presented themselves. What he would have done for a truly waterproof glue. We can actually build "composite" boats, from dissimilar metals, coat and protect them, with reasonable assurance they'll survive. He couldn't, but surely would have loved to, because he knew how much weight, money and labor he could save.
     

  12. boat fan
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    boat fan Senior Member

    That was the original " gist " of the thread , I thought.( correct me here if wrong )

    Why does the planking have to be " load bearing " ? It keeps the water out....

    Floors in houses have been constructed like this for centuries , and still are.

    Planking = floor boards ( or panel flooring )
    Longitudinal stringers = floor joists
    Frames = bearers

    Seems like perfectly good engineering to me ....and VERY appropriate to a sharpie , no less.
    Make the planking as thick or thin as you like. Adjust your spacing and size of longitudinals , and frames accordingly.


    So how is that any different to "longitudinal notched stringers with load bearing planking....? Except ...Less work?

    I`m always amazed how these threads drift off the original context......I thought these ( to notch or not ) were the "original apples" ?
     
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