Fastening Plywood Sheathing

Discussion in 'Wooden Boat Building and Restoration' started by SamSam, Feb 21, 2006.

  1. SamSam
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    SamSam Senior Member

    Is this advice outdated? It comes from a second edition, 1978 book. Sam

    FASTENING PLANKING TO FRAMES
    In the SHEET plywood planked boat, it must be emphasized that fastening of the plywood planking to the athwart- ship frames, either on the side or the bottom, is NOT recommended. This may come as a surprise to many, but after the following explanation, the reasoning should become clear. Note that the statement above refers specifically to sheet” plywood, and not to the other methods of plywood planking that are covered later.
    Perhaps an analogy will describe the situation that exists with the flat plywood planking panel. Imagine that the sheet of plywood is a sheet of common corrugated cardboard with the corrugations running longitudinally. If we took this sheet of cardboard and laid it around the framework much like would be done with the planking, it would no doubt conform to the frame members just like the plywood planking would do. In fact, we could theoretically “fasten” this cardboard to all the longitudinals without any problem.
    But what would happen to our cardboard if we drilled a series of holes for fastenings across the sheet thereby perforating the cardboard at each of the frames? Obviously, the cardboard would crisply fold and even possibly fracture right across this per-
    foration. Now imagine doing the same thing to a plywood panel put under a lot of stress by laying it over our hull. While the plywood may not actually “fold” or even break like the cardboard, nevertheless, a localized weak point would be set up by the screw holes placed across the panel at each of the frames. It may be that such a panel would hold. But it cannot be denied that the panel has been weakened, and if such a panel were overstressed just once (and it need not be necessarily at that exact point), the panel could fail in use.
    Of course, this situation is worse where the bending is most severe. But even in the flat areas of a hull, the practice of fastening to the frames should be avoided. On the properly designed plywood boat, there are enough longitudinal members in the structure to receive fastenings, making the few that could be driven into the frames simply not necessary. While the frames, and especially the side frames in many boats, are used to give contour or shape to the planking, the planking should still not be fastened to these frames. In fact, in many cases, it is not even desirable for the frames to actually contact or mate to the inside surface of the planking. In many cases, such a contact will protrude severely against the planking, thereby forming a sort of stress concentration, or "hard spot” as it is called, and could eventually cause the panel to fail. A possibility of a void in the plywood panel at this point could cause a fracture at the first time the stress is too great. In short, fasten the planking ONLY to the longitudinals (and at the ends of panels of course), but NOT to the athwartships frames. Relieve any frames during fairing which protrude unfairly against the planking.
     
  2. DanishBagger
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    DanishBagger Never Again

    Sounds like a sound advice. Thank you.

    I have a question, though, wouldn't the structure of the bulkheads etc make up for the weakness? I mean, it seems true when the sheet is not connected to anything, but since it is alredy bent on, wouldn't that make up for it?

    Andre
     
  3. nero
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    nero Senior Member

    Good reason to bond it with epoxy and fillets without screws.
     
  4. lewisboats
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    lewisboats Obsessed Member


    That's what I would say too, if it needed to be bonded.

    Steve
     
  5. SamSam
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    SamSam Senior Member

    I don't know much about wooden boats. Are screws not used much anymore? In the pictures in the book screws seem to be about every 2" on the edges. This is from the Glen-L book "Boat Building With Plywood" by Glen L. Witt and Ken Hankinson. I'm thinking one of the reasons is because the ply might not bend evenly or "fairly" over the frames if it is weakened by numerous holes. The other reason I'm thinking is that say you have a fairly thin skin on the hull and you run over something smooth that doesn't rip or gouge the wood but pushes /flexes it up some. If there were no athwartship frames it would just do that until you were past. (I'm conviently ignoring the transom) If there were frames, it would seem the pressure on the ply would skyrocket in the last few inches as the boat was ramped up and over the underwater object by the solid hard spot of the frame. Screws or not, the pressure would be concentrated, if weakened by holes, maybe problematic. ??? Sam
     
  6. PAR
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    PAR Yacht Designer/Builder

    Ply isn't noticeably weakened by fastener holes if proper spacing is enforced. Clearly you've limited engineering experience with plywood structures. There are several types of plywood construction methods, most boil down into a few of categories, "over frame", "composite" or "taped seam". These three engineering concepts have stood the tests of time and are well established in the industry, though they rely on quite different principles to function properly, they all incorporate plywood's unique properties into the structure matrix (some more so then the others)

    Engineers have developed safe bending limits (diameters) for given thicknesses and types of ply, bending beyond these limits (often done) can lead to some of the issues SamSam has pointed out. Most reasonable designers do not exceed these limits often, without substantial reinforcement in the highly stressed areas.

    Where or how did you arrive at these conclusions SamSam, particularly if you "don't know much about wooden boats"?
     
  7. DanishBagger
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    DanishBagger Never Again

    He read it in a book from 1978. It's a quote.
     
  8. SamSam
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    SamSam Senior Member

    Post #1 was a cut and paste from the book. Post #5 are guesses as to why the authors were so adamant about not fastening the ply to the frame and also an attempt to answer DB's question in post #2. (I didn't mean that there were NO frames, just that they were not touching the ply sheathing.) Anyway, is the advice from the book outdated? Is there any sense in following this advise with todays construction methods? Did this advise EVER make sense? Sam
     
  9. safewalrus
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    safewalrus Ancient Marriner

    Hey Sam that seems about right, now that everybody has one of these magic battery driven scre drivers and can put more nails in properly in a day than a whole army of chippies could put in in a week using the old method! (you still cant beat a good hammer!)
     
  10. PAR
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    PAR Yacht Designer/Builder

    My bad (I'll have the other half spank me latter), I miss understood the post. Sorry SamSam.

    When Ken Hankinson and the Glenn-L folks wrote the book, it possible the concepts were reasonably untried, though I can't imagine why not. Stressed skin structures over frames, ribs, bulkheads, etc. had been done for generations pervious to the printing in several industries including boatbuilding.

    The analogy of the corrugated cardboard was a poor one, but I understand the concerns as it does relate to other materials used in boat construction. It's this very issue that has bulkhead construction in 'glass boats landing on a foam block, rather then tabbed directly to the hull shell. This wasn't always the case and hard spots appeared in 'glass construction, before it's implementation.

    In traditional plywood over frame construction you have to screw/nail the structure, including the plywood planking to the frames, if only to hold while the goo sets up (though the goo isn't necessary most of the time) In tape and seam construction, no it's not necessary to have any fasteners (my designs do) the same is true in composite construction, as the adhesives are the fastening method. Personally I like the idea of bits of metal between me and the part falling off, even if it's a touch of over kill from a technical stand point.

    I rarely use nails larger then a brad, preferring screws and bolts. A big old machine bolt, nut and fender washer pattern perforating a gusset/frame combo at the deck line, while making a hard slosh to windward just gives me a warm fuzzy feeling. I know the glue is stronger then the wood, but the 1/2" diameter bolts on 5" centers helps calm my nerves sometimes. I guess I'm getting old, I haven't done a back flip from the spreaders in a decade either.
     
  11. hartley
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    hartley Junior Member

    re post from samsam ,here again we have sarcastic answers from so called experts ,rabbiting on about "plywoods unique properties into the structure matrix" and "engineering concepts" give us all a break will you .I thought the question was a simple one ....on a plywood boat do you fasten the sheet plywood to the frames or not? the answer iis you never repeat never fasten sheet plywwod to the frames only repeat only to the stringers or longitunals that is of course if you want a fair hull...cheers hartley
     
  12. PAR
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    PAR Yacht Designer/Builder

    A glued joint will create a hard spot, just like a mechanically fastened joint, be that a plywood panel to frame, stringer or what ever.

    I hung two 23' scarfed together plywood planks on a boat today. Each of the 36 frames received fasteners, as did the edges of the planks (#6 & #10 screws on 2 1/4" centers) I guess I should hang up my screw gun after installing over 350 screws per plank?
     
  13. Oyster
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    Oyster Senior Member

    Interesting
     
  14. hartley
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    hartley Junior Member

    well mr par you have me wondering ,are we speaking the same language ?perhaps you call apples oranges over there.I am not talking about plank on frame construction as has been practiced for decades,I am talking about plywood on frames and stringers.the frames I am talking about in say a 30 footer would be sawn frames or laminated in the dimensions of 3x2 inches mimimum,stringer sizing 2x1 and a half inches and plenty of them .the frame spacing would be 2'6 to 3 feet,yet you are talking about 36 frames ,must be one hell of a boat .perhaps you are meaning "ribs" you know those puny steam bent things .in my opinion this method of construction(plywood over ribs) is a very ***** way of doing things suitable only for small dinghy's .....just my 2 cents worth cheers
     

  15. nero
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    nero Senior Member

    Par,

    On strip planked hulls, will the tabbed plywood hulls create hardspots? If so, once they are faired out are they prone to reappear later on? And would making a wood flange or adding 45/45 biaxel glass between the hull/bulkhead joint reduce this tendancy?

    The boat building books I have do not address this subject for strip planking.

    Thanks
     
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