Substitute for Clench Nailing?

Discussion in 'Wooden Boat Building and Restoration' started by adt2, Mar 22, 2012.

  1. adt2
    Joined: Sep 2009
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    Location: Magnolia, Texas

    adt2 Senior Member

    I'm getting ready to start a small skiff with my son, and the plans call for a bunch of copper clenched nails where the planks overlap.

    Disregarding the obvious "soiled tradition" reasoning, is there any reason I can't substitute something easier/more efficient? Any suggestions for replacing 1-7/8" copper clench nails with something else?

    EDIT: I ordered #6 x 1" stainless steel screws to substitute for the clench nails since I couldn't think of a good reason not to. Will let you know if the boat sinks when I drop it in the water.
     
  2. PAR
    Joined: Nov 2003
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    PAR Yacht Designer/Builder

    Screws are not as strong as clench nails, (amazingly enough) on most small lapstrake craft. This is because the end that "bites" is quite small, once you count the tip threads, which don't grab anything.

    Lets take a 1/2" plywood lap. It's beveled, but for discussion, lets call this plank now .4" thick along the faying surface at the lap. The adjacent plank is also .4" thick along the faying surface, so married, not counting any bedding, the lap is .8" thick, right? This means a 3/4" screw or it'll punch through. About 1/5th of the inserted length of a 3/4" flathead wood screw is the tip, which is threaded, but doesn't really grip anything. It's job is to keep the screw centered in the pilot hole. This means of the .75" of grip range, you now have .6". Of this .6" grip range, .4" is in the other strake, leaving you with only .2" (less then a 1/4") of holding power in the other strake. Not much, maybe two threads on a #8 screw, which can't hold diddly.

    Simply put, all this means is screws are a lousy way to hold laps tight, in all but the lightest of lapstrakes. Screws can be used at frame or other structural element locations.

    Rivets (roves) are commonly employed, which are slightly stronger then clenches, but can't be tightened up like a clench, if the need rises. They're more costly and a two person installation job except on new construction. I prefer clenches because they're easier to install and remove when the time comes.

    Through bolts or more appropriately for small craft, machine screws are often used. This is my usual choice. I may still use screws in the frames, but through bolt the laps between the frames. A #4 or #6 machine screw (miniature bolt with a screw type head) will hold much longer and is considerably stronger then a rivet, clench or screw.

    The only time I've seen screws work, is on glued lapstrake builds and on very small pulling (rowing) boats. The screws were flathead of course and their sole purpose was to hold the laps together as the sealant or glue cured.

    What design are you building and how close is your frame spacing (if appropriate)?
     
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  3. gonzo
    Joined: Aug 2002
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    gonzo Senior Member

    I agree completely
     
  4. hoytedow
    Joined: Sep 2009
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    hoytedow Bananas

    Ditto.
     
  5. adt2
    Joined: Sep 2009
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    adt2 Senior Member

    A small (15') lapstrake skiff (the Macomber 15 from the last Small Boats issue). I'll be gluing the planks, so the screws will mainly be holding the planks together while the goop cures. I will probably fill the screw holes with epoxy before driving the screws.

    The frames are spaced about 18" O.C. and the planks are bolted to each frame. Screws are 3" apart, so there are five screws between bolted frames, per plank, plus epoxy or some other kind of glue.
     

  6. PAR
    Joined: Nov 2003
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    PAR Yacht Designer/Builder

    On a glued lap build, you don't need the lap fasteners, except to hold the joints closed as the glue cures. If building over a male jig, I wouldn't bother with the screws, just well fitted planks, some duct tape, maybe a few brads on unruly areas. I hate filling temporary fastener holes.
     
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