Missing stringer fix?

Discussion in 'Boat Design' started by LTDboatdesign, Nov 5, 2012.

  1. LTDboatdesign
    Joined: Oct 2010
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    Location: Bainbridge Island WA

    LTDboatdesign Junior Member

    I have recently been looking at a boat that a vague friend of mine has had enough of. Classic, started building it and now doesn't want to finish it. Anyway I am thinking of taking over the project but am concerned that the scantlings are insufficient. It is a 27' monohull sailboat. Single chine construction, 3/4 in. ply bottom with hefty frames every 22 inches. The frames seem fine, but if I stand on the ply in between the frames I can feel it flex just a little bit.

    Should I be worried?

    Also could i possible just epoxy stiffeners in between the frames? Not quite the same as a stringer but would that work?

    Sorry pretty vauge, I think the boat was designed by George Buehler, but it has been modified to some extent.

    Thanks for any input.
     
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  2. PAR
    Joined: Nov 2003
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    Location: Eustis, FL

    PAR Yacht Designer/Builder

    We can assume this is a Hagar, George's first design (I think). The planking scantling is 3/4" as a minimum, with likely two layers of 3/8" being necessary in some locations. Frames and floors are on 24" centers, with intermediates on 12" above the chine. He also shows 2 bilge stringers per side, evenly spaced from the keel of 1.5" x 2.5".

    Are the bilge stringers missing? If so, you can scab them in, but I wouldn't epoxy them unless you laminated them up from 3/4" stock. Personally, I would just place the 1x3's on edge and glue and screw them between the frame bays and skip the bother of laminated on the flat deal. Buehler designs are ridiculously over built, you probably don't even need the stringers, though the weight of their inclusion isn't going to affect anything: it'll still be slow as sin.
     
  3. bpw
    Joined: May 2012
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    Location: Cruising

    bpw Senior Member

    Plenty of plywood boats with hulls of similar or less thickness than that with no framing other than the bulkheads and interior, sounds pretty stout. Surprised you can feel the flex, my bridge-deck is 5/8 ply spanning a bit more distance than that and it is solid as a rock.

    Did they use marine ply? I know lots of older books say AC exterior is fine, but exterior ply has gotten a lot worse in the last 20 years.

    Remember the hull is only a small portion of the work, so probably not worth taking on unless the hull is very close to what you want.
     
  4. PAR
    Joined: Nov 2003
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    PAR Yacht Designer/Builder

    Bpw, it's helpful if you understand the way Buehler designs his boats.
     
  5. LTDboatdesign
    Joined: Oct 2010
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    Location: Bainbridge Island WA

    LTDboatdesign Junior Member

    Thanks for the input guys. about the flex, it isn't very much, but if i stand on one foot in the bay it will flex maybe an 1/8". I just didn't know if such flexing was worth worrying about or if it is pretty standard.

    Just curious PAR, why not use epoxy to bond in the stringers? I only ask because the outside of the hull has already been fiberglassed and i would rather not put a bunch of screw holes in it.

    Thanks again
     
  6. bpw
    Joined: May 2012
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    bpw Senior Member

    Can't say I am an expert on his designs, but have looked at some of his construction drawings, your description of "ridiculously over built" seems pretty spot on.

    I am just surprised he is seeing that much flex in 3/4 plywood over a 22 inch span, more than I would expect with good quality ply.
     

  7. PAR
    Joined: Nov 2003
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    Location: Eustis, FL

    PAR Yacht Designer/Builder

    I'll bet the planking is APA Exterior grade stock, which will be relatively flimsy, compared to a BS-6566 or BS-1088 sheet. An 1/8" of flex isn't going to cause an issue I don't suspect, but the stringers do have another function, besides panel support.

    If it was me, I'd just piece in vertical stringers, between the frame bays, gluing and screwing (unfortunately) as you go. Filling the exterior screw holes will just be a tedious pain in the butt and will not compromise the sheathing (assuming it's done properly). Sheathing on a plywood hull skin, is just for abrasion protection, unless it's quite thick. Assuming it's just a light sheathing, counter sink the screw heads and fill flush with thickened epoxy when you're done. If the hull is painted, you'll need to do some touch up, but an afternoon with a paint brush isn't too costly.

    The reason I'd install the scabbed in stringers on their edge, is to compensate for their lack of continuity. Originally, these would have been over the frames or notched into them, forming a continuous stringer system. Frame depths should be 3.5", so a 1x4 on edge, glued and screwed will provide sufficient stiffness, without having to go insane trying to fit full length stringers. The vertical orientation of the pieced in stringers, will help "bridge" the frames and support the planking between them, much like the way bridging is used on floor joists in a house. The bridging keeps them from twisting under load. In this case, they also offer planking support between the frame bays. Lastly, provide limber holes so moisture can drain to the low point and get pumped out. A 3/4" hole is a minimum.
     
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