Wood-Epoxy Strip-Plank Construction

Discussion in 'Wooden Boat Building and Restoration' started by adt2, Feb 8, 2011.

  1. adt2
    Joined: Sep 2009
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    adt2 Senior Member

    I've been reading Dave Gerr's excellent The Elements of Boat Strength, and I'm curious what the general opinion around here is of his work and theories. Specifically, his wood scantling formulas, and more specifically, his thoughts on wood-epoxy strip-plank construction for large (50'-60') vessels.

    I built a big Excel spreadsheet with most of his scantling formulas, and based on some rough numbers (55' LOA, 18' Beam, 10' Depth (not draft)) it indicates a plank thickness of ~1-1/4" over frames spaced 7' (!) apart and sheathed on the exterior side only with something like 120 oz/sy fabric. Frames called for are roughly 3x9 material on the bottom, and 3x9 on the sides tapering to 3x4 near the sheer.

    Frames seem plenty hefty, but the spacing seems high, and the plank thickness seems low...but I'm a new guy and I have no idea whether they're reasonable or not. Just wondering what others' opinions of Gerr's work are. Thanks in advance.
     
  2. Landlubber
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    Landlubber Senior Member

    ...my opinion....Dave Gerr is one very clever and astute man, he has vast experience in what he does, and has many seriously clever co workers/friends that he can also discuss matters with, I am sure he would be a good guide for boatbuilding matters...along with Nigel Calder.
     
  3. gonzo
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    gonzo Senior Member

    His numbers are about the same as using Herreshoff's formula. The strip planked boats are similar in scantlings to a Downeaster.
     
  4. PAR
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    PAR Yacht Designer/Builder

    Actually, the Herreshoff rule will yield slightly lower figures. Had 'ol Capt. Nat been alive today, I suspect his rule would have been whittled down to 25% less then that of Geer's.

    I see the problem as definition. There are at least a dozen different types of strip plank. Dave lists 5 or 6 of them going on memory (I haven't read "Elements" in about 10 years). It's important to establish which strip system you'll employ, some require full framing systems and are treated much like carvel, while others don't require internal framing in a traditional sense at all.

    Clearly a 7' frame spacing isn't right, though 7' partitions could be, given your hull length and some of the strip plank types. For the most part, modern epoxy strip methods don't use "frames" but rather rely on bulkheads (this conversation is sounding familiar) spaced relatively equally along the length of the hull. In fact, if memory serves me again, Dave refers back to a GRP rule for transverse elements (ring frames, bulkheads, etc.) on these types of strip builds. This said a 55' hull would likely employ a minimum of 8 transverse stiffeners (bulkheads), which roughly places these "frames" on 7' centers. Naturally, there are "qualifiers" (there always are). Powerboats may need more in specific areas, sailing craft may need additional "framing" at masts, etc.

    It might be more helpful if you further define your project. The general dimensions suggest a hefty powerboat. Will your build method use the 'glass as part of the structure, or just as a protective sheathing? Are you employing a plank on frame rule to a modern epoxy strip build? Again it's not good to mix without a very good understanding, though I'm jumping to conclusions that I shouldn't, without further definition of your project and the build method.
     
  5. adt2
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    adt2 Senior Member

    This is now two separate threads in which you've given me roughly this answer. The designer I've hired has also questioned my 7' spacing number. When I ask questions about your (and his) answer, though, it's not because I don't believe you; it's because I don't understand how I misinterpreted what I thought was a pretty clear explanation of how to figure this information by Mr. Gerr.

    He lists three types of wood-epoxy hulls: strip-plank, sheet-plywood, and cold-molded. In discussing the strip-plank method, he says that you can use structural bulkheads in lieu of frames, sized according to the rules given for FRP bulkheads, but spaced according to frame spacing given elsewhere. He then gives formulas for determining the sheathing weight and the plank thickness, which is reduced because of the heavy exterior sheathing.

    He goes on to discuss other modifications to previous formulas for double-diagonal and other types of strip-planking, but I'm only interested in the 'classic' strip-planking method already covered. (PAR - see my summary of how I arrived at my numbers in the other thread; too much to re-post here.)

    As far as my own project, it is in the very early stages of design, and I had always planned on trying to use sheet-plywood. But Mr. Gerr's book has really piqued my interest in strip-planking, which I think will be easier to build alone (a long skinny plank is a lot easier to manage than a 70-lb sheet of plywood). She'll be roughly 55' x 18' x 5.5', displacement cruiser, liveaboard-type trawler for a retired couple. I've got 20 years until retirement, so there's plenty of time to take my time and make sure I get it right. I'll mostly be working alone, so planks vs. plywood sheets is attractive. The 'glass sheathing will be for both abrasion resistance and structural support.

    I'd always seen frames on ~3' spacing, so that's what I was prepared for...but Gerr has got me thinking about specifics now, and, as I said, if I've misunderstood his "rules," I'd like to know where I went wrong.
     
  6. PAR
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    PAR Yacht Designer/Builder

    I'm not an expert on Dave Geer's scantling rules, other then the clever approach he's taken with it, but these types of details are best left to the designer. Again, I can rattle off at least a dozen different strip plank methods, each will have different scantlings for the same design.

    Maybe it would be helpful if you describe "the classic strip planking method". Strip planking is the back yard builder's friend. It's an easy, less precise method that permits a pure novice to build a round bilge boat. The method is old, at least the traditional versions of strip planking are. These are usually treated just as carvel, over frames, floors stringers, the lot. The other methods are treated by their longitudinal stiffness or as a core in a sandwiched composite. The more modern methods require a lot more 'glass work, which isn't the most pleasant thing too. The more traditional methods require more wood working and pieces installed.
     
  7. adt2
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    adt2 Senior Member

    Per Mr. Gerr, "classic wood-epoxy strip-planking" is simply strip-planking with each successive plank epoxy-glued and edge-nailed to the one below it. The completed planking is then covered with layers of 'glass and epoxy, essentially forming a monocoque hull structure. He gives a whole litany of reasons why he prefers it.

    I like the idea of it because I'm working alone, and a long skinny plank is going to be a whole lot easier to manage single-handed than a big piece of plywood. Plus, strip-planking just seems more "shippy" than plywood. My designer is currently leaning toward 3 layers of 1/2" marine ply; I would just like to know the pros and cons of 1-1/2" strip-planking instead.
     
  8. CaptBill
    Joined: Jan 2010
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    CaptBill CaptBill

    'Strip planking' is not even what this really is other than talking of the method. This results in a one-piece hull. You could easily end up with a 'engineered work of art'. Good luck with it
     
  9. arnehult
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    arnehult New Member

    I have quite a bit of 0.63' x 5" x 84+" sliced hemlock available; suitable for strip/cold molding. Wood is clear (occasional defects), dry and unitized for shipping.
    Originally produced for door skins, so lengths are 84" usable after trim.
    Contact if interested?
     
  10. oldsailor7
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    oldsailor7 Senior Member

    I would really like to see a tutorial here on double or triple diagonal planking using strip veneers. :cool:
     
  11. PAR
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    PAR Yacht Designer/Builder

    Double planking is as the name suggests and is two layers of planking, one typically is fore and aft while the other is on a bias. Triple diagonal is similar, though the ornamentation of the planking can be one of several combinations, the most common is the first layer is on a bias, the next the opposite bias and the last fore and aft.

    With strip planking, it's a different animal. The strips are fore and aft, but the diagonal layers are veneers and on the bias to offer cross grain stiffness.
     
  12. JRD
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    JRD Senior Member

    Oldsailor, Gougeon Brothers wooden boat building book covers building with veneer and epoxy in a heap of detail.
     
  13. oldsailor7
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    oldsailor7 Senior Member

    Thanks JRD and PAR.
    I built a Crowther 38 Impala in double diagonal planking so I am familiar with the process. I really asked that question for the benefit of others, as not everybody can afford to buy the Gougeons book. :eek: A quick run down on this thread would help.
     

  14. nzboy
    Joined: Apr 2011
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    nzboy Senior Member

    Strip plank

    I think your dream is the same as mine .For someone building alone strip plank hybrid is the way to go Build George Buehler style upright with a large keel as a basis frames 1-1.2m apart 8 by 3 .2 by 2 strip plank with a layer of 12mm ply cold moulded and then glassed for professional look.Frames then form bulkheads and cabin divisions Have a look at desiel duck website of course design is in the eye of the beholder these 2 ideas are my preference at this point in time
     

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