Need Help with Wood-Strip Composite Question

Discussion in 'Boatbuilding' started by northboundtrain, Mar 29, 2016.

  1. northboundtrain
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    northboundtrain Junior Member

    Hello,

    I’ve been a longtime reader of this forum, but this may be my first post. My question is how a wooden strip-plank hull with both external and internal biaxial E-glass with epoxy resin sheathing behaves as a composite layup. How well do the different materials—with different moduli of elasticity—work together?

    According to Gerr’s Elements of Boat Strength, a boat with a scantling number of 2.5 would require a strip-plank core 7/8” thick using a wood species like Douglas Fir with and density of 32 lb/cu ft and three layers of 17 oz biaxial (approx 0.09” thick) on the outside and two layers (approx 0.06”) on the inside. For the sake of comparison, a balsa-cored hull with the same scantling number would require a ¾” thick core and a mat-roving layup thicknesses of 0.14” on the outside and 0.10” on the inside. The difference between the two systems, as Gerr notes, is that “Unlike the very low-strength (low-modulus) cores used in conventional cored-FRP construction, the wood strip-plank core itself provides substantial longitudinal strength, as well as local flexural strength and impact resistance. Furthermore, the wood-strip core has vastly greater sheer strength . . . [Thus] the interior and exterior laminates are thinner for wood-strip-cored construction . . ."

    Clearly then the “wood-strip-core” as Gerr calls it is more than just a core separating two skins as in FRP cored construction. It’s working in conjunction with the laminates and doing a similar job. My understanding is that the strip planking and sheathing should thus have a similar elasticity moduli so that they work together as opposed to one at a time, as it were. And this is where I’m having difficulty finding good information. According to Wikipedia the Young’s modulus in GPa of E-glass is 81, while for Douglas Fir it’s 13.

    I’m also not aware of any real-world testing done with the strip-plank-biaxial-sheathing composite that could provide definitive answers.

    This question all relates to a choice of whether to build with the strip-biax method, which I’m quite drawn to, or the seemingly more common strip with diagonal veneers.

    Any insight into all this would be greatly appreciated. Thanks.
     
  2. gonzo
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    gonzo Senior Member

    If you are using Gerr's method, this is not a high performance boat, and his numbers work fine. Whatever you do, avoid mix and matching methods. His are based on experience and boats that have been successful for decades.
     
  3. upchurchmr
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    upchurchmr Senior Member

  4. Jamie Kennedy
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    Jamie Kennedy Senior Member

    The E = 13 GPa is not so bad in terms of stiffness for weight. Also, to be fair you have to compare the E-glass and resin combo, since the Douglas Fir is itself a Cellulose and Lignin, or Cellulose and Lignin and Epoxy combo. When you do this you will find them well matched in terms of stiffness for weight, with the core being lower density, and the skin being high density to effectively increase the area moment, like a virtual I-beam. The skin also needs somewhat higher %elongation, which it is with glass/epoxy. Douglas fir, or even Sitka Spruce, are not the lightest of cores, but they do some of their share of the work, whereas lighter cores do not. Overall it should end up somewhat heavier, somewhat thinner, somewhat lower in material cost, but perhaps more durable over the long term.
     
  5. northboundtrain
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    northboundtrain Junior Member

    The biaxial is 17 oz +45/-45, two separate layers of 8.5 oz uni-directional stitched together.

    The boat will be a 38+/- ft medium displacement cruising sailboat.
     
  6. northboundtrain
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    northboundtrain Junior Member

    I did see that the specific moduli—i.e., elasticity modulus divided by density—for E-glass and Douglas Fir are much closer together than their elasticity moduli. I'm not an engineer and I haven't quite wrapped my mind around the concept of specific modulus. Is it more relevant than elasticity modulus by itself?
     
  7. gonzo
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    gonzo Senior Member

  8. TANSL
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    TANSL Senior Member

    For me this sentence does not make any sense, I recognize my limitations. Could you develop this theory a little more in depth ?. Thank you.
     
  9. northboundtrain
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    northboundtrain Junior Member

    I looked at the Gougeon Brothers book again and saw the test panel chart done for relatively thin strip-composite laminations (p. 275). I remember now reading this a while ago. While it doesn't address thicker panels with larger strip dimensions and heavier biaxial sheathing, the strength figures are certainly telling. Adding fabric to both sides of the strips results in tremendous strength increases.

    I haven't seen any direct comparisons between strip-composite and strip-veneer, but I suppose going with established scantling rules as Gonzo suggests without over-analyzing or questioning proven methods is prudent enough.

    Thanks for the responses.
     
  10. PAR
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    PAR Yacht Designer/Builder

    Additionally to the establish Geer rules, you need to focus on a single build method. I can think of at least a dozen different strip plank build types, each with their own scantling requirements. Geer lists a few of them, so if using his rules, apply them only to the appropriate method.
     
  11. Steve W
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    Steve W Senior Member

    Par, the name is Gerr. You have been spelling it wrong for years on here. Nobody else seems to want to correct you so I guess I will.

    Steve.
     
  12. Steve W
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    Steve W Senior Member

    The first builder I remember using glued strip planking with the addition of structural glass in and out therefore making the strip planking a true core was Jim Young in New Zealand and there was an article written about it in Sea Spray magazine and he did present the results of testing he had done to compare it to a typical cold molded panel. The testing was typical backyard style where a Young 43 sailboat in the yard was used as an immovable object to jack against and the panels were supported at the corners and a hydraulic jack used between the keel and the panel with a scale or load cell used to measure the failure. It was probably forty years ago so I'm not sure I will be able to find the article but someone else may be able to.

    Steve.
     
  13. Mr Efficiency
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    Mr Efficiency Senior Member

    How do you get a continuous glass lay-up on the inside ?
     
  14. PAR
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    PAR Yacht Designer/Builder

    Oops, I knew it was Gerr, but my fingers seem to like two E's instead.
     

  15. northboundtrain
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    northboundtrain Junior Member

    You strip over temporary molds, sheath and finish the outside of the hull, then remove the molds and sheath the inside like laying up a fiberglass hull inside a female mold. Then install all the interior structure.

    Van de Stadt calls it the "strip-core" method and has an article you can buy that explains the process.
     
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