Flats boat hull/transom design calculations

Discussion in 'Boat Design' started by yetigrey0864, Jun 18, 2007.

  1. yetigrey0864
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    yetigrey0864 Junior Member

    I am planning to build a small flats boat 12'-14' using a sandwich hull to be powered by a tiller steer outboard. I am a ME, so I was hoping a NAME could point me in the right direction for a text that would aid in the basics of marine design (transverse and longitudinal sections and calculating their stresses, transom stresses and design, hull stresses and design) to help develop the necessary section strength calculations, as I am pretty clueless about hydro forces. I work at a NAME firm (working primarily with steel and large vessels) as the IT/ME intern, and have gotten many suggestions for advanced texts, but I am afraid I need to start at step one here. I plan to develop the design in Freeship and have access to AutoCAD/Shipconstructor/Maxsurf at work and hope to have a design prepared in about 4 mos. (I need to build in the fall when the humidity is low down here on the bayou) that I will gladly share once completed. I just need to develop some of the basics of the hull and transom structures before creating a set of drawings and beginning the process of the design analysis. I plan to use the Hewes Redfisher as a preliminary model, but will adapt the hull as necessary to benefit from using a 20HP or less till steer outboard. Also I was wondering if anyone could point me in the right direction to find the basic specs (density, Tensile strength, Elastic Modulus, Youngs Modulus) for the standard available fiberglass weaves and marine foams. I appreciate any feedback on the subject. Thanks.
     
  2. alan white
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    alan white Senior Member

    Common sense and a bit of "rule of thumb" might get you there too. What you might find useful would be layup schedules and basic formulae.
    I can recommend Dave Gerr's "The Nature of Boats" as a ready access to design formulae and scantlings. As far as layup schedules go, there are a couple of designers here who could assist.

    Alan
     
  3. fcfc
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    fcfc Senior Member

    I think you are in the wrong direction. Forces on small boats are so variable, that you do not compute them. Hydro forces and powering are one thing, but hard docking, beaching / grounding , jumping in the boat (or falling in) , overloading, trailering (specially loading/unloading from trailer) are realities of small boats.
    And you 'd better never know what your kids will do with your boat :p

    So you have to rely on "rule of thumbs" rules to tell what is acceptable strength for general usage.

    One place to find this is in ISO rules 12215 series. There is a specific annex in 12215-5 that deal with structure for small boats below 6m.
     
  4. tom28571
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    tom28571 Senior Member

    Dave Gerr's "Strength of Materials" covers scantlings for all kinds of boats and materials.

    That said, unless you just want to do the work, work from past practice in boatbuilding. Your boat is so simple that calculations are just guilding the lilly.

    fcfc is correct.
     
  5. alan white
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    alan white Senior Member

    Thanks, Tom---- yes, Strength of Materials.

    A.
     
  6. yetigrey0864
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    yetigrey0864 Junior Member

    canted bow frames?

    I was able to find the annex, which was indeed very helpful, and contained most of the information regarding the materials, so thanks very much for that. And, very true about the hydrodynamic forces, so analyzing the hull frame as a static system is probably more than enough for this type of application, so I imagine that using the hull frame strength plus .5 of the hull shell's tensile strength will provide an adequate safety factor for longitudinal and transverse stresses, and using the annex's information (excellent recommendation again) to calculate compression stress and shear normal to the hull should be a good starting point. In regards to the lily, I am not trying to guild it, just make sure that it doesn't crack somewhere along the way. I would like the boat to be light, as good planning could save thousands by reducing unnecessary weight and lowering the power reqs, so with that being said I probably will do a bit of guilding regardless. Which brings me to my next question (humor it a little) in regards to the bow frames. I was thinking of using cant bow frames in a radial (xz, top view) layout to add a little on the strength/weight side of things and don't mind the extra time involved in the design - but are there any significant drawbacks to doing this? Thanks again for the feedback.
     

  7. alan white
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    alan white Senior Member

    Stiffening that area (assumed meaning frames lying perpendicular to hull skin) is actually stronger. A single thickness FG hull will flex where flatter sections are located. If lightweight is desired, and that particular area would be prone to "oil-canning", frames as you describe would stiffen things up. A frame called a "hat case" section can be used to maintain light weight, and can be formed around plastic to reduce the chance of wood rot. Its strength depends not on any core material (often lightweight semi-flexible tube for curved sections), but on the fiberglass tape formed over it, which provides stiffness through sectional height. For instance, 3/4" pvc water pipe is "staged" by springing into place with dabs of epoxy top, middle, and bottom. When cured, tape is layered over the pipe in perhaps three layers 5", 4", and then 3" tape). Very stiff and strong, allowing a lighter hull thickness.

    Alan
     
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