What would be the scantlings for...

Discussion in 'Class Societies' started by TealTiger, Nov 26, 2009.

  1. TealTiger
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    TealTiger Junior Member

    Hello All,
    If you picture a blue water aluminum cruiser (motor/sail/multi) in the 40' - 60' range. Now we imagine the hull above the waterline to be comprised of perfectly flat, and vertical sections about 6' X 6' (please bear with me). There would only be transverse framing and longitudinal stringers supporting the span of the 6' panels. Bulkheads/soles/decks or other major structural elements would only be at the edges of the 6' panels. What scantlings would I need to support that 6' X 6' panel against the predominantly ‘secondary forces’ or the open ocean?
    Thank you very much for reading this far and considering my question.
     
  2. Tad
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    Tad Boat Designer

    Tiger....

    One can say nothing about scantlings unless we know the global loads on the boat....it's length, beam, depth, displacement, speed, and operating sea state, also we need to know the local pressure loads, specifically where the panel is and it's orientation in the overall vessel.

    40' to 60' is a huge range.........
     
  3. baeckmo
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    baeckmo Hydrodynamics

    If you look at how the various scantling rules are built up, you find that the skin thickness is often expressed as a function of material strength, LOA and speed (or pressure as f. of speed), while the strength of backup structure is a function of panel size, pressure and material strength.

    So, in your case you have primarily to settle on a hull size, material specification and speed in order to have the necessary input to calculate the dimensions. Note that it may not be possible to use the minimum allowable skin thickness for the hull size, if you stick to a panel size of 6´ x 6´!
     
  4. TealTiger
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    TealTiger Junior Member

    Thank you baeckmo & Tad

    I now see how disp. is important. It affects inertia, which affects how much force can be dissipated moving the hull away from the wave. Maybe lateral resistance is also important as it affects how firmly the ship is ‘stuck to the water’. I don’t see how length and beam matter though, at least not directly. Are the used to help approximate disp. or lat. res.?
    In addition, I see how speed is a factor; it multiples the force of an oncoming wave. Since I *am* thinking about a multi with a L:B of about 12:1 and that factor diminishes to zero directly abeam, maybe it is negligible in such cases.

    In any event, is it a naval engineer who could calculate this (given all the necessary data)? Where does one find the right rind of person for hire? Additionally, do either of you have any idea how big a job this is for someone with the right qualifications.

    Thank you again.
     
  5. TeddyDiver
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    TeddyDiver Gollywobbler

  6. TealTiger
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    TealTiger Junior Member

    Wow :)

    I had know idea this info would be available without a Naval Engineering degree. I'm sure I sound like a dummy but I have read a few books on boat design, including (all?) three of Marchaj's, a couple on construction, and even worked in an aluminum boatbuilding shop for a while. Just never came across anything specific on scantling.

    Thanks again.
     
  7. Tad
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    Tad Boat Designer

    For a powerboat......
    By far the largest part of the loads imposed on the structure will derive from the speed of the vessel through (and over) the water. Momentary impact (point) loads on the hull bottom are also dependent on sea state, deadrise angle, and longitudinal position along the hull. A 25 LWL boat traveling at 30 knots might see a bottom pressure load of 25 psi, the same boat at 60 knots might see 54 psi. That's 8000 lbs on a square foot of bottom.

    To design the structure you must be able to predict the highest speed the vessel will achieve. To predict the speed you must know the form, weight and power of the boat.

    There are two main elements in any hull structure, the skin which keeps the water out, and the supporting structure (stiffeners). The stiffener layout takes various forms, all transverse, all longitudinal, or a mix of transverse and longitudinal. The spacing of the stiffeners divides the skin into small sections called panels. The location, geometry, and area of these panels determines the load (which may be) imposed on them, and thus the makeup of the skin.

    For a modern sailboat.....
    The highest loads on the structure will probably derive from rigging loads and perhaps point loads from appendages. There will also be hydrostatic and hydrodynamic loads on the hull as with the powerboat above. Global bending (from both rig and bending in waves) in the hull/deck could be a concern.....or not....
     
  8. TeddyDiver
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    TeddyDiver Gollywobbler

    Just follow my link and press the Buy button.. then you have scantlings :D
     
  9. rxcomposite
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    rxcomposite Senior Member

    Because there is nothing specific about the scantlings. It is (as Tad has explained) a combination of elements.

    So unless you know the load/pressure (constant, repetitive, slamming), how it will be supported(cantilever, fixed,ect), where (below waterline, above waterline, forward,aft,ect) then it is not specific. just a general description of the "skin" and "support structure" which is called "scantlings".
     

  10. TealTiger
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    TealTiger Junior Member

    Every once in a while I think, 'who recommended Boat Strength to me?' I love that book! My understanding has developed so much from my numerous re-readings of that book. I own two copied; one pristine, one heavily annotated ;) I finally looked the culprit up. Thank you TeddyDriver.
     
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