Rhino 5 - material thickness

Discussion in 'Software' started by rwatson, Jul 6, 2013.

  1. rwatson
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    rwatson Senior Member

    That's what some people are trying to tell me, but its only out of tradition and convenience.

    I explained a few posts ago, that a large number of boat designers have to work from the displacement lines, in. In fact it is essential for ply and glass designers.

    I am not coming 'unstuck', just amused how people cannot see past tradition to logic.

    Would you design to the mould lines if you had 20mm of hull clearance to a container ? I doubt it.
     
  2. TANSL
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    TANSL Senior Member

    I certainly did. To do otherwise would lead to complications and possibly facilitate errors.
    The fiber boat builders with female mold, probably, for some items (not all), prefer to work with external lines. All other boat builders certainly should do most of his work with the moulded lines. This is the tradition that we all respect, not to be a tradition but because, as is best, it has become tradition.
     
  3. rwatson
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    rwatson Senior Member

    So, logic prevailed . But you got upset with me doing exactly the same thing ??


    The vast majority have to - as fiberglass doesn't have the traditional framing layout of other types of builds. And an increasing number of moulds are machined by 3d mills - and of course they use the displacement lines.

    http://homepages.ihug.co.nz/~jacksonj/boat_hull.html


    Look, this 'tradition' only exists because the time and effort to do displacement line drawings wasn't worth it with many traditional building methods. Now, with digital tools, it is the easiest way to start, because the external hull is usually quite smooth, but the thickness of the 'skin' varies.

    I was surprised to hear that at least one steel project (Australian warships), the displacement lines were the first to be done, as I said before, for tank models and publicity models before the mould lines were defined.

    Likewise, some local contacts in the big aluminum ferry business told me they always start with the displacement lines first, because of an important part of the feasibility study involving plate costing, as well as concept drawings for the client.

    The third area, which is a significant area, is the plywood boat plans. This is especially so for trailer-able boats that are often have target exterior envelopes, as well as needing to do 'unrolling' of hull skins.

    People who get all pedantic about 'the only way' have the onus of presenting sound logic, not just promoting a meaningless mantra.
     

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  4. TANSL
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    TANSL Senior Member

    rwatson, I'm sure it's true what you say. However, a case, or fifteen, your friend told to you, can not invalidate the general rule. Always, of course, there are exceptions to the rule.
    It seems to me impractical, but it is just an opinion, to begin work on a project with the displacement lines when not yet know the thickness of the hull and deck. But no doubt there is some reason, in some cases, to justify this approach.
     
  5. DCockey
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    DCockey Senior Member

  6. MikeJohns
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    MikeJohns Senior Member

    But it's still not an issue You simply allow for the total width limit when specifying the max beam section if you already know your skin thickness.

    I don't know what sort of material and thickness you getting NC cut ? I presumed probably incorrectly that it was alloy?

    And also have you completed the structural requirement side of the design;
    The reason I'd work from the moulded line is because it suits the design spiral best since you often don't know the final hull thickness until you have your structure optimised and have your global loads. ie the shell thickness is defined last and often changes depending on frame spacing and loads and regulations.

    But if you have the design already you really can work any way you like.
     
  7. rwatson
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    rwatson Senior Member


    Davids link to Sponbergs explanation (thanks David ) is one I read before, but its exactly what I have been saying.

    I have had my plywood hull FE analysed, and the starting point had to be the displacement line, due to exterior constraints.

    But for the design cycle - I don't believe that working to the mould line is that much easier easier for scantling calculations. For a start, most CAD packages don't do a good job of FE analysis, so you do the engineering calcs outside the drawings, using the Cad dimensions, don't you ?

    And the whole thing started when I was getting a handle on representing material thicknesses, for accurate cnc cutting. Whether the design was done inside/out or outside/in, I STILL have to get accurate measurements, as you say ........

    Exactly !! Shame that certain individuals didn't think it through.


    The inferences from Ad Hoc, and the rabid proselyting from Tansl of 'mould line' starting points, were just plain irrelevant, and as we have found out, not even that accurate.
     
  8. MikeJohns
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    MikeJohns Senior Member

    Ray
    Exporting the CAD model to the FEA package is usually the way I work if doing specific design. I have FEA packages that take simple surfaces that are shelled very simply by the FEA package. Changing material thickness of the skin is then a matter of changing a thickness in a dialogue box. But that only works if you work to the moulded line. All our software tools for vessel design are set up that way. For sheet material work it really simplifies the design process.
     

  9. rwatson
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    rwatson Senior Member

    Exactly, thats how most designs go. The FEA analysis would find it much easier to work to the mould line, certainly, because they are usually such static, simple graphical tools. And it would have to assume that the imported measurements are the mould line, as there would be no ability to assume a skin thickness ( which you wouldnt have worked out yet ).

    But then, you dont import the FEA design back into CAD do you ? You simply adjust the Cad drawings to match the engineering results. ?

    In which case, if your design is to have differing skin thicknesses, you have to alter them on the interior to leave a flush exterior one, and adjust the framing sizes to suit.

    That would not be necessary for planked, and other uniform 'skin' materials, IF the outside dimensions weren't critical.

    So, as I said previously, many times, working to the mould lines is just a case of 'time and effort saving', not some mysterious secret that is the 'only right and proper way'.

    ... and for many situations, isn't sufficiently accurate.
     
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