Planar Sheer with Orca3d

Discussion in 'Software' started by ndar, Aug 22, 2016.

  1. ndar
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    ndar Junior Member

  2. TANSL
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    TANSL Senior Member

    Hi ndar, nice video.
    The technique of cutting a surface by a plane is well known.
    My question : generally decks have camber and sheer (many times parabolic), so why the needing to create a planar sheer line.
     
  3. bhnautika
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    bhnautika Senior Member

    Yes ndar a very good video, and I would recommend it to those studying design.
    TANSL here is a real example, same boat from different angles. Note the subtle bump in the sheer at the bow.
     

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  4. ndar
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    ndar Junior Member

    Thank you gentlemen,

    Planar sheer line has to see with lofting, in the traditioanl sense: Having a planar sheer will ensure that the sheer stringer will be laid easily, with need to be twisted.
    This makes it valid too when fairing the hull with a computer software. It is always easier to fair a curve when it is planar

    I don't think it has to do with the deck camber though. It is only once the sheer line is sorted that the designer will tune the deck profile at CL and transversal camber.
    BR
     
  5. TANSL
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    TANSL Senior Member

    Fortunately shapes of a ship not depend on how easy is the loftman work. They have more to do with aesthetics and the seaworthiness of the ship. Loftman is who has to do whatever is necessary to get correct shapes. Twisty stringers are never a problem.
    Not easy to find sailboats with planar sheer.
     
  6. PAR
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    PAR Yacht Designer/Builder

    If you know how to draw a sheer properly, you don't get a powderhorn, unless you want it. This is yacht drafting basics. It's also an error that can be made when lofting and/or when lining off the sheer batten. Some use a planar sheer as a crutch, probably because they don't know how to correct for this, but it's rare to see a planar sheer on a yacht. The yacht pictured above has an intentional powderhorn, which was fairly common styling treatment in it's era.

    Pictured below are two boats. The first shows the sheer done by a novice (my assumption). The attached image is the exact same design, built by me about 10 years ago. Note, I've accommodated it to alleviate this effect on the sheer. The plans for this design, didn't come with traditional lines or offsets (damnit, why do people do that), so I worked with a batten when tuning up the various lines, including the sheer. Simply put, you can't fix it, if you don't know what to look for.

    [​IMG]
     

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  7. TANSL
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    TANSL Senior Member

    In my opinion, and I would not bother anyone, we are facing a classic case of one thing modeled with Rhino versus what should be a boat, which Rhino can not teach. As always, the cartoonist who draws with Rhino something like a boat does not have to be a boats designer. But this should not be taken as an offense, they are simply different professions.
     
  8. bhnautika
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    bhnautika Senior Member

    This is where I get confused between the physical and the metaphysical. So how do I get the battens and the curves into the computer?
     
  9. PAR
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    PAR Yacht Designer/Builder

    Fairing on the computer or with a batten on a loft floor, is essentially the same thing, though does use different approaches. There's nothing metaphysical about this, it simple geometry.
     
  10. bhnautika
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    bhnautika Senior Member

    Sorry they cannot be the same, battens have both material and dimensional properties which govern the way they curve in 2d space. Even the difference between the batten material and size from board to floor can make slight changes. A computer curve has none of these characteristics apart from the weighting of points and degree, so you need tools to give feedback on the curvature of a line which could be or is in 3d space.
     
  11. PAR
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    PAR Yacht Designer/Builder

    Obviously if you've sprung a few battens before, you'd realize the correlations in 3D. The problem as I see it, is you don't understand the problem fully, so can't address it on a computer screen. It's one thing to have the physical skills to manipulate lines on a screen and wholly another to understand why you need to do so. For example a wide bowed boat would experience a much more dramatic flat spot on the sheer, compaired to a flatter sheer on a fine entried boat. Typically this is called "expansion" or "projection" and it's performed in a number of locations around a yacht, though the sheer and transoms are most common places to need to address this issue.

    I'm not trying to insult you, but some education is necessary, before you draw anything is the usual cure. Chapelle's "Yacht Design & Planning" is a common reference (around page 350 I think). There are other texts that cover it (Greg Rossel's book "Building Small Boats" is one), as well as practical experence that reinforces it. Simply making this correction on the loft floor is the fastest way to understand what's going on and subsequent drawings on the screen will look right once built.
     
  12. bhnautika
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    bhnautika Senior Member

    This is not about whether I have sprung a few battens or not (pure conjecture). Its about the physical world of laying down curves on a drawing board or loft floor using a batten, adjusting said batten to allow for the finished 3d shape. Its having the tool set to do the same thing in the 3d space of the VR of the computer. Computer modelling of hulls is more like the past when hulls were carved out of wood then cut up to get the lines. The hull was there to see, so any anomalies could be fixed, then came the lines. Drafting lines first took some of that visualisation away.
     
  13. SukiSolo
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    SukiSolo Senior Member

    Actually if you know how to draft it's not a problem. Most hulls are drawn then built rather than carved blocks and then the lines taken off. Partly because it is easier to do the basic maths on the shape. Nothing wrong with using either approach, after all the one great benefit of 3D modelling is the visualisation of a hull and deck etc.

    If you understand properly how one curved line affects the shape, you should be comfortable manipulating it. However you do need to understand how each curve in any of the three basic projections acts upon another, when defining a form.
     

  14. bhnautika
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    bhnautika Senior Member

    SukiSolo I did not mean it in the literal sense it was a reference to the past, why draft the lines then create a surface when you can create a surface and have the lines generated automatically.
     
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