3D CAD Hull Design - Origin Location?

Discussion in 'Boat Design' started by DCockey, Mar 3, 2018.

  1. DCockey
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    DCockey Senior Member

    Some questions for those who design hulls using 3D CAD such as Rhino: Where do you locate the hull relative to the hull? Is the vertical location of the coordinate located at the design waterline, below the hull and appendages, or elsewhere? Is the longitudinal location at a fore or aft "perpendicular", at the bow or stern end of the hull, away from the hull, or elsewhere? Does anyone use a transverse location for a monohull which is not on the center line of the hull? If you design catamarans what transverse location do you use?

    What is the reason for you choice of origin location: Employer practice? What you were taught? Standard practice based on your experience? Customer / classification society / regulatory authority preferences or requirements? Personal preference? Another reason?
     
  2. 3dig
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    3dig Junior Member

    On a mono hull, Ive always used the point were the bottoms, and transom come together at the stern, the lowest, most aft part of the hull. Cats I use the same point, but midway between sponsons.
    Not sure what I would do on say a Proa? or non symetrical type hull. Reason is, its nice to have a standard plane on centerline to use when sketching, and my ORCA hydrostatic software suggests this as a starting point.
    Ive 'built' and 'floated' hundreds of hulls with great accuracy this way.
     
  3. jorgepease
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    jorgepease Senior Member

    I, without any formal training gravitate to 0 being the waterline, stern, centerline of hull. It seems to give me comfort in the 3D space and makes for a fast start when drawing first lines. I could see where an employer might set a standard but otherwise I can't see why it would matter and it's easy fairly easy to change at any time.
     
  4. Ad Hoc
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    Ad Hoc Naval Architect

    Baseline is also the lowest part of the hull, this is the datum for all vertical dimensions.
    Longitudinal, is always about amidships.....except on small monohulls.
    DWL is the objective displacement draft.
    FP and AP are the 2 extremes on the hull, of the DWL.
    Amidships is 1/2 the Lwl, which is located on the DWL.
    Transverse is vessel centreline.
     
  5. TANSL
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    TANSL Senior Member

    I would not want anyone to be upset by my opinion, which, based on my experience, is the following :
    • Baseline is also the lowest part of the hull : Not always
    • Longitudinal, is always about amidships.....except on small monohulls. not a general rule
    • DWL is the objective displacement draft. : objective for lightship, full load ...??
    • FP and AP are the 2 extremes on the hull, of the DWL. : AP use to be at the rudder axis
    • Amidships is 1/2 the Lwl, which is located on the DWL. : amidship = Lbp/2
     
  6. gonzo
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    gonzo Senior Member

    From the building point of view, a baseline that is outside of the hull makes measurements and setting molds easier. I find that with hulls that are built upside down a baseline above the hull can be practical.
     
  7. DCockey
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    DCockey Senior Member

    Additional questions: Does your baseline always coincide with the x-axis in your CAD system? Where do you locate the baseline if the keel is not straight or not parallel with the DWL?
     
  8. TANSL
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    TANSL Senior Member

    According to the International Convention on Load Lines, the perpendiculars must be taken at the end points of the "L" length, with the length "L" of the total length measured in a flotation whose distance to the high edge of the keel is equal to 85% of the minimum strut of track, or the length measured in that float from the bow face of the stem to the axis of the rudder stock, if this second magnitude is greater.
    The baseline and the waterline should be parallel to each other and horizontal. It is necessary to place the ship so that, in the load condition considered "normal" or "more frequent", with respect to that baseline / flotation, its trim is zero. There is nothing written, as far as I know, about how to draw the baseline so, I suppose, the designer can place it where it is most comfortable. As the designer, generally does not know if the boat is going to be built upside down or not, it is not necessary to take into account this circumstance.
     
  9. DCockey
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    DCockey Senior Member

    My specific interest in creating this thread is where and why is the coordinate origin of the CAD system located when designing the hull, not debates about definitions for regulatory purposes.
     
  10. DCockey
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    DCockey Senior Member

    TANSL, do you design hulls in 3D CAD, or do you use the traditional 2D lines drawing methodology? Where do you locate the origin of the CAD coordinate system?
     
  11. DCockey
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    DCockey Senior Member

    If the builder is known then the construction procedure is probably also known.
     
  12. TANSL
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    TANSL Senior Member

    I do not know how important it can be how I design but YES, I do all the calculations using 3D models and I use several CAD packages, Rhino included (I know that this preoccupies you a lot). I use models of wires, surfaces and solids. Each of them can be more or less useful depending on what conditions or studies need to be carried out.
    I put the baseline at the lowest point of the hull, about half the length, more or less, but not exactly there. From there I measure the distance to the deck on the side, which is the depth, and at 85% of the depth I trace a water line that defines the Lwl. The aft end of that line is, for me, the FP and, if there is a rudder, the rudder stock is the AP. If there is no rudder, I place the AP at the most aft point of the aforementioned water line.The origin of coordinates is the intersection of the baseline with the AP.
    Naturally and the designer must design some aspects of his boat depending on the builder but in the early stages of design, what I call design engineering, that is totally irrelevant. To carry out the development engineering, construction plans, the designer can not do it if he does not know the construction procedures of the builder, as well as its technical and human resources.
    But if you calculate the hydrostatics thinking that the ship is built face down, with the baseline according to that position, I do not know what those calculations will be used for when the ship does not float upside down. Of course, in case of rollover it is useful to have that data.
    I may not be a builder, but I have never heard that the baseline depends on the position in which the ship is built. I am always willing to learn something new but this, NO, I will not learn it.
     
  13. DCockey
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    DCockey Senior Member

    Thanks. Interesting and more information than I was seeking. (My primary question was where the origin is located.)

    I've seen some small boat designs with the offsets measured down from a baseline above the boat. These are for boats intended to be built upside down.
     
  14. TANSL
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    TANSL Senior Member

    Yes, I have also taken measures like this, from existing boats, but when you make the body lines plan, or the 3d model, you do not draw it head down, you place it as you think it will float uy with the baseline at the bottom.
     

  15. Ad Hoc
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    Ad Hoc Naval Architect

    Hmmm..let's see ...

    Then you should read the questions and try to comprehend what the poster is asking rather than throw endless points back most of which are unrelated, often meaningless, as you consistently show on all threads you post.

    And there is your ignorance on what hull design is and thus how one defines the datums.

    Read the question!

    And what you are quoting is for boats that wish to be built under the 24m Loadline rule. That has zero, yup zero influence on where YOU places datums inside the 3D model, as the OP was asking.

    Hopeless....utter nonsense.
     
    Last edited: Mar 4, 2018
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