help needed with hull software

Discussion in 'Software' started by Rambler001, Feb 2, 2014.

  1. Rambler001
    Joined: Feb 2014
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    Rambler001 New Member

    Hi there,

    i'm new here and after building some model boats i want to make a design of my own.
    I've tried using delftboat software but my knowledge is incuficient to make my idea work. So i have an idea for the hull above the waterline but that is were my knowledge ends.

    So i'm wondering if someone here can help me out making my hull in a program wich i can use to make a rc model

    thanks Bart
     
  2. PAR
    Joined: Nov 2003
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    PAR Yacht Designer/Builder

    Welcome to the forum.

    DelftShip, is one of the easiest software packages there is. The real problem isn't manipulating the software, but understanding where and why you need to manipulate it. The software will happily draw up, the most unsuitable set of shapes you can envision, so you do need a working understanding of hydrodynamics, which you appear to be lacking. This said, for a pond model, you can play around and learn a lot, just through trial and error. You'd be best advised to use a known set of hull shapes, below the LWL and stylize the topsides to your liking, as the fastest way to get on the water with a design that works reasonably well.
     
  3. rwatson
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    rwatson Senior Member

    I dont think Delftship or Freeship are 'easy' to use at all. They are non intuitive, poorly documented and badly supported compared to more expensive versions.

    They do have some great features that cost thousands in other packages, like the hydrostatic calculations.

    I ended up spending $1000 on Rhino, which is popular due to its releative ease of use and drawing power. Even so, you are looking at a months practice with the excellent tutorials available. Then you don't get the handy computations like displacement, balance and other useful hydrostatic information.

    Welcome to the frustrating world of design software - there is no easy answer.
     
  4. DCockey
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    DCockey Senior Member

    The Hydrostatics command in Rhino calculates volume displacement, center of buoyancy, center of flotation, waterplane area, wetted area , etc.
     
    Last edited: Feb 2, 2014
  5. TANSL
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    TANSL Senior Member

    What is meant by initial stability and what its value is used for?. Are you, may be, talking about the initial transverse GM?. Thanks
     
  6. rwatson
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    rwatson Senior Member

    Thats a new one for me.

    I made a simple hull, and tried the command, but it refuses to SELECT the shape.

    I cant 'surround' it with the cursor, and as soon as I click on one surface, it goes straight to the report. There are no Youtube demos or Rhino tutorials on the topic either.

    I must be missing something simple.
     

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  7. rwatson
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    rwatson Senior Member

    Second time lucky - I must have had some extraneous features in the first attempt.

    This worked better, and i used the Symmetric Option, which doesn't require a Mirror done of the hull.

    I will need practice with this.

    The report has all the basic info required, for sure, but I didn't see any 'Initial Stability' on this report.
     

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  8. DCockey
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    DCockey Senior Member

    The submerged portion of the hull surface must be "watertight". Use ShowEdges and select "Naked Edges" to find gaps.

    Simple way to use Hydrostatics
    1) No surface selected
    2) Hydrostatics
    3) Change the Waterline Elevation to the desired value if needed
    4) Select the hull surface
    5) Enter

    If the Hydrostatics command returns a report showing only wetted area but not volume then the surface is not watertight or the waterline is at the bottom of the surface.

    From Rhino Help:

    Hydrostatics


    Where can I find this command?Toolbars

    Mass Properties
    Menus

    Analyze

    Mass Properties

    Hydrostatics
    Shortcut


    Hydrostatics command

    Reports hydrostatic values for surfaces and polysurfaces.

    Hydrostatic values include wetted surface area, waterline length, maximum waterline beam, water plane area, and center of floatation.

    Steps

    Select surfaces or polysurfaces.

    Options

    WaterLineElevation

    The water plane must always be horizontal in world coordinates. (This is a limitation of the command, not a statement of a physical principle.) Its location is defined by specifying the depth of the origin in world coordinates.

    Symmetric

    Only half of the model needs to be given. The calculation results are doubled or adjusted as appropriate to represent the full model.

    Longitude

    The symmetry plane is either x=0 (when y is longitudinal) or y=0 (when x is longitudinal). The longitudinal direction (from bow to stern or front to back) must be either the direction of the x-axis or the y-axis.

    Value

    Volume Displacement

    Volume under the water.

    Center of Buoyancy

    Centroid of the volume displacement.

    Wetted Surface Area

    Surface area under water.

    Waterline Length

    Length at water line. The longitudinal bounding box extents of the water plane area.

    Maximum Waterline Beam

    Maximum beam at water line. The transverse bounding box extents of the water plane section.

    Water Plane Area

    Area of the cross section at the water plane.

    Center of Floatation

    Centroid of the water plane section These are the values for the whole model even if only a half model is given.

    Save As

    Save the information to a file that you can use in spreadsheet programs.

    Notes

    To get displacement information there must be no naked edges below the waterline except in the case of Symmetry = Yes, in which case there can be naked edges on the symmetry plane.

    If the waterline falls on a singularity (place in the surface where points converge like at a pole of a sphere), the command will fail. Move the singularity point a fraction away from the water line.

    See Also
    Analyze an object's mass properties.
     
  9. rwatson
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    rwatson Senior Member

    Were you using GHS data Tansl ?
     

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  10. rwatson
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    rwatson Senior Member


    Many thanks for that David - a whole new world to explore. Looks like my days of wrestling with Free/Delft Ship are over ... hooray !!!! :p
     
  11. DCockey
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    DCockey Senior Member

    My mistake - Hydrostatics does not report moments.

    To get initial stability:
    1) Create a surface inside the hull at the waterline. This can be as simple as first creating a rectangular plane at the waterline elevation larger than the hull, and then trimming the plane with the hull.
    2) Use AreaMoments and select the waterline plane to get the moments of the waterline plane.
    3) Use Hydrostatics to get the volumetric displacement
    4) BM (metacentric radius) at zero heel is:
    Transverse Moment of Inertia of the Waterplane / Volumetric Displacement​
     
  12. rwatson
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    rwatson Senior Member

    Maybe not :(

    Looking up the GHS geometry file options lead me to a new company called Creative Systems. http://www.ghsport.com/home/index.htm

    No pricing info, not much explanation, and no distributors in Australia.

    Must be a mega-dollar thing.
     
  13. DCockey
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    DCockey Senior Member

    GHS geometry has nothing to do with the Hydrostatics command nor the AreaMoments command or any other analysis calculations in Rhino.
     
  14. DCockey
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    DCockey Senior Member

    I've created stability curves for Rhino hull shapes using a combination of the HYdrostatics command and an Excel spreadsheet. The hulls surface is rotated incrementally. At each increment the the waterline elevation is adjusted until the displacement has the desired value and then the location of the center of buoyancy is copied and pasted into an Excel spreadsheet. The desired stability parameters are then calculated and plotted in Excel.
     

  15. Mike Graham
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    Mike Graham Junior Member

    GHS is one of the more sophisticated tools for stability and other hydrostatics analysis. It's very powerful and flexible, but not very modern or easy to use and it's not cheap. They also make a program called BHS, which is cheaper, though I don't know by how much (it probably still costs more than a seat of Rhino). There are lots of GHS users in Australia, but I don't know how they went about buying it.

    The GHS geometry format is a way of storing tables of offsets for various ship compartments. It's supported by tons of hydrostatics and hydrodyanmics codes that aren't GHS.

    If you're interested in using another piece of software to get stability information about your Rhino hull model, the obvious choice is Orca3D. It's more affordable and has a nice interface and integrates well with Rhino. It can't do analyses of the level of sophistication that GHS can, but that isn't necessary in many cases.
     
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