From lines plan to 3d hull form

Discussion in 'Software' started by ivansalasj, Jul 7, 2004.

  1. ivansalasj
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    ivansalasj Junior Member

    Hi,

    I need to create an existing hull form using offsets from a lines plan that i have i was wondering if anybody can give me a wee hand to do so. I have maxsurf academic and also autoship.

    Thanks for your help
    Ivan
     
  2. Andrew Mason
    Joined: Mar 2003
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    Andrew Mason Senior Member

    I have added a new tutorial to the Maxsurf Academic site that covers the creation of a set of three dimensional data points from a linesplan. The title of the tutorial is Maxsurf Tutorial 5.

    You may also wish to download a file that covers some of the fitting tools, this is named Maxsurf Example 3.

    Please see -
    http://msacademic.formsys.com/tutorials
     
  3. DavidG
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    DavidG Junior Member

    I use the following method in Autoship;

    Trace the stations onto drawing film as cleanly as possible, ensure that there is a centreline and waterline on the plan. Scan this and convert to a .dxf file.

    Using an offset table (if there is one) or build an offset table identifying critical points (sheerline, keel line, chines, stem), and enter this points into your Autoship model, fit curves to these points.

    Import the .dxf sections and move them to the required positions.

    Loft surfaces that fit the above (it is helpful to set up the station spacings to match the imported curves.

    Obviously there are some tweaks to the system depending on the complexity of the hull, but I think that Autoship excels at this type of work.

    You could possibly improve the process with a digitising tablet, but I havn't tried.
     
  4. SailDesign
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    SailDesign Old Phart! Stay upwind..

    I use A'Ship this way as well. Works well, and is very easy to tweak with. Then it all gets sent to Rhino for the pretty stuff.

    Steve
     
  5. Andrew Mason
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    Andrew Mason Senior Member

    I would caution against using methods that loft surfaces accross a series of profiles (including our own Prefit program) if the intention is to do design for production. Lofting methods are fine for generating a quick model for hydrostatics or for generating a basic hull for rendering purposes.

    Unfortunately the surfaces that result from the lofting process tend to be of poor quality, with areas of unfairness and multiple small inflections. More importantly, the control point nets that results are usually irregular and employ too many control points, making them unsuitable for later fairing or design modification.

    These problems tend to be hidden by programs that have good rendering, as the shading model used (Gouraud or Phong shading) tends to smooth out anomalies in the surface, making it difficult for the user to spot problems.

    If you doubt this is true, import an IGES NURBS file of your lofted surface into the demo or academic version of Maxsurf and perform a Gaussian curvature analysis in the rendering window. I can guarantee you will be surprised at the generally poor quality of the surface.
     

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  6. DavidG
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    DavidG Junior Member

    Lofting Lines

    I would agree with Andrew, I dont think I have had much success using programs where you press the button to loft a surface through a series of lines or points, there are too many control points, the surface rarely is the intended shape and it is hungry on processing time. However I understood that the recent version of Prefit was intended to address this.

    What I do is set up boundary edges and "rule" a 5 by 5 surface between the curves, set up station lines at the same spacing as the imported .dxf lines, and fair the surface until the station lines are coincident with the .dxf lines. Then use gausian curvature to check for fairness.

    David
     
  7. PNACS
    Joined: Jul 2004
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    PNACS Naval Architect

    Markers (offset points) can be feeded in to Maxsurf Pro and Surface can be fitted using " fit to markers" . But It is very time consuming and tedious process.
     
  8. Andrew Mason
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    Andrew Mason Senior Member

    David

    I assume you are referring to the Fit to Markers command in Maxsurf. This is enabled in Maxsurf for people who have purchased Prefit, but is actually a completely different method to the old Prefit lofting approach.

    The Fit to Markers command utilises a Genetic Algorithm to do the fitting. It effectively treats the fitting task as an optimisation problem that has multiple constraints and objectives, namely fit, fairness, net spacing regularity, net orthogonality. The results can be very good, however the time taken can be large. Where it can work extremely well is where you already have a surface that has been fitted using another method, such as the old Prefit or even manual fitting. The Fit Surface to Markers can often dramatically improve even a good manual fit.

    The pics below show an surface and net created by the Fit to Markers function. Obviously we had a very good dataset to start with, but you can see that the net of control points is what you would expect a good human designer to come up with. It is very different to what you would expect to see from a typical lofting method.
     

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  9. ivansalasj
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    ivansalasj Junior Member

    Dear Andrew Mason,

    Thanks for you tutorial on the website. I am now following your instructions and i am having some problems specifying the station spacings and also once i have inserted the markers i require, how do i digitilize the markers.

    Thanks again for your help
     
  10. ivansalasj
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    ivansalasj Junior Member

    Once i have all the markers set up at their corresponding station, what is the best way to create a surface that i can export to a CFD program, as you have probably guessed i am quite new using Maxsurf, i started with autoship but never got really into it so im trying my best now

    Thanks
     
  11. Andrew Mason
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    Andrew Mason Senior Member

    Ivansalasj

    The general answer to your question would be to use Preft or the Fit to Markers function, however here we are stepping outside the realm of free software.

    Within Maxsurf Academic you need to fit the surface manually, and although this can be done fairly quickly by an experienced user, it can be time consuming the first couple of times.

    The key tips here are -

    1. Use the Display->Markers->Show Markers for Currrent Station command to display just the markers for one section at a time. If you find that the markers dont display properly using this option you need to make sure each marker is correctly associated with its station position. To do this, select the markers for the station and select Markers->Marker Properties. Enter the correct Station Index for the group of markers and click OK.

    2. Locate the corner points of the surface first. For a single surface design, these are commonly top of stem, bottom of stem, top of transom, bottom of transom. If you have marker points at these corners, the Markers->Snap Control Point to Marker is the quickest way to achieve this.

    3. Fit the edges (e.g. sheerline, stem, centreline, transom) next. You can do this manually or alternatively by selecting marker points that lie on an edge and using the Markers->Fit Edge to Markers command.

    4. Next fit the interior of the surface. A good way to start is to use the Markers->Smooth Interior Controls. Although this is not a surface fitting method, it will quickly lay out the interior control points in roughly the right locations based solely on the positions of the edge control points.

    Now using the control point net and the single section display in the Body Plan, drag control points to get the surface closer to the interior markers. Switch sections often and try to keep the overall surface net regular and fair.

    If you need to get a numerical value for the quality of fit, use the Markers->Measure Surface Error command. This will give you a measure of the RMS error between the surface and the marker points.

    Hope this helps.
     
  12. ivansalasj
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    ivansalasj Junior Member

    Thank you very much for your help Andrew and all i really appreciated.
     
  13. Davor
    Joined: Jul 2004
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    Davor Naval Arch.

    3D-hulls

    Hello,

    I am working on ship's dynamics, and made a program that creates 3D models from hull data file. If you are interested to see few models give me E-mail address to send it to you. Program is written in Fortran and VRML.

    By the way if you are interested in resistance and propulsion visit my web page www.sea-power.net

    rgds

    Davor


     

  14. ivansalasj
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    ivansalasj Junior Member

    Dear Davor,

    Thank you very much for your messege, i would defentely like to see some of your examples, what kind of hull data does your program require to create a 3d hull form?, i am still looking for a good way to be able to create my models since i am also working on ship dynamics/motions and i would use the hull forms in a motions program to give me some results. I was initially using maxsurf academic but i need something a bit more pro to get the results tha i require

    Thanks again,
    Ivan
     
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