Import Offset on DELFTship

Discussion in 'Software' started by PixelTweet, Jun 21, 2018.

  1. PixelTweet
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    PixelTweet Junior Member

    Hi!

    I'm currently facing an issue when trying to import offset .txt files to DELFTship, and I haven't found any answers on previous threads.

    I have read the documentation on the .txt file format for the offsets, but there is an important information missing: the location of the "flat of bottom". Not only am I unsure of what this information actually is, I also have no idea where to put it in the file !

    I have tried a few things, but unsuccessfully...

    Thank you for any answer you can provide !
     
  2. TANSL
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    TANSL Senior Member

    I suppose that with "flat of bottom" it means the coordinates (half breadth) of the points in which each frame becomes tangent to the flat bottom (points "1", "2", "3", ...), if there is a flat bottom.
     

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  3. PixelTweet
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    PixelTweet Junior Member

    Yeah, that's also what I think, but my main problem is to find where to put that dataset inside the offsets format taken as an input by DELFTship...
     
  4. TANSL
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    TANSL Senior Member

    Maybe the DelftShip User's Manual explains something about it.
     
  5. PixelTweet
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    PixelTweet Junior Member

    Well, actually, no. The rest is specified and explained with examples, but this very information is missing...
     
  6. TANSL
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    TANSL Senior Member

    This is what I have found in the help of DELFTship. I think that may be enough.
     

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  7. PixelTweet
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    PixelTweet Junior Member

    This is just the DELFTship documentation, and as I told you the information that I'm looking for is missing ! There are examples showing the location of the data in the file for every dataset except the "flat of bottom" :/
     
  8. TANSL
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    TANSL Senior Member

    I do not understand what you want to say, what is the information that is missing, but if it is missing, and the Manual does not talk about it, maybe it is because it is not necessary ????
     
  9. baeckmo
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    baeckmo Hydrodynamics

    /...........the Manual does not talk about it, maybe it is because it is not necessary ???? /Good point!
    If you are importing a textfile for a roundbottom ("softchine") hull, you have a set of coordinates (numbered from keel and upwards) for a group of transverse sections (numbered from aft and forwards). The part of a section you call "flat of bottom" is just a part with no curvature, the section coordinates still start with the Y-value for the keel line (for a monohull generally Y1=0). If there is a "flat of bottom", the next Y-coordinate(s) will have the same Z-value as the keel coordinate, until the lines start to rise towards the round chine.

    Note that when entering coordinates for transverse sektions, the number of points may vary between sections.
     
  10. PixelTweet
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    PixelTweet Junior Member

    Thank you for for answers, although it doesn't solve the problem...

    In addition to the required information, which is as baeckmo explained a double-entry table of offsets for a set of waterlines and a set of stations, a few other sets of information can be added, including the coordinates of the deck line, the forward and aft contour lines (z=0) for each waterline, the keel contour line for each station, and finally the "flat of bottom" (I'm not the one calling it like that, it's in DELFTship's manual). The location in the file of all these additional sets of information is shown with example, except for the "flat of bottom"...

    I am indeed trying to get the model of a roundbottom, with a flat (no curvature) bottom (a series 60 ship). I have the offsets for the flat bottom (i.e. for each station the y-coordinate until which there is no curvature), and the offsets for 25 stations and 7 waterlines. However, if I simply put my set for the flat bottom at z=0 (1st waterline), DELFTship doesn't create a surface for the bottom. If either I omit the data for the flat bottom or put it at a very low z-coordinate (close to 0), the bottom isn't actually flat (see pictures, with what DELFTship gives and what I'm trying to achieve).

    Also, your last remark isn't quite right baeckmo.... It is true if the input are just stations (curves build with a set of points with the same x-coordinates), for example with the "surface" file input (see DELFTship manual), but not for a table of offsets, as it is a double-entry table, which mean that for every station there must be a point (y-coordinate) at each waterline (the offset y can be 0 if at said station the considered waterline is under the x-local draught).

    For the pictures: one is the original bodyplan, which was accompanied by the offset table I'm using. The next is the bodyplan obtained with DELFTship, which as you can see doesn't have a flat bottom at all. The last picture is also from DELFTship, but shows both the offset input (pink dots) and the resulting stations (green lines). The pink lines are built by DELFTship using the offset input.
     

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  11. baeckmo
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    baeckmo Hydrodynamics

    ".......for every station there must be a point (y-coordinate) at each waterline (the offset y can be 0 if at said station the considered waterline is under the x-local draught)". This is your problem when using coordinates defined by waterlines (ie fixed Z-values). With a horizontal part of a section, there are unlimited number of Y-values for a single Z-value, ie the surface is undetermined.

    You have to add one (or more) coordinate along the flat between the keel and the start of curvature in order to "lock" the spline into a horizontal straight line. If you show your coordinates for one station as an example, it may be easier to see where the problem hides. One trick is to add one or two waterlines at Z=0.1 mm and Z=0.2 mm.
     
  12. TANSL
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    TANSL Senior Member

    Instead of trying to create a single surface for the whole hull, you could create a flat surface, for the flat bottom, and one or more sufaces for the bottom and sides.
    On the side there is also a flat vertical area that, as it is intended to do now, will be difficult to achieve.
     
  13. PixelTweet
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    PixelTweet Junior Member

    @baeckmo
    That is exactly why a flat bottom can be specified in the offset input, so that DELFTship knows that there is a flat bottom. It doesn't accept several sets of offsets on the same waterline anyway, And I already tried the trick of adding line at 0.001, 0.002, 0.003... But it would take an infinity to actually get the tangent right, and in the end my bottom isn't flat because of the slope created artificially!

    @TANSL
    The problem is then of the tangent continuity at the junction between the flat bottom and the round chine, which I guess DELFTship is able to deal with when you tell him there is a flat bottom in the input offset file... The sides of the ship aren't an issue here, as the offset definition is enough to get them vertical.

    Overall I think the problem is already dealt with through the "flat of bottom" dataset if only I could get it to the right place in the input file!
     
  14. TANSL
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    TANSL Senior Member

    As in other CAD programs, I suppose that, with the offsets table, DELFTship can generate some surfaces and you can always improve them and complete them as much as necessary.
    Once that is done, ask for the offsets table of the new hull and analyze the file generated by the program. I do not know if that will be useful but I can not think of anything else.
     

  15. baeckmo
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    baeckmo Hydrodynamics

    Ok, back to notes 2 and 6. Have you imported the Y-values for the lowest waterline (Z=0) correctly? As I understand it, they should describe the tangent points where the bottom flat connects to the radius (as in TANSL's example), not the keel position. When the "line seven" is set to "1" for a flat bottom, I expect the program to automatically add the keel point, ie Y=0; Z=0. This means that you should not have to input any other separate info in order to get the bottom flat.

    BTW, In the Delftship example file, the station at 6.00 starts with a Y-coordinate of 0.073 at the 0.015 waterline, but all other stations start at Y=0. Is that a typing error, or is there a flat part of the bottom, or an opening? Since the line seven in the input says "0", there should not be a flat. The spline that corresponds to the offsets has an inclination at the first point; a flat would connect to the radius through a sharp "chine".
     
    Last edited: Jun 23, 2018
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