Problems relating Layer Symmetry property

DELFTship forum Feature requests Problems relating Layer Symmetry property

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    • #36787
      john langworthy
      Participant

      I am trying to determine hyrdostatics and resistance for a heeled hull.

      I am therefore using the mirror command to create both sides, then setting the layer properties to non-symmetric, prior to rotating the model about the centre line.

      I have noticed the following, which dont seem quite right:
      Prismatic ceofficient is dislayed has half the value it should be.
      When using the resistance calculation, most parameters are extracted correctly, but midship area is doubled.

      Similarly, the hull transformation sets the current displacment to double the value it should be when modeling the whole hull and not using symmetry.

      Looks like this property isnt being checked everywhere in the code that it needs to be.

      Cheers

    • #36788
      giorgio zuppin
      Participant

      Hello, mate, i’ll be brief ’cause i’m sweating like a pig… 40°C here…
      Have a look at the Forum, Hydros & stability, there’s a post of mine: <>
      It will explain, but Only if you run the free DelftShip prog.
      It happens that most of the calcs works only for symmetric designs, so…
      If it’s not the case, let me know.
      see you in coldest days.
      Jurgen.

    • #36789
      john langworthy
      Participant

      Thanks Jurgen, but its not really a solution for me.

      I am using the pro version and expect it to just work. I think the software is giving me duff information.

    • #36790
      giorgio zuppin
      Participant

      Well, that’s weird, i’ve got the same issue as yours… an old model, I wanted to obtain hydros data for a heeled hull.
      ( my first reply was about the sub ’cause it shows perfectly causes and remedy…)
      At the time I was aware that the pro version will accomplish the task, but it seems unfortunately not…
      I even asked a confirm to the admin, but no reply, due to this AND to the fact I don’t use the prog for commercial purposes,
      I still run the free version, anyway I really want to know if there’s a bug, or a common customer fault…
      Keep in touch please.
      Jurgen.

    • #36791
      Marven
      Keymaster

      It seems to me that you might have forgotten to disable the symmetry setting for the layer after mirroring it. This can easily be verified with the following rough check:
      Multiply the waterline length with the beam and draught and a constant factor 0.7. This should result in a value close to your displacement. If the layer symmetry setting is incorrect then the value is close to twice the expected displacement.

    • #36793
      john langworthy
      Participant

      Hi Marven, Thanks for replying. The symmetry check box is being set correctly. I dont think it is being applied consistently in the code. I’ve done some tests, and get mixed answers for a given model.

      Prior to mirroring:
      Symmetry enabled.
      (From Design hydrostatics report)
      LWL 9.317
      BWL 2.146
      Design Draft 0.5
      Displacement 4.184
      Midship section area 0.727
      Prismatic coefficient 0.591
      This all looks spot-on.

      After mirroring without a deck
      Symmetry disabled
      (From Design hydrostatics report)
      LWL 9.317
      BWL 2.146 (Correct)
      Design Draft 0.5
      Displacement 4.184 (Correct)
      Midship section area 0.00 – Why is a deck needed?
      Prismatic coefficient 0.00 – Why is a deck needed?

      After mirroring with a deck
      Symmetry disabled
      (From Design hydrostatics report)
      LWL 9.317
      BWL 2.146 (correct)
      Design Draft 0.5
      Displacement 4.184 (Correct)
      Midship section area 1.453 WRONG!
      Prismatic coefficient 0.2957 WRONG!

      Wetted area and other calcs are fine.

      As far as I can see, Am is being calculated as twice the value it should be, becuase symmerty is being assumed, this is then being used to calculate Cp, which is coming out at half the value it should be.

      The incorrectly calculated midship area value is also being propagated into the Resistance calculation dialog when the Extract data from model check box is checked.

      Interestingly, the section area table in the design hydrostatics report gives the correct values for midship area.

      Also, for a mirrored hull, when switching on waterlines and stations, they appear on both sides of the hull (as expected). Buttocks however only appear on the port side. Is that intentional?

      Hopefully, the attached jpeg demonstrates all the above.

      Attachments:
    • #36794
      Pepijn van Schaik
      Participant

      Hello jlangworthy,

      Due to the holidays, Martijn is limited available at the moment. Perhaps you can send me your file. (pepijn@delftship.net) so I can have a quick look.

    • #36797
      john langworthy
      Participant

      Thats not needed.

      Go file new.
      Create a deck and transom (it wont calc Cp if after mirroing if you dont, but thats a separate problem)
      Mirror about centre line.
      Turn off symmetry.
      Obsever Cp to be 0.212 – half what it should be.

    • #36798
      Pepijn van Schaik
      Participant

      Hello jlangworthy,

      We are going to have look at it and will try to reproduce it. We will get back to you asap.

    • #36800
      Pepijn van Schaik
      Participant

      There indeed seems to be a little bug for the calculation of Cp when mirroring the hull. Thank you for mentioning. Next week we have planned a new release and this will be included in the bugfixes. Please note that this has (had) no influence on the normal hydrostatic calculations.

      I do want to clarify something else related to the reason why you encountered this bug. I understood you want to calculate the resistance for the hull under heel. I assume that is the reason why you want to mirror the model and want to rotate the full hull model for that calculation. If I assumed that correctly I do want to warn you that this is not a valid approach since the basis of this resistance calculation currently included in DELFTship has nothing to do with heel and is an upright calculation. Because of that it doesn’t make sense to try to simulate that. Of course resistance calculations under heel do exist for some series but they are not included in the current software. It might be in the future but unfortunately I cannot guarantee anything.

    • #36801
      giorgio zuppin
      Participant

      Even if I’m only a concerned spectator, have to say your answer was exaustive.
      Please keep it on…
      Jurgen.

    • #37534
      mustafa
      Participant

      Hi, new to forum.

      I am trying to design a 640 sailboat for the yachting club of my university.
      I quickly got some progress but unfortunately stuck in heeling the hull. I understood the process to do this in the way marven told in this topic and another topic related to heeling. It was an old topic and although i commented on it i couldnt get answer. Since this topic is more recent i hope to get an answer to my problem. It seems that the symmetry button in the Layers-Edit, is enabled in my program and i can not change it to disabled, because it is closed. ı tried every scenario of operation but it is closed. I can not turn it off. Some help will be appreciated.

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