How to compare effect of hull form (only) on motion?

Discussion in 'Hydrodynamics and Aerodynamics' started by ras, Dec 15, 2021.

  1. ras
    Joined: Aug 2021
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    ras Junior Member

    Good time of the day!
    I would like to compare two hulls of similar size but different forms (significantly, including different WL areas/KM), and evaluate how forms (only) affects its motions/RAOs , excluding or minimizing effect of loading on motion. So far as loading should be out of consideration, I can select VCG arbitrarily. So, if I select VCGs (different for each hull) so that GMs of both hulls will be equal, then results of analysis gives me RAOs, which depends only on form and not on loading. Am I right? Or better select equal VCG?
    Thank you for your advice!
     
    Last edited: Dec 17, 2021
  2. gonzo
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    gonzo Senior Member

    Are you using any particular software, or trying to decide which software is adequate?
     
  3. Ad Hoc
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    Ad Hoc Naval Architect

    Then you need to create a systematic series.
    You would use 1 hull form as your datum/baseline, run a series of RAOs.
    Then make, the modifications that you seek to explore, on that datum hull form and compare the results of the RAOs to the baseline hull.
     
  4. ras
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    ras Junior Member

    I am using Maxsurf, and goal is to compare 2 hulls of similar dimensions , but different forms.
     
  5. ras
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    ras Junior Member

    Exactly. I am doing this, but would like to be sure that results are correct (not affected by position of VCG). I am using fully loaded hulls (up to design waterline) but with arbitrary VCG. Of course, any changes in VCG affects resulted RAOs. So, if I would like to minimize (exclude?) effect of VCG on changes in motion, shall I use same VCG for all hulls? Or better to use same GM ( for that I need to use different VCG because the hulls have different KMs)?
     
  6. Ad Hoc
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    Ad Hoc Naval Architect

    Using the same model/hull....but with subtly different mods to each variant... does this for you.
    You do not need to introduce another hull form.

    And for completeness, since:

    You need to ensure that the hull you are using for your sensitivity analysis, is within the limitations of the software selected.
    Otherwise, how can you verify it is correct?
     
  7. ras
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    ras Junior Member

    Unfortunately (as I mentioned before) the difference between hulls is substantial (it is unavoidable). Most noticeable is difference in waterline areas, and therefore in GMs.
    Sure. That is what I need to follow.

    So what is your opinion on my question about VCG?
     
  8. Ad Hoc
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    Ad Hoc Naval Architect

    see above.

    That's why you use one hull only as the basis and make minor changes to that hull, to ascertain the differences in motions.

    You're over thinking this...
     
  9. ras
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    ras Junior Member

    I am very much appreciate your advises, but somehow have a feeling that we are talking about different things.
    I have a 2 hulls given as a task to compare its responses as a result of its shapes- I do not have a privilege to amend it freely("make minor changes"). And I need to exclude the variable (VCG) from consideration of effect of hull form on motion. Using "one hull only" mean that I have to use fixed VCG for both ships, is not it? For hulls with different WL it will results with significantly different GZ/GM/moments- is it correct way to minimize influence of loading?
    Could you please elaborate your opinion on this problem?
    Thanks
     

  10. ras
    Joined: Aug 2021
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    ras Junior Member

    What is your opinion on this problem with VCG? Appreciate your advise also.
     
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