Righting arm curve

Discussion in 'Stability' started by Mirko, Feb 28, 2007.

  1. Mirko
    Joined: Apr 2004
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    Location: Milan, Italy

    Mirko Junior Member

    I have a problem, with the right arm curve. All we known that the GZ curve is plotted keeping the same immersed volume for all the transversal inclinations. My problem is: fixed the underwater volume, the transversal inclination, do I have to change the trim or the draft to achive the fixed volume.

    Thanks.

    BYE

    Mirko
     
    Last edited: Feb 28, 2007
  2. Eric Sponberg
    Joined: Dec 2001
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    Location: St. Augustine, FL, USA

    Eric Sponberg Senior Member

    Mirko,

    Yes you do. Many software programs for hydrostatics and stability have automatic draft, trim and heel functions based on a certain initial condition when calculating stabilit. For example, in the program I use, ProSurf, I input an initial condition, and this can be draft or displacement, and LCG or Trim. Draft determines displacement and vice versa, and LCG determines trim and vice versa. Then as the boat rolls over to check stability, the computer will automatically adjust the momentary draft and trim to achieve the correct displacement and LCB position to correspond with weight and LCG. This all happens very quickly, within 10-15 seconds, for a 180-deg roll. To do it by hand, however, is extremely laborious. Also, if your computer does not have automatic draft, trim and heel capability, then you have to iterate the calculations yourself, and that also gets very laborious.

    Eric
     
  3. Mirko
    Joined: Apr 2004
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    Mirko Junior Member

    Righting Arm Vs. Heel

    Hi, and thanks for your reply. Obviously I don't want to carry out stability calculus by hand. What I have been doing is building an excel spreadsheet for the calculation of stability properties of a floating object shaped like a barge, for this reason the problem I posted occurred in my mind. Do you know any kind of algorithm for the increment of the trim and the draft which can help me? Due to the small movement of the center of buoyancy in the longitudinal direction I thought that I may neglect the trim changes and carry out the calculus of the rhighting curve at a fixed trim varying the draft.

    Do you agree?

    Bye

    Mirko
     
  4. Eric Sponberg
    Joined: Dec 2001
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    Location: St. Augustine, FL, USA

    Eric Sponberg Senior Member

    For a rectangular barge, there should be no changes in trim, so I guess you could ignore that. As for any other normal hull shape, computer typically will iterate draft and trim until a suitable solution is found. How you do that in a spreadsheet is probably best done by programmers such as yourself or others more talented in that area than I.

    Eric
     
  5. subman
    Joined: Aug 2007
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    subman New Member

    Mirko,

    I am trying to do the same thing as I want to validate a piece of stability software, if you post your sheet so far maybe we can work on it together?

    cheers,
     

  6. Mirko
    Joined: Apr 2004
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    Location: Milan, Italy

    Mirko Junior Member

    Hi Subman,

    I don't have worked to my spreadsheet for months because I found several errors so I gave up.

    If you don't have started yet I would be pleased to collaborate with you.

    Let me know your intention.

    Bye

    Mirko
     
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