Problems with IMS stability data

Discussion in 'Stability' started by wavemaker, Jan 26, 2007.

  1. wavemaker
    Joined: Oct 2006
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    Location: Delft, The Netherlands

    wavemaker New Member

    Hello there. Can somebody please enlighten me on the units used for IMS stability data ?

    I'm am doing some indicative calculations for this boat: this boat (alternative link), an IMX 40. We're considering moving weight form inside the interior to the keel. I am now trying to make some quick calculations to find out how big the effect is on our stability.

    Just to demonstrate, I take RM90, which is 41,6 . According to the IMS Rule book (rule 711 on page 47) this number is a righting moment divided by the degrees of heel. So if multiply that with 90 degrees, I arrive at a righting moment of 3744. I assume the units are Nm for now. Now divided by the sailing displacement (7574) times the acceleration constant of gravity (9,81) I arrive at the GZ value. The result is 0,05 m . Way to low of course for a cruiser/racer like an IMX-40 on its side. Now I could imagine that the moment is expressed in kNm, but that would be mean that the GZ is 50 m. Probably a bit of the mark as well ...... ;-)
    So beyond that it would be guessing on my part .

    So what is up here. What units does the IMS use for this number ? Or have I misinterpreted the definition ?

    Also, can somebody confirm that RM / RMC is just a GM value. It seems so.

    Thanks for any help.
  2. Guillermo
    Joined: Mar 2005
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    Location: Pontevedra, Spain

    Guillermo Ingeniero Naval

    If I'm not wrong, righting moments are in kg*m, not in Nm. Don't use "g".
  3. rayk
    Joined: Nov 2006
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    rayk Senior Member

    Try it like this. Bold is mine.
  4. wavemaker
    Joined: Oct 2006
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    Location: Delft, The Netherlands

    wavemaker New Member

    Guillermo and rayk, thanks very much. That makes sense. Is there some guide where the units of the IMS data are listed ? It's not in the ORC Rule guide that I linked in my first post here.

  5. Windvang
    Joined: Jan 2006
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    Location: Rotterdam,The Netherlands

    Windvang Yacht Designer

    Al other units are Kg., m, m2, sec/mile ore unitless ratio's.
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