# Problems with IMS stability data

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

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### wavemakerNew 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.

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### GuillermoIngeniero Naval

If I'm not wrong, righting moments are in kg*m, not in Nm. Don't use "g".
Cheers.

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### raykSenior Member

Try it like this. Bold is mine.

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### wavemakerNew 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.

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### WindvangYacht Designer

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

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