# Stability Help!

Discussion in 'Stability' started by lady pirate, Jul 19, 2012.

1. Joined: May 2011
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Hi!

I'm currently doing a spreadsheet for stability. I have a problem with the center of gravity for my on-going design, my cgs due to total weight are not exactly amidship. Most of the books that I have read, CGs are always amidships. What happens if this is the case? Please help! Thank you very much!

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### TANSLSenior Member

I think you got an error there. The cg does not have to be amidship. I would say something else, is unlikely to be there.
You need take care of the relative position of center of buoyancy and cg to get the trim/heeling of the ship as desired.

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3. Joined: May 2011
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Hi, thanks! In reality, I know it's not always amidship. Maybe to make it clear, I'm confused that since my CG is not amidship, I assumed I have an Overturning Moment already in referenced to amidship, so what I did, I checked for Buoyancy first then computed my Righting Moment and compared it to my "assumed overturning moment" to check if my barge is stable. Or I just need to check that my metacentric height is just above my CG for it to be stable, no need for moment comparison? Thank you.

4. Joined: Sep 2011
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### TANSLSenior Member

For any object, not only for ships, the equilibrium is only achieved when the resultant moment of all forces acting on it is zero (This does not mean, however, that the equilibrium position is correct). In short, the heeling moment due to the ship's total weight must be equal and opposite to the righting moment produced by the pressure of the submerged hull.
The metacentre, on boats that float, must always be above the c. of g. (At this point I do not know what happens to the submarines). There are several regulations requiring also that the GM has minimum values​​.
My advice is to delve more in naval architecture studies, because you have some doubts that are very "disturbing".
Besdt Regards

5. Joined: May 2011
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Thank you again TANSL, I have indeed very "disturbing" doubts. I have started my program from scratch and was able to compute for the submerged volumes with matrices of draft levels. I was able to get several draft levels at bow and stern that gives me an almost equal weight to my barge weight, however I know this is not yet final as I have to check my stability. The problem starts here. I am able to compute my RMs due to buoyancy from the different draft levels, but I don't know where I will compare this with.

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### RAraujoSenior Member - Naval Architect

Do you mean "the CG is not amidships" or "the CG is not in the centreline"?
As TANSL, very well, said it is only by luck you will have the CG amidships - the CG will vary with the loading condition of your vessel.
If the CG is not in the centreline then you will have list and the stability characteristics wil be decreased to the side of the list...

7. Joined: May 2011
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Hi RArujo, my CG is not on the geometric center line of my barge to to different loads distribution.

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### Stephen DitmoreSenior Member

Transversely or longitudinally?

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