# Find Centre of buoyancy

Discussion in 'Stability' started by BelgiumLearner, Dec 1, 2011.

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### BelgiumLearnerJunior Member

How to find the center of buoyancy on a semi-submerged object?
Assume a square shaped barge with towers on both sides (PS & SB). Deck already submerged.
Can someone please give me a hint....

regards,

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### jehardimanSenior Member

Do it by parts, just like a constant density center of mass problem...

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### BelgiumLearnerJunior Member

It does not ring a bell yet.... but i will try to find something about that problem....

Regards.

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### BelgiumLearnerJunior Member

Google is not my best friend .....

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### gonzoSenior Member

The center of buoyancy is the same as the center of gravity of the submerged section of the body.

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### BelgiumLearnerJunior Member

Absolute correct Gonzo, but i'm searching for an semi-submerged object.....

I responded to quick this morning.... it was stil early...

The center of buoyancy can be at the same location as the center of gravity.... then there is a indifferent stability. Most of the time you want the center of gravity below the center of buoyancy...

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### DCockeySenior Member

Center of buoyancy is the centroid of the submerged volume, whether the vessel is partially or completely submerged.

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### BelgiumLearnerJunior Member

Thanks DCockey, but how to find it for a submerged body like described in the starting post

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### DCockeySenior Member

Same as for a floating body.

Are you using software and the software doesn't seem to work for the particular configuration?

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### BelgiumLearnerJunior Member

No... i'm just trying to find out how stability works on semi-sub rigs and/or barges....

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### DCockeySenior Member

Basic principles are the same any floating vessel. Righting arm equals the horizontal offset between Center Of Buoyancy and Center Of Gravity. Static equilibrium is when COB and COG are aligned with one over the other. The equilibrium may or may not be stable depending on what happens at a small inclination angle. If the COB moves horizontally than the COG then the configuration is stable for small perturbations. If it moves less then the configuration is not stable for small perturbations.

Some of the assumptions used as short-cuts for ships may not be valid for other configurations. For instance the fore-aft stability may not be much greater than the transverse statibility.

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### DCockeySenior Member

Do you know how to find the Center of Buoyancy for a "normal" type of vessel? If so what do you think is different for the configuration you described?

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### BelgiumLearnerJunior Member

For a single hull ship, i use Simpsons rule. For a square i use 1/2 * draft.

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### DCockeySenior Member

Use Simpson's Rule the same way as for a ship. Only difference is for submerged portions the top surface of the submerged portion is used rather then the waterplane, but that is the same as for a ship with a submerged bulb on the bow.

Any more specific questions?

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### BelgiumLearnerJunior Member

Thanks David,

Will continue tomorrow....

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