Centre of Buoyancy - Clarifying a Doubt

[jnanesha.ks]

Hi,

I am having a doubt with centre of buoyancy. I referred some books but still the doubt is not clarified.

Centre of buoyancy is the point where the mass of displaced water acts upon (upwards). When a steel part is completely immersed in sea water, the shape and volume of water displaced is same as that of the body. Hence the centre of buoyancy coincides with centre of gravity of the body? If so, there will be no tilting action takes place. Am I correct?

Thanks in advance,

Jnanesha KS
jnanesha.ks at quest-global dot com

 

[john.phillip]

If the fully immersed body is homogeneous it will be stable as you predicted. If the body has off-center internal cavities, the mass distibution will be uneven giving it a righting arm in respect to the center of bouyancy.

 

[jnanesha.ks]

Thank you for the reply. Well, I understand the phenomenon of non-homogenous structure behavior. The subject body under question is a assembly of several items. I have a 3D model built. Basically it is a subsea equipment used for oil exploration. The tool used to build 3D model is Pro/Engineer.

I have calcualted the COG and got x,y and z dimensions for it. Now I need to find where COB comes when the body is completely immersed. I have worked on simple calculation of COB around 14 years ago when I was in university. Since many years passed by and not in touch with these things I have forgot how to find out COB.

I hope anyone of the members would help me out.

Thanks and regards,
Jnanesha KS

 

[nvn]

jnanesha.ks: Perhaps try thread https://www.physicsforums.com/showthread.php?t=289528".

When a steel part is completely immersed in sea water, the shape and volume of water displaced is the same as that of the body. Hence, the centre of buoyancy coincides with the centre of gravity of the body?

Yes, if it is a solid steel object (and also has no trapped air underneath it).

If so, there will be no tilting action that takes place. Am I correct?

That is incorrect. Since, in your example, the centre of buoyancy coincides with the centre of gravity, the object is instable, meaning it can easily rotate, somewhat spontaneously.

Now I need to find where the COB comes when the body is completely immersed.

The centre of buoyancy of your completely submerged body will be the centroid of the entire volume of the body.

 

[jnanesha.ks]

Thanks for those links. I went through them.

OK. centroid of volume is the answer. However the pain is that the assembly which I have is not a homogeneous in nature.

 

[nvn]

jnanesha.ks: That's right. Therefore, on a scratch copy of your CAD model file, you can change all materials (including all enclosed air) to only one, solid material. Then, you will let the software compute the centre of gravity of this uniform volume, which is the centre of buoyancy (point B), because your assembly is completely submerged. If the centre of gravity from your original model file (point G) is not directly below point B, then your completely submerged body is instable, and therefore will rotate.

 

[jnanesha.ks]

Many thanks.