Centre of Buoyancy - Clarifying a Doubt

  • Thread starter Thread starter jnanesha.ks
  • Start date Start date
  • Tags Tags
    Buoyancy
Click For Summary

Discussion Overview

The discussion revolves around the concept of the center of buoyancy (COB) and its relationship to the center of gravity (COG) in submerged bodies, particularly in the context of a subsea equipment assembly used for oil exploration. Participants explore the implications of buoyancy in both homogeneous and non-homogeneous structures.

Discussion Character

  • Technical explanation
  • Conceptual clarification
  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • Jnanesha KS questions whether the center of buoyancy coincides with the center of gravity for a fully immersed steel object, suggesting that this would prevent tilting.
  • Some participants propose that if the body is homogeneous, it will be stable, but if it has off-center internal cavities, the mass distribution will be uneven, potentially leading to a righting arm.
  • Jnanesha KS indicates that the assembly in question is non-homogeneous and seeks assistance in calculating the COB.
  • One participant clarifies that the center of buoyancy for a completely submerged body is the centroid of its entire volume.
  • Another participant suggests using CAD software to compute the COB by treating the assembly as a uniform material, noting that instability occurs if the COG is not directly below the COB.

Areas of Agreement / Disagreement

Participants express differing views on the stability of submerged bodies based on their homogeneity and internal structure. There is no consensus on the implications of these factors for tilting or stability.

Contextual Notes

Participants reference the need for specific calculations and modeling techniques to determine the COB, highlighting the complexity introduced by non-homogeneous materials and internal cavities.

jnanesha.ks
Messages
9
Reaction score
0
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
 
Engineering news on Phys.org
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.
 
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
 
jnanesha.ks: Perhaps try thread https://www.physicsforums.com/showthread.php?t=289528".
jnanesha.ks said:
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).
jnanesha.ks said:
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.
jnanesha.ks said:
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.
 
Last edited by a moderator:
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.
 
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.
 
Many thanks.
 

Similar threads

High School Does Buoyancy Apply When Not Fully Submerged?
  • · Replies 7 ·
Replies
7
Views
2K
Undergrad What is the location of the center of buoyancy and how is it determined?
  • · Replies 14 ·
Replies
14
Views
10K
Undergrad Buoyant Force Definition & Explanation - Archimedes' Principle
  • · Replies 1 ·
Replies
1
Views
4K
How Do You Calculate the Volume of a Floating Object?
  • · Replies 10 ·
Replies
10
Views
12K
Undergrad What Are the Complexities of Buoyancy and Common Misconceptions?
  • · Replies 48 ·
2
Replies
48
Views
12K
Graduate Buoyancy: Archimedes' Wooden Spheres
  • · Replies 1 ·
Replies
1
Views
4K