Center of gravity of submerged object

[Bob456]

I need to know the center of gravity of a machine in water. Can I assume that the center of gravity is the same in water as out of it. There are no voids in which air can be trapped and none of the materials used are buoyant.

 

[dooron]

centre of gravity "centre of mass?" they are the same, the point at which the force acts as if the mass is a single point. it is independent of place. space, water etc. the same in all. note that you do have a centre of boyancy "lift" that acts as well, it may cause overturning etc. this centre is the centre of mass of the displaced water, which may be a little more difficult to figure out.

 

[prex]

Bob456,
as pointed out by duron, the center of buoyancy is not necessarily at the same location as the center of gravity of the object, unless some conditions on mass distribution are satisfied: in the particular case where the object has constant density over its parts (perhaps this is what you mean by your last sentence?) then the two points coincide.
Hence in general the center of action of forces acting on the object due to weight plus buoyancy is not at the center of gravity, but will be there in the special case above.

prex

http://www.xcalcs.com

Online tools for structural design

 

[NickJinks]

Centre of boyance is effictvly the centre of volume, which is independent of mass, where as the centere of mass is dependent on dencity of the materials

 

[TheTick]

The center of buoyancy of a floating object is also dependent on shape and orientation. This is what allows a boat to right itself when it is tipped.

To reach an audience with better understanding, perhaps it would be best to repost this query in a shipbuilding related forum.

 

[Bob456]

Thanks to everyone for there help. It looks like I will be taking a swim.

 

[colinp]

Hi Bob456,

If you are trying to find the center of mass of a submerged object for statical stability purposes then it will not matter whether the object is in a fluid when you do your calculation or measurement. The center of mass is a property of the body and is the same anywhere. The center of buoyancy is at the center of the immersed volume. If the body does not penetrate the free surface than the location of the center of buoyancy in relation to the body will not change depending on the body's orientation.


BUT WATCH OUT -

If you are planning on accelerating a body through the water dynamically you will need to consider the liquid added mass. When an body is accelerated in a fluid the fluid flow around it also accelerates. Accelerating this fluid requires a force. The additional force required is not the same as the steady state hydrodynamic drag and is proportional to the acceleration. The body will behave as if it has additional mass, so for convenience of calculation we must include an "added mass" term. Depending on the shape and the acceleration of the body the liquid added mass may be on the same order as the mass of the body - or even higher.

The easiest way to calculate the liquid added mass is through model testing, if you can build a full scale model.


Good luck - sounds like you have a cool project.
-colin