COG of assembly in seawater

[jnanesh.sajjan]

Hi,



I remember last time I asked same question and didn't get answer.



I have an assembly of 200+ parts made up of various materials. I want to
calculate the COG location when it is resting on seabed.



Any idea?





Thanks and regards,



Jnanesha KS



T 91-80-4119 0900 Ext: 233

F 91-80-41190901
M 994 500 4179

Quality Engineering and Software Technologies

No 55 l QuEST Towers l Mahadevpura l Whitefield Main Road
Bangalore l 560048 l www.quest-global.com
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[peter.stockhaus]

Try this, add your assembly toan assembly you will use for analysis. Create two parts in this assembly with identical protrusions that encloses your assembly. Make the density of these parts the density of sea water. Subtract the parts in your assembly from the first part (be sure to remove any volumes of the part that would be air in your assembly). Once you have removed all the parts of your assembly from the first part, remove the first part from the 2^nd. This part would represent the displaced water. In your assembly create an analysis feature for the mass properties of the assembly in the dry condition. In the 2^nd part create an analysis feature for the displaced water. Finally do an analysis to subtract the displaced water from the dry assembly to calculate the COG for your wet assembly.

Good luck

Peter Stockhausen
Infotech Aerospace Services

[A.DelNegro]

This might get you close if all your part are metal, i.e quite a bit
heavier than water, but the CG will be quite different if there are
parts that are lighter than water as the will have a weight that is not
in the direction of gravity. I addition, all the air spaces will also
have weights in this upward direction.



Tony





J. Anthony DelNegro, P.E.

Product Development Engineer

Micropore, Inc.

350F Pencader Drive

Newark, De 19702

[jnanesh.sajjan]

Well, FYI, the lightest part of my assy has density more than that of seawater. So I guess, I could take a chance 😉


- Sending via mobile. Please pardon the brevity and possible typos -

Thanks and regards,

Jnanesha KS

T 91-80-4119 0900 Ext: 233
F 91-80-41190901
M 994 500 4179
Quality Engineering and Software Technologies
No 55 l QuEST Towers l Mahadevpura l Whitefield Main Road
Bangalore l 560048 l www.quest-global.com

[gkbeer]

It's a nonsense question. The mass properties of your assembly are the same.
On land, underwater, or in space.

On Tue, Oct 19, 2010 at 3:05 AM, Jnanesha K S
<->wrote:

> Hi,
>
>
>
> I remember last time I asked same question and didn’t get answer.
>
>
>
> I have an assembly of 200+ parts made up of various materials. I want to
> calculate the COG location when it is resting on seabed.
>
>
>
> Any idea?
>
>
>
>
>
> Thanks and regards,
>
>
>
> Jnanesha KS
>
>
>
> T 91-80-4119 0900 Ext: 233
>
> F 91-80-41190901
> M 994 500 4179
>
> Quality Engineering and Software Technologies
>
> No 55 l QuEST Towers l Mahadevpura l Whitefield Main Road
> Bangalore l 560048 l www.quest-global.com
>
>
>

[jnanesh.sajjan]

Hi Glenn,



Yes, even I had same thought in mind. Thanks for confirming.



Good day,



Jnanesha KS

[iturner]

The mass properties would be the same but if one of the components is
buoyant then surely it is like a negative mass and would alter the COG



Ian Turner

CAD Manager

Cobham Mission Equipment

[jnanesh.sajjan]

Hi,



Well, after much discussion over the mail, phone and personal - It is
concluded that COG location remains same irrespective of media. However
the weight will differ due to presence of media. (in seawater the body
weighs almost 87% of original value)



To evaluate the stability of a floating or fully submerged body,
location of COB is calculated.



Thanks,

Jnanesha KS

[kdemont]

Jnanesha,
While it is possible that the COG location does not change in your specific
case, I would agree with Ian, that a boyant component (unless located at the
COG) would change the COG

[puetzc]

I would propose the following thought experiment:

Construct a balance beam with spheres of equal mass, but different volume equidistant from the balance point. Ignoring the mass of displaced air, the beam would balance. If that beam were to be submerged in water, where the mass of displaced water could not be ignored, the larger sphere would rise, as the moments would no longer be equal.

[TimothyField]

The COG is going to be the same whether the assembly is in water, air, or a vacuum. The buoyant force of the medium in which it resides will be in addition (even if it acts in the opposite direction) to the force of gravity

Timothy

[jnanesh.sajjan]

Exactly! It is evident (i referred some text books on physics/mechanics) that cog remains same. Whatever unbalanced forces (due to act of buyancy
) generates moment about a PIVOT. If the system is stable, after succesful momen act, body comes back to original position (cog below cob). If body is displaced upside down, then also it is stable but cob is below cog. If moment act makes a neutral result (body becoms horizantal) cob and cog lies on a same horizantal line.

Cheers!


- Sending via mobile. Please pardon the brevity and possible typos -

Thanks and regards,

Jnanesha KS

T 91-80-4119 0900 Ext: 233
F 91-80-41190901
M 994 500 4179
Quality Engineering and Software Technologies
No 55 l QuEST Towers l Mahadevpura l Whitefield Main Road
Bangalore l 560048 l www.quest-global.com

[puetzc]

I stand corrected. Wikipedia redirects "Center of Gravity" to "Center of Mass." If we use proper units (since gravity actually has nothing to do with the physical property we meant to discus), the confusion disappears. Gravity and its cousin buoyancy are not relevant.

I learned something this Friday - it's a good day.

[ptc-593619]

Unless, of course, you're caught in that devilish tractor beam from the
Klingon starship!

[MikeFoster]

The COG is unchanged regardless of the medium. The force due to gravity acts through the COG. The COB (center of buoyancy) is the location through which all the buoyancy forces act. The relative locations of the COG and the COB determine the stability of the object.

Thanks,

Mike Foster
ATK