9 Replies Latest reply on Jan 28, 2013 4:57 PM by Rick McWilliams

# Assembly's center of bouyancy

I am working with high school students and the design of submersible ROVs. Each vehicle contains many components of varying volumes and densities. There are some components specifically designed for floatation while others are for ballast. My question is; can SolidWorks be used to locate the center of buoyancy and the amount of buoyant force in an assembly?

Before it is said, I know that I can use SolidWorks to find the volume of the individual enclosed parts and knowing their mass I can find individual buoyant forces. The sum of those forces should be the total buoyant force, but to find the center of buoyancy it now becomes quite a practice in vectors. I want to be able to have my students make many design iteration and always know whether the vehicle is balanced fore to aft and neutrally buoyant.
• ###### Assembly's center of bouyancy
Is this correct: center of buoyancy = center of mass when submerged for an object of uniform density?

You could do a Save-as --> Part in a way to save only outside surfaces, then fill in the interior. This would give you the center of displaced volume.
• ###### Assembly's center of bouyancy

SolidWork does not posses this capability.
• ###### Assembly's center of bouyancy
Roland, thanks but I don't think that works.
Eddie, I am afraid that this is the correct answer. As much as i don't want to hear it.

Thanks for the quick replys guys!!!
• ###### Assembly's center of bouyancy
Dug up one of my old naval science textbooks...
Center of buoyancy IS center of mass of displaced water.

turn assembly into part (Save as --> Part)
fill all voids until everything is solid
set density to same as water (1g/cc for fresh water)
get CG of filled part, this will be center of buoyancy when submerged
It really is that simple

actually, location of CG will be same regardless of density, but with density set to same as water you can get mass of displaced water, which you will need to calculate total buoyant force

You may continue to think it doesn't work. You can even say I am wrong. You can't say Dutton's is wrong.
• ###### Assembly's center of bouyancy

As was stated before, buoyant force is equal to the weight of the displaced water, centered at the CG of the displaced volume of water. There is no need to add all the vectors of the individual components.

Keep in mind that the center of buoyancy is usually not the same as the center of mass, especially if there are hollow components. Thus, an untethered submerged object at rest will be stable when the center of mass "hangs" below the center of buoyancy.

When the center of mass is not in vertical alignment with the center of buoyancy, the object will experience a righting moment proportional to the cross-product of the difference between buoyant force and weight and the distance between the two centers.

Take a submerged object held in a random orientation. If the center of mass is not aligned with the center of buoyancy, the object will experience a righting moment. An equal and opposite counter-moment will be required to hold the object in that orientation.

The stability of a floating object also depends greatly on this righting moment. A boat will not capsize as long as the center of mass stays below a point where the righting moment does not get reversed, which would result in the boad being flipped in the opposite direction.

Plenty of additional info available on Wikipedia. Most of it looks kosher.
• ###### Re: Assembly's center of bouyancy

Old thread I know, but I wrote a macro recently that does just this. It will create a 3D sketch point at the Center of Gravity and the calculated Center of Buoyancy. I will upload here in the hopes it will help someone as I've seen this question asked a few times.

Cheers,

Chris

• ###### Re: Assembly's center of bouyancy

I look forward to trying your macro.  I have done bouyancy and righting moment calculations manually by slicing at the water plane.