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center of buoyancy


New member
does Pro/E have the ability to do buoyancy calculations? In other words, can you change the environment that an assembly is in (from air to water) to do an analysis?


New member
I saw a buoyancy problem solved for a floating intertube using Behavioral Modeling in Pro/E 2000i. Unfortunately I don't have a copy of that analysis. However, with a little equation work and some analysis features, it shouldn't be too hard to duplicate. Just make sure the sum of the forces equals zero.

The buoyancy equation is:

Buoyancy Force = specific weight * volume of body

at equillibrium:

Buoyancy Force = Weight Force of body

Changing Buoyancy to a depth is:

depth=(fluid pressure-atmospheric pressure)/(specific weight of fluid)

I think pressure can be converted to a force at the object's centroid by this equation. If the numbers don't make sense, you will have to look up this equation.

Force of fluid pressure = specific weight * depth to centroid * surface area of object

Here are some average specific weights:

specific weight of water = 62.4 lb/ft^3

specific weight of sea water = 64.0 lb/ft^3

specific weight of air = 0.0765 lb/ft^3

Weight is applied at the object's center of gravity. Buoyancy force is applied at the object's center of volume. These do not have to be the same location, but they will line up vertically in steady state.

Good luck.


New member

just wondering if you ever got this to work? I'm trying to do something similar at the moment and was wondering if you had any success?

Or if anybodyelse has any ideas about how to approace a problem involving an object floating in water??


Edited by: james.lynch


New member
I see I have sparked a bit of interest anyway, 60 views in the last hour..

I see I'm not the only one interested in this..

let's get this one solved!


New member
Try this:

1. Create a horizontal plane (water line) offset from your horizontal reference.

2. Set the part density and calculate the mass. Write down the mass value (M).

3. Change you density to, say, water.

4. Create a "Cut/Use Quilt" using the "water-line" plane to remove the upper (dry)portion of your initial part.

5. Create a "Datum Analysis" to compute mass properties and request an out parameter for the mass. Let' call it "mass"

6. Run a optimization study with your goal being mass = M (M from step 2) and your variable being the vertical offset of your water-line.

7. Get the value for the offset


New member
One more thing. All the above apply if the part has 2 planes of symmetry (FRONT and SIDE). For all the other cases the analysis will be more complex since you need to make sure that CG and CB are on the same vertical.


New member
Look at one sided volume measurements and sensitivity (as i recall) studies in standard Pro/E functionality. You can solve this right out of the box.


New member
Is there any kind of Center of Volume calculation you can have Pro/E do just
like it does Center of Gravity. I know you can set all the densities to 1 and
use that but its quite a hassle to keep switching the densities back and
forth. Especially when you have dozens of parts.


New member
Could you do a solid surf copy of all the parts and copy them into one part and solidify them, set the density of this one to 1 and do you calculation? when the parts change, simply regen the new part..

Should that work?



New member
I am not sure if that would, i havent ever used that feature (i am semi-new
to Pro/E). But i am designing something where the center of volume and the
center of gravity need to be as closed to each other as possible if not on top
of each other. And i know through the model mass analysis you can get the
center of gravity in the form of a cordinate system point, i was wondering if
there was the same thing but instead of showing COG it would show COV or
show both simultaniously.


New member
Can you explain a little more about your problem? Also, how do you measure the "center of volume"? (is it the COG when all density is =1?)

you can measure the COG easily then if you use what I said in the last post to measure the COV, then use an alalysis feature to measure the distance between teh two. then using BMX vaiy the dimensions you are allowed to with the goal of minimisign the distance between the two!

If you are not comfortable using one part to solidify all parts then make a surface copy of each part solidify them individually and make a new asm and measure that!

hope this helps,

If you could upload a simple example of your asm (two or three parts) I will have a go at it for you.. I'm curious to see if it works!



New member
The asm in question is about to be submitted for a patent therefore i
can't post it at this time. The COV would be the COG when all the
densities are equal in value.

Right now all of the parts are in part renderings and inserted into an
asembly drawing with no actual work done in the asembly drawing except
for positioning. In each part drawing each part is defined under "mass
props" for a specific density which is carried over into the assembly
drawing to provide the basis for the COG calculation. So if i were to
create a 2nd assembly drawing that references the same parts it would
still have the same densities and not a density of 1 for a COV calcuation.

Also I have been adding both parts and changing dimensions to help
bring the COG and COV closer. So using the feature to make a new solid
from copied surfaces would be quite painstaking to do everytime i made a
change and wanted to check the COV and COG.


Active member
creating two different analysis features COG and COV and then a third analysis feature relation can do what you want provided you have BMX license.

There is a tip on how to differentiate COG and COV and that is the option of "calculated" and "assigned" in the mass properties calculation.



New member
I dont believe i have the BMX license nor do i know what it is... I am working
off of my school's machines that i think just have the basic Pro/E


New member
If you are working with an educational (university) lisence then you have BMX. it stands for Behaviour Modelling eXtension. it is exceptionally powerful!

take a look at the PTC website in the Educational rescourcelibrary, there is good information there!

your lecturer also will have access to ALL of PTC training material and if you ask mybe they can give you access to the BMX trainiing one?


Can you expand please on your tip about "calculated" Vs "Assigned"? if you assign a COV/COG will it update as your model changes? I assume not..




New member
Hi All,

Just remember to have you COG lower than your COB or it will float upside down.Our term is turtle because it will not flip back over...



New member
danielc said:
does Pro/E have the ability to do buoyancy calculations? In other words, can you change the environment that an assembly is in (from air to water) to do an analysis?

Hello Sir,

this velmurugan from chennai,india. now i am looking for how to get center of buoyancy value directly like center of gravity. help me sir, how to do,

with regards,



New member
From what I can remember your COB is the exact COG of the displacement volume. Your design will dictate further calculations that may be needed. For instance if you are designing some form of boat then you have take into account COB in the transverse cross section at various points along the ships length and also the longitudinal metacentric height ( derived from the longitudinal cross section). Its been a few years since my pops and I built a day sailer, but for a stable object the COB is straightforward.

It can geta little more complicated depending on what is is you want to achieve right now. But your COG of the displaced water volume will give you the COB.



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