Center of Buoyancy versus Center of Gravity

[MFJewell]

Some of you may have seen my other thread about vortex breakers for a floating skimmer I am designing (see link below). I have been playing with the design to get the inlet velocity down and currently have a manifold with 8 inlets (see below). The get the flow distribution into the inlets even, I have an offset in the connecting piping to the manifold that causes my Cbx and Cgx to be ~ 1 inch apart. My Cgy and Cby are only .005" apart, and my Cgz is below the Cbz, so I am good there. The connecting piping will be 2" SS pipe using OPW swivel joints. Due to this design, the skimmer should not rotate in the XZ plane. That said, I still keep revisiting this and am contemplating a change to the manifold to get the entire skimmer balanced in the X. Am I over thinking this one?

Edit: The skimmer is a total of 70 inches wide in the X direction.

http://www.eng-tips.com/viewthread.cfm?qid=421829

 

[LittleInch]

Yes.

Nice sump design though - glad you took on board what we were saying though I would make those two drains nearest the centre 1" or 1 1/2" to equalize out the flow a bit otherwise 50% will go through those two.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[SwinnyGG]

Personally, I'd want the COG and COB to coincide in all planes... but my perfectionist tendencies also frequently result in first-draft designs that exceed what is necessary and have to be pulled back to reality ha ha.

If you are concerned about the COG/COB relationship, I would just add a piece of plate to one end as ballast to pull them back in line.

 

[MFJewell]

Personally, I'd want the COG and COB to coincide in all planes...

Totally with you on that one. It's why I can't seem to leave it alone.

 

[MFJewell]

So I did a quick calc on angle of twist of the connecting piping based on reaction torque for not have Cbx and Cgx aligned and it turns out to be .0147 degrees. This would equate to .01" of displacement at the far end of the skimmer (in the x direction). Guess I'll leave it alone.

 

[chicopee]

I would change the diameters of the manifold inlets to equalize the flow and reposition the outlet of the manifold to the center line in the X-X axis in order to match the vertical positions of Cbx and Cgx. An alternate method is to add more weight on one side to accomplish the same objective.

 

[MFJewell]

Already changed inlets. Innermost are 1.25", next is 1.5", outers are 2". The outlet of the manifold in centered. The outlet of the OPW swivel joint is not. The outlet of the OPW joint is connected to a 38" piece of pipe, then another OPW joint. Since it is all hard piped and I am using swivel joints that only allow rotation in one plane, it shouldn't matter if the Cgx and Cbx are slightly off (as shown in my last post). If I move the manifold to get the Cgx and Cbx aligned, it would be offset be a couple of inches from center (the swivel joint weighs 11 pounds and the connected piping adds about 5.5 pounds). I did consider making the top plate for the left pontoon out of 13 gauge (the rest is 16 gauge). That would add 3.31 pounds at ~ 5.75") and get it almost perfectly balanced. Not sure it is worth it though.

 

[MFJewell]

Just a quick update. The skimmer just finished fabrication and the shop superintendent took it home to toss in his pool. Sits level in the water and submerged depth is just about dead on my calculated value. Now just need to install it at the plant and flow water through it.