Re: [PSUBS-MAILIST] Center of gravity Vs. Center of Bouyancy

[Lew Clayman <lew_clayman@yahoo.com>]

In surface vessels, the center of buoyancy is that point where the displaced volumes fore&aft,
port&stbd, and up&down all balance.  Subs must have such a point, but I'm too newbie to know if
anyone cares about it.

Center of gravity is, as Ed states, similar with respect to weights.  Put a big lump of lead (for
example) up in the nose, and CG moves forward, but as the outer form of the sub is unmodifed the
center of buoyancy is unchanged.

Left uncontrolled, a surface vessel (and I presume a sub also) will come to rest with CB directly
above CG.  Thus it's important that the vessel be designed such that this balance is upright and
level, or pretty close anyhow.

Because a surface vessel - or indeed a sub at the surface - submerges a different volume at every
change of pitch or roll, there are endless worries about the differing CBs at each angle.  A sub
does not behave this way; it displaces the same amount at every position because the whole thing
is submerged.  Maybe this is a reason why it's less important in subs.  But I'm really asking this
of the experienced, not stating this.

For small surface vessels, and I suppose for small subs, crew weight is an important and moveable
component of overall weight, so CG moves every time the people move.  In a sub, where crew
movement is so much more constrained than on a surface vessel, this is less important and CG is
more reliably fixed at or near the nominal design location.  Another reason, maybe(?) why
determining CG is more important on a sub than on a surface vessel... on the surface, in a small
boat, if the trim is a little too far to port, you simply sit to starboard.

Do I make any sense here?

-Lew



--- Ed Greany <crest25@attglobal.net> wrote:
> Adam,
> 
> I don't think I have heard of "Center" of bouyancy unless you are
> referring to "Neutral" bouyancy. Neutral bouyancy is that point where a
> sub neither raises or descends but its weight is equal to that of the
> water it displaces for a particular depth. This is what every sub
> strives for while underway and not intentionally diving deeper or coming
> up. It is a constant depth where the diver/sub capt neither has to add
> ballast nor discharge ballast.
> 
> Center of gravity is the centerline midpoint at which the trim fore and
> aft is balanced if you were to suspend it from a cable in the air at
> that point. It is like a balanced scale = both ends level. This is
> usually close to the middle between the bow and stern but because
> equipment, passengers, or ballast is heavier toward one end, it is not
> usually exactly midway.
> 
> Ed
> 
> Adam wrote:
> > 
> > Hi,
> > 
> > Thanks for the responses on the active SONAR guys.
> > 
> > Now, I haven't purchased a principles of sub-design book yet because
> > they are kinda hard to get where I live.  So here is a rudementry
> > question.
> > 
> > What is the difference between center of gravity and center of
> > bouyancy?
> > 
> >
> 
> 


=====
"Yo no soy marinero / Soy capitan"
          - Traditional Mexican Lyric (La Bamba)
=====

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