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

[Ed Greany <crest25@attglobal.net>]

Lew,

You said:
"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."
and
"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."

I suppose your point refers to a very small sub - perhaps one person -
but on an overall scale and especially larger boats, the center of
gravity or "Trim" for a better word to use is very important. The center
of gravity is useful when determining where to weld the hook to raise
the craft in the middle by a winch or crane so it balances fore and aft.
Trim, on the other hand which I believe what you are referring to is the
movement of ballast (water or weights) to level the sub both on the
surface and submerged. Yes, it is most important in a very small craft.

I recall on the big boats, when we wanted to "get back at" the Diving
Officer (submerged) we would gather all the crew we could aft in the
engine room. Then, we would all race to the forward torpedo room. When
you have all this weight moving 400 feet all at once, it will require
moving some water (aft). So the command to trim from forward to aft is
given. About the time the boat is trimmed again, all the guys would race
aft again and the cycle starts over. Pretty soon, the OOD senses what
his Diving Officer doesn't and makes the announcement over the 1MC
intercom system. The gigs up and every scats. Who me? Naw, never!

Ed

Lew Clayman wrote:
> 
> 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
>