[PSUBS-MAILIST] Stability & Buoyancy

Sat Nov 30 01:55:16 EST 2013

I don't know specifically about minisubs because I've never worked out 
the metacenter for one or seen it done, but it is very difficult in 
actuality to have the CB below the CG, and as a practical matter it can 
only be accomplished in a low freeboard, low reserve buoyancy craft - in 
other words, a dangerous one.

Military subs definitely use the same criteria as surface vessels for 
determining stability on the surface. There's ample evidence of that in 
the textbooks.

My understanding is that MBTs ARE emptied completely on the surface in 
naval submarines, initially with compressed air to reach the surface and 
then with low-pressure blowers or with bilge pumps. Some subs, however, 
are not equipped with Kingston valves - in other words, the bottom of 
the ballast tank is always open to sea - which of course means that 
there would have to be enough water in the tank to provide a hydraulic 
seal for the air trapped there. Supposedly, Russian subs have Kingstons 
and British and American ones don't, but don't hold me to that.

Because of the potential for "tank burps" if the sub rolls or pitches 
enough to allow the bubble to escape, any sub that I build will have 
Kingston valves or the equivalent.

Best,
Marc

On 11/30/2013 1:59 PM, jimtoddpsub at aol.com wrote:
> Marc,
> When you say the CB is usually below the CG when on the surface, are you
> speaking about military subs or including small subs as well?  I'd like
> to see how some of the K-boats calculated out both surfaced and submerged.
> I found this US Navy training film from WWII.  It shows the ballast
> tanks wrapping around the bottom of the pressure hull very much like the
> illustrations that Alan provided.  But apparently the tanks were
> generally not blown all the way down.  In my mind I had not pictured
> the MBTs extending below the pressure hull on the old military subs.
> http://www.youtube.com/watch?v=e7ixYuCObUk
> Jim
> -----Original Message-----
> From: Marc de Piolenc piolenc at archivale.com <mailto:piolenc at archivale.com>
>
> On the surface, the center of buoyancy is USUALLY below the center of
> gravity, but this does not make it necessarily unstable. That's why the
> metacenter of the boat needs to be determined.
>
> When fully submerged, the CB must be above the CG, and it is very easy
> to determine whether this is so.
>
> Marc
>
> On 11/30/2013 10:54 AM,jimtoddpsub at aol.com  <mailto:jimtoddpsub at aol.com>  wrote:
>> Hi Alan,
>> Re: "the centre of buoyancy moving upward past the centre of
>> gravity... "  This implies that somehow the centre of buoyancy had been
>> /below/ the centre of gravity which would be really, really scary.  The
>> ABS rule (per Cliff's spreadsheet) is that the CB must be at least 2"
>> /above/ the CG when the sub is submerged.  In the event the drop weight
>> is released, the CB must still be at least 1" above the CG.  Frankly,
>> that narrow a spread doesn't meet my comfort zone.
>> When the sub is surfaced, any portion above the water line is now dead
>> weight since it is no longer displacing any water.  When that
>> same portion was submerged it was contributing buoyancy.  Therefore the
>> above-the-water-line portion contributes to the CB moving downward.
>> Offsetting that is the fact that the main ballast tanks were
>> contributing little or no buoyancy to the extent they were full of
>> water when the sub was submerged.  Once they are filed with air they
>> move the CB upward.  If the tanks are fore and aft as on the K-boats,
>> they are located even with the top of the cylindrical hull.  However
>> remember that the portion of the ballast tanks now above the water
>> contributes no buoyancy.  With the fore and aft tanks, the tanks don't
>> contribute much to lateral stability (anti-roll); you're dependent on
>> the CB/CG spread for lateral stability.  I'm purposely staying away from
>> any direct discussion of metacenter for now.
>> My MBT's are fore and aft.  My original plan for setting design
>> procedures for adding saddle tanks was this:  Calculate where the
>> surfaced water line would be _/if/_ I installed the saddles at 4:00 and
>> 8:00 positions, then actually install them higher so that the top of the
>> saddles would be right at the water line.  This would give me maximum
>> lift and freeboard since no part of the saddles would be above the water
>> line.  However Alec correctly pointed out that having a portion of the
>> saddles above the water line contributes to anti-roll since the
>> down-rolling tank would then provide extra displacement and buoyancy to
>> push that side back up (handy if someone steps on that side of the
>> sub).  The lower your tanks, the greater your freeboard, but less CB/CG
>> spread.  The higher your tanks, the greater your surface stability, but
>> you sacrifice freeboard.  The design challenge is finding the optimum level.
>> Jim
>> -----Original Message-----
>> From: Alan James <alanlindsayjames at yahoo.com  <mailto:alanlindsayjames at yahoo.com>>
>> To: psubs.org <personal_submersibles at psubs.org  <mailto:personal_submersibles at psubs.org>>
>> Sent: Fri, Nov 29, 2013 6:41 pm
>> Subject: [PSUBS-MAILIST] Stability & Buoyancy
>>
>> Hi everyone,
>> While reading through some background info for the G.L.
>> certification document, I came across this link with a good
>> explanation of stability & buoyancy
>>http://maritime.org/doc/fleetsub/chap5.htm
>> There is a phenomena described, where submarines can be
>> unstable to the point of turning over during the transition from
>> surface to diving & vice versa.
>> This has to do with the centre of buoyancy moving upward past
>> the centre of gravity. As it approaches the centre of gravity the
>> submarine is at it's most vulnerable point.
>> This would be different for individual designs & a worry if
>> you dropped your emergency drop weight.
>> I had been told by a pilot that he never stuffed around while
>> descending & liked to drop as quick as he could.
>> I didn't have a full understanding of this & thought there may
>> be others in the same boat.
>> Regards Alan
>>
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