[PSUBS-MAILIST] Stability & Buoyancy

[jimtoddpsub at aol.com]

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

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 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>
> To: psubs.org <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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