[PSUBS-MAILIST] Stability & Buoyancy

[jimtoddpsub at aol.com]

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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