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<div><font size="3" style="background-color: transparent;">Hi Alan,</font></div>
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<div><font size="3">Re: "the centre of buoyancy moving upward past the centre of gravity... " This implies that somehow the centre of buoyancy had been <em>below</em> 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" <em>above</em> 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.</font></div>
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<div><font size="3">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.</font></div>
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<div><font size="3">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 <u><em>if</em></u> 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<font face="Arial, Helvetica, sans-serif">).</font> 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.</font></div>
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<div><font size="3">Jim</font></div>
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<div style="color: black; font-family: arial,helvetica; font-size: 10pt;">-----Original Message-----<br>
From: Alan James <alanlindsayjames@yahoo.com><br>
To: psubs.org <personal_submersibles@psubs.org><br>
Sent: Fri, Nov 29, 2013 6:41 pm<br>
Subject: [PSUBS-MAILIST] Stability & Buoyancy<br>
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<div id="yiv3971601525yui_3_13_0_ym1_1_1385768974238_2554">Hi everyone,</div>
<div id="yiv3971601525yui_3_13_0_ym1_1_1385768974238_2554">While reading through some background info for the G.L.</div>
<div id="yiv3971601525yui_3_13_0_ym1_1_1385768974238_2554">certification document, I came across this link with a good</div>
<div id="yiv3971601525yui_3_13_0_ym1_1_1385768974238_2554">explanation of stability & buoyancy</div>
<div id="yiv3971601525yui_3_13_0_ym1_1_1385768974238_2554"><a href="http://maritime.org/doc/fleetsub/chap5.htm" target="_blank">http://maritime.org/doc/fleetsub/chap5.htm</a><br>
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<div id="yiv3971601525yui_3_13_0_ym1_1_1385768974238_2554">There is a phenomena described, where submarines can be </div>
<div id="yiv3971601525yui_3_13_0_ym1_1_1385768974238_2554">unstable to the point of turning over during the transition from</div>
<div id="yiv3971601525yui_3_13_0_ym1_1_1385768974238_2554">surface to diving & vice versa.</div>
<div id="yiv3971601525yui_3_13_0_ym1_1_1385768974238_2554">This has to do with the centre of buoyancy moving upward past</div>
<div id="yiv3971601525yui_3_13_0_ym1_1_1385768974238_2554">the centre of gravity. As it approaches the centre of gravity the</div>
<div id="yiv3971601525yui_3_13_0_ym1_1_1385768974238_2554">submarine is at it's most vulnerable point.</div>
<div id="yiv3971601525yui_3_13_0_ym1_1_1385768974238_2554">This would be different for individual designs & a worry if</div>
<div id="yiv3971601525yui_3_13_0_ym1_1_1385768974238_2554">you dropped your emergency drop weight.</div>
<div id="yiv3971601525yui_3_13_0_ym1_1_1385768974238_2554">I had been told by a pilot that he never stuffed around while</div>
<div id="yiv3971601525yui_3_13_0_ym1_1_1385768974238_2554">descending & liked to drop as quick as he could.</div>
<div id="yiv3971601525yui_3_13_0_ym1_1_1385768974238_2554">I didn't have a full understanding of this & thought there may</div>
<div id="yiv3971601525yui_3_13_0_ym1_1_1385768974238_2554">be others in the same boat.</div>
<div id="yiv3971601525yui_3_13_0_ym1_1_1385768974238_2554">Regards Alan</div>
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