[PSUBS-MAILIST] Stability & Buoyancy

[Jon Wallace]

I'm somewhat a novice with CG and CB but it does occur to me that the 
illustration in the document shows a circumferential ballast tank which 
from the perspective of CB must be the worst type to employ.  The K-350 
design with both drop weight and battery compartments well below the 
ballast tanks provide an extremely low CG with CB well above it at all 
times.


On 11/29/2013 9:54 PM, 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
>

-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://www.whoweb.com/pipermail/personal_submersibles/attachments/20131129/868bff63/attachment.html>