[PSUBS-MAILIST] Stability & Buoyancy

[jimtoddpsub at aol.com]

Yeah, Jon, the concept of a keel tank seems a bit spooky.  Trim tanks are often at the bottom of the boat but not below it, and their function has more to do with trim weight than with buoyancy. 

To clarify what I said:  "The lower your tanks, the greater your freeboard, but less            CB/CG spread."  That's true when the tanks are intersecting the water line.  Once they are completely below the water line, moving them any lower isn't going to give you more freeboard, but it will lower your CB.   Not good.

Jim


-----Original Message-----
From: Jon Wallace <jonw at psubs.org>
To: Personal Submersibles General Discussion <personal_submersibles at psubs.org>
Sent: Fri, Nov 29, 2013 9:10 pm
Subject: Re: [PSUBS-MAILIST] Stability & Buoyancy


          
    

      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
        
 
      
    
    
  

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