[PSUBS-MAILIST] Stability & Buoyancy
jimtoddpsub at aol.com
jimtoddpsub at aol.com
Sat Nov 30 00:03:35 EST 2013
Joe,
That brings up another design standard. If everything except the pressure hull itself floods, the release of the drop weight must result in positive buoyancy. So if both battery pods and all ballast tanks flood, you should still be able to surface by releasing the drop weight. If the drop weight also sticks, you're screwed!
Jim
-----Original Message-----
From: Joe Perkel <josephperkel at yahoo.com>
To: personal_submersibles <personal_submersibles at psubs.org>
Sent: Fri, Nov 29, 2013 10:27 pm
Subject: Re: [PSUBS-MAILIST] Stability & Buoyancy
This issue, explains the existence of the snorkel in the K-350. Imagine for a moment, the loss of a MBT, or the flooding of a battery pod.
I tell you, a lot of thought went into this little boat. Every time I look to exceed the parameters, I see better the original intent. Every time!
Joe
Sent from Yahoo Mail for iPad
From: jimtoddpsub at aol.com <jimtoddpsub at aol.com>;
To: <personal_submersibles at psubs.org>;
Subject: Re: [PSUBS-MAILIST] Stability & Buoyancy
Sent: Sat, Nov 30, 2013 3:45:36 AM
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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