Re: [PSUBS-MAILIST] battery acid? & battery issue follow up

In a message dated 2/17/04 3:00:06 AM Pacific Standard Time, coalbunny@vcn.com writes:

Shawn, that is something that I have wondered about- will the ballast
shift? I have read how a number of scientific units used lead or steel
shot. Is there anything that prohibits that being used? IIRC at one
time even mercury was used. Of course we know the environmental
implications from that. One thing I thought of using that should be
cost effective and would be environmentally friendly is the use of sand
or gravel. But since I have no real unit to use that ballast method
with, I have no idea if it'll work.
Carl

Sorry for the delay in my reply. Mind you, I'm no authority on ballast but I've learned a tremendous amount over the last year from this group, personal research and through the process of designing the ballast system for my project. I'm working on a white paper to organize what I've learned in the hope that it might help shorten the learning curve for new members just starting their designs. I consider it a token repayment for the knowledge I gleaned from our current set of white papers and articles, and would like to (eventually) see a whole series of overview-like white papers covering the different aspects of submersible design. To answer your question, yes, ballast can shift, both intentionally and unintentionally.

In classic water ballast tanks, whenever the attitude of the sub deviates from level the center of mass will shift as the water seeks the lowest point in the tank. The magnitude of that shift depends to a large extent upon the size, shape and orientation of the ballast tank, the presence and type of any baffling in the tank, the amount of "tilt", and the ratio of water to air in the tank. More specifically (though still generally speaking):

Long horizontally-oriented tanks suffer the most shift; round tanks the least.

Baffling helps reduce the amount of shift, but can complicate filling/venting.

The greater the tilt, the greater the shift. As discussed in previous threads, careful consideration should also be given to how the tank venting system can be affected by shifts in the ballast.

The fuller the tank is the less shift there is.

You'll experience similar shifting in gravel, sand, lead/steel shot disposable ballast to one degree or another for similar reasons. Worst case occurs if the shift becomes so large and/or pronounced that it requires either full-vent or -blow to correct. However, these considerations are generally less critical for non-hydrobatic subs because the range of inclination, hence the likelihood/magnitude of shift, is fairly narrow.

In my case, I needed a ballast system that would work properly no matter what orientation the craft goes through. I chose to use a pressure-compensated bellows arrangement to constrain the shift of water.

Warm Regards

Shawn