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Shawn, The problem is clear, but I’m
curious about your solution. Pressure compensated bellows? Could you elaborate?
I think you might be doing something similar to Thanks, Alec -----Original Message----- In a message dated
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 ***** |