From: "Alec Smyth" <Asmyth@changepoint.com>
Reply-To: personal_submersibles@psubs.org
To: <personal_submersibles@psubs.org>
Subject: [PSUBS-MAILIST] non-slosh tankage (was "battery acid")
Date: Thu, 26 Feb 2004 16:00:07 -0500
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 Pierre, if you're familiar with his sub.
Thanks,
Alec
-----Original Message-----
From: NeophyteSG@aol.com [mailto:NeophyteSG@aol.com]
Sent: Thursday, February 26, 2004 3:37 PM
To: personal_submersibles@psubs.org
Subject: 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
*****
"Call nothing thy own except thy soul.
Love not what thou art, but only what thou may become.
Do not pursue pleasure, for thou may have the misfortune to overtake
it...
Live in the vision of that one for whom great deeds are done ..."
Man of LaMancha, D. Wasserman