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RE: [PSUBS-MAILIST] Ambient Variable Ballast Tank
Jon,
The system is very basic- one plastic tank forms a seat in the back (shaped
like an orange peel) and another forms a "bench" just under the bow dome.
Passengers sit port and starboard toward the bow. Each tank has a cap on it.
There is a ball valve with a 4 ft length of clear plastic hose for filling
these tanks. Just pop the cap, stick the hose in, and open the valve- that's
it. If I'm diving alone, I put the water in the front tank. If I have
passengers, I fill the back first.
But as far as balance goes, it really doesn't matter with my sub. The hull
is 50" in diameter with over 1,200 lbs of keel. It's only 72" long (and that
is mostly hemi heads). I usually add water so that it is sitting level on
the surface, but move up to the "bench" in front on the bow dome when I
reach the bottom. Tilting the bow down just improves the view.
I have a pump that can be used to expel the water, but usually just hand the
ballast tanks out through the hatch after the last dive(they're heavy
though!) and dump the water out. When in doubt, go simple!
In the case of the 600, a series of crescent shaped plastic tanks to fit in
the floor between frames would create a nice flat surface for a passenger to
sit.
Regards,
Greg
-----Original Message-----
From: owner-personal_submersibles@psubs.org
[mailto:owner-personal_submersibles@psubs.org] On Behalf Of Jon Wallace
Sent: Friday, May 25, 2012 10:40 AM
To: personal_submersibles@psubs.org
Subject: Re: [PSUBS-MAILIST] Ambient Variable Ballast Tank
I was going to stay with an external VBT just to keep the interior as
empty as possible but the way you guys have described the internal VBT
with either a bladder or conforming to the tank head contour is worth
looking at. By the way Jim, the system I described could only be used
externally. The primary purpose is to achieve neutral buoyancy at the
surface to account for load variation (occupants/equipment) and in most
cases that is probably enough. It also makes more sense than
fabricating and plumbing for an external ambient VBT now that I think
about it because the AVBT doesn't get you any more functionality (I
don't think) then a bladder at the surface. I'd have to include an
overpressure valve on the AVBT so the pressure could adjust as you came
back up into shallow water. However, that overpressure valve would
prevent removing excess water from the tank if it was overfilled
accidentally and made the sub negatively buoyant. In fact, once flooded
with any amount of water you could only remove the VBT water by draining
it once the sub was out of the water unless you had a way to lock the
overpressure valve. Starting to sound too complicated.
While obtaining neutral buoyancy at the surface is one purpose of a high
pressure VBT, the other purpose is to allow picking up objects (weight)
at depth and then readjusting to be neutral again. This wouldn't be
possible with an internal bladder in most cases unless that bladder had
a lot of unused volume so it limits you to not adding weight (picking up
100 pounds of rocks for example) during operations. However K-boats
don't have appendages to pick anything up anyway so again the bladder
becomes a really simple VBT.
The other thing to be careful of with an internal bladder is placement
so you don't disrupt balance. Kittredge put the K-250 VBT in center of
the sub right under the pilot seat. Greg, you must have some way to
control ensuring the fore and aft tanks in your sub have the same amount
of water in them to maintain balance. Do they just fill equally
naturally or do you have a water pump to adjust the levels fore and aft
if necessary?
I thought of a high pressure fiberglass tank but as I recall from
previous discussions they are expensive although those were the 3000psi
versions. I'll check pricing on the fork lift tanks.
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Weare, NH 03281
603-529-1100
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