Re: [PSUBS-MAILIST] Ambient Variable Ballast Tank

[Jon Wallace <jonw@psubs.org>]

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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