Re: [PSUBS-MAILIST] ABS - 3/17.1.1 - Normal Ballast System

[VBra676539@aol.com]

Hey folks, I got back in--don't know how, but the powers that be (or be not) 
at AOL have done something, or Jon did. Whatever, thanks.

Ballast systems:

I read through most of the posts from the past few days. Interesting stuff. 
Someone from HBOI said that VBTs are for payload changes. Well, sometimes yes, 
sometimes no. The JSLs have 4 vertical VBTs built into their main tanks. They 
are air compensated and operated at ambient pressure plus whatever the 
operating pressure is set for to blow the system and operate the valves 
(pneumatically controlled flood/vent valves). That's a pretty cushy idea, but is monstrous 
hard on your air supply. Which explains why they have all those fiber wound 
aluminum T-bottles hung everywhere. Most PSUB sized vessels won't have that 
luxury, or that requirement.

VBTs, strictly speaking, are variable BUOYANCY tanks, not variably BALLAST 
tanks. They are supposed to be adjustable at sea (or lake, or pond) to 
accomodate minor or inadvertant changes away from neutral buoyancy. It allows you to 
change the sub's weight without changing its displacement. It's a handy little 
option. Everything from temperature changes in the water to bottom mud in the 
battery box can make you wish you had one--or a bigger one.

Major payload changes should be planned for so they can be done without 
impacting the normal (read small volume) VBTs that most of us have. A few chunks of 
ballast lead added to the design prior to construction can then be replaced 
by new toys, goodies, science packages, girl friends, cases of beer and so on 
WITHOUT impacting your ability to acquire and/or adjust for neutral buoyancy 
with that dinky little old VBT you built.

MBTs are, as specified, for freeboard and safety. However, they work plenty 
well as VBTs once you get down a hundred feet or so. You have to keep adjusting 
the bubble, of course, but nothing moves your sub vertically quite as 
quickly, unless you got horsepower fever, and more batteries than I do.

Drop weights:

It all depends, like the man said. The JSLs drop a big box of railroad 
batteries and you'd best believe the sub leaves--RIGHT NOW--when it goes. The Perry 
boat that sits right beside the Sea Links in their shop (PC-1204) has a big 
steel tray under the belly (usually) and it does, in fact, weigh the same as the 
difference between flooded and dry in one pod (about 900# if memory serves). 
That lead is NOT used for payload changes (except when they sling jettisonable 
payload packages in its place). On the other extreme, the little Delta has no 
floodable spaces outside the pressure hull itself, and so has a fifty pound 
chunk of lead up front they can drop just for fun.

The thing to remember is that your sub's STABILITY is paramount, whatever it 
is. You need to figure it in all configurations, including without your 
dropweight if you have one. And by the way, the sub doesn't necessarily need to stay 
upright when you drop the weights. The PC-14s that Perry built were designed 
with a single jettisonable battery pod instead of a drop weight per se. And 
they were designed to ROLL OVER at the surface. You would pressurize the hull 
(if you survived the rollover, one assumes) and open the conning tower hatch, 
which would have been converted to a temporary diver lock out trunk by then. No 
one ever had to do it, but ABS bought the design and certified the boats that 
way. You got to think outside the box, some days. That's all there is to it.

Best Regards,
Vance Bradley



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