Re: [PSUBS-MAILIST] VBT positioning

["Martin Sanderse" <sanderse@protoproduction.com>]

----- Original Message -----
From: <VBra676539@aol.com>
To: <personal_submersibles@psubs.org>
Sent: Monday, July 16, 2001 8:01 AM
Subject: Re: [PSUBS-MAILIST] VBT positioning

> I was hoping you'd get around to saying this, as I think it answers the
> original question. Hard (pressure resistant) variable buoyancy tanks,
whether
> mounted inside or outside, vent overboard. Soft tanks are filled using
> ambient pressure as the motive force and vented inboard (no other way, is
> there?) then pumped overboard. And there are a couple of interesting
> variables, including a pressure compensated, externally mounted hard
system
> which allows you to use a much lighter SS or aluminium (that's aluminium
for
> you, Harry) tank or tanks.
>
> Either way, the term Variable Buoyancy Tank (VBT) is commonly used, and
means
> what it says. It is a system for altering the sub's buoyancy without
changing
> it's weight. And keep in mind that hard tanks are positive and quick for
> those changes and soft systems are generally very slow and very precise
> (relative to the former).

With that in mind, I was considering using two bladders, one internal, and
one external, fluid filled, with a small hydraulic pump operable in either
direction.  I would of course have to provide limit switches of some sort on
the bladders to prevent myself from bursting either one of them.  There
would be a change in volume inside the hull as the internal bladder filled
and emptied, but it's a small % of total internal volume.

What I find more problematic is the main ballast tanks for surface
freeboard.  if I locate them high for surfcae stability, I invariably run
across the problem where I could trap air somewhere, making it impossible to
descend with the VBT volumes I'm thinking of.  If I make the VBT larger,
then as I decend and rise, the air in the main tanks would compress or
expand, requiring more VBT adjustments than what seems a stable situation to
me.

So far I haven't figured out how to get the main ballast center of bouyancy
directly above center without a bunch of valves. (as some appear to have
done)






rs



>
> Best Regards,
> Vance
>