Re: [PSUBS-MAILIST] VBT positioning

["Captain Nemo" <vulcania@interpac.net>]

----- Original Message -----
From: "Martin Sanderse" <sanderse@protoproduction.com>
To: <personal_submersibles@psubs.org>
Sent: Tuesday, July 17, 2001 7:57 AM
Subject: Re: [PSUBS-MAILIST] VBT positioning


> 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.

How big of a hull; and how big are the bladders?


>
> What I find more problematic is the main ballast tanks for surface
> freeboard.

Are these hard tanks or bladders?

  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.

How does the positioning of the tank relative to the hull effect your
ability to vent them?  How / where is the air being trapped?


  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.

In designs where the ballast tanks aren't valved top and bottom (I've seen
some that were just open-vented at the bottom) the pilot usually adds or
bleeds-off a little compressed air to compensate for changes in depth
pressure.  Any reason why you can't or don't want to do that?
>
> 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)

Directly above center of what?  And why do you want to avoid the use of
valves?

Pat