Re: [PSUBS-MAILIST] VBT positioning

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

----- Original Message -----
From: "Harry Spillett" <harry.spillett@bigfoot.com>
To: <Personal_Submersibles@psubs.org>
Sent: Sunday, July 15, 2001 10:51 PM
Subject: [PSUBS-MAILIST] VBT positioning


> I have noticed that on all most all existing designs I have seen the
> variable buoyancy tank is sited inside the pressure hull.
> Is this so that it does not have to withstand diving pressure?

Hi Harry,

If I understand the type of VBT system you're describing, they are protected
from depth pressure by the hull and associated vent and drain valves.
However, if flooded at depth, the tank will experience depth pressure
internally as an expansive force.  Generally speaking, welded steel tanks
have greater tolerance to expansive than compressive forces, and therefore,
an internally mounted VBT could be made of somewhat lighter gauge metal
than, say,  one designed to survive outside at the hull's design operating
depth.   This MIGHT be one reason the designer chose an internal tank, but
not the best one, IMOHO.

There are other (better) reasons why someone might choose an internal tank.
For example: the internal BT in a K-250 also serves as the pilot's seat, and
appears situated at-or-near the boat's pitch axis, enabling variations in
ballast-weight without destabilizing pitch / trim.



>Ore is there
> another reason perhaps to do with the internal volume of the pressure
hull.

Yes.  A ballast tank carried inside the pressure hull doesn't add it's
volume to the overall positive displacement of the boat.

For example: if I have a 40 cubic foot pressure hull and an empty
externally-mounted 4 cubic foot ballast tank, the positive displacement of
the boat equals 44 cubic feet.  If, however, I've got that tank inside the
hull, the boat's positive displacement is only equal to 40 cubic feet.
Tank volume and location come into play when calculating the boat's overall
weight, displacement-buoyancy,  freeboard / ballast-volume requirements, and
hydrostatic center-of-gravity.

If, say, the gross weight of the example boat (above) is equal to 44 cubic
feet of sea water (2816 #), and the desired surfaced freeboard is equal to
that provided by 10% of the pressure hull's displacement;  I'll want to
mount 4 cubic feet worth of ballast tanks outside and below the waterline on
the 40 cubic foot pressure hull if, when the tanks are  flooded, I want the
boat is to submerge at neutral buoyancy.

If, however, the design calls for the gross weight of the boat  to equal the
displacement of only 40 cubic feet of sea water (2560#), and I still want a
10% freeboard, I can place a 4 cubic foot tank tank inside the hull, where
it's volume won't add to the overall displacement; and when it's flooded,
the water-weight it takes on will overcome  the 10% positive buoyancy /
freeboard, enabling the boat to submerge.

Above, we're seeing another advantage: design simplicity.  In both cases, we
needed  4 cubic feet of variable ballast.  For reasons of balance, the
design might require us to divide that into two tanks if situated outside
the hull; while the same amount of ballast could be carried in a single
internal tank which would be easier to design, build, and operate.
>
> Following on from this, when the VBT is flooded is the air released vented
> into the pressure hull or to the sea.

Generally, the ballast tanks are vented to the atmosphere when surfaced and
the on-ballasting process begins; or to sea when submerged and the boat is
bring trimmed.  And / or, if you're deep enough, you might be able to just
open the bottom valve and let the water level in the tank rise via depth
pressure without opening the top vent at all; depends on the system you're
using.

But to flood a ballast tank at depth and vent tank air inside the pressure
hull would raise internal hull air pressure beyond the one atmosphere
usually maintained in dry subs.  I'm not saying it can't be done; and there
may be reasons why you might want to do that; but in most cases it goes
outside, if it has to go anywhere at all.

And, usually, when compressed air is let into the tank to force ballast
water out, the flow vents overboard, too.

>
> Once again I hope I have followed correct procedure
and look forward to your
> thoughts. It is comforting to know there is a pool of like minded people I
> can turn to for advice rather than pull my hair out alone!
>
> Regards
>
> Harry Spillett
>

Good luck with your project, Harry.

Pat