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Re: [PSUBS-MAILIST] Ballast Question
Hello all. Sorry for coming in late on this,
especially if what I am about to cover has already
been articulated by someone else.
The ballast tanks you are referring to are called
"soft" tanks. That is, they are open to the sea and,
as such, subject to changes in displacement caused by
pressure at depth.
Now, if you hope to have any depth control at all you
should not use these tanks to control your descent. If
your sub is a dry ambient vehicle then you must
calculate its diplacement and, forgetting about the
soft tanks for a moment, ballast it for neutral
buoyancy. That is, if the sub were placed in the water
with the soft tanks completely flooded, the sub would
float just barely below the surface.
The soft tanks are best used only to gain freeboard,
that is, to raise the sub out of the water enough to
allow surface navigation or recovery, or the use of a
hatch for entrance and egress without swamping.
So, when you dive, you flood the ballast tanks
COMPLETELY. This will eliminate the problem of
constantly adjusting for changing displacement. Then,
with the sub floating almost neutral just below the
surface, you can use thrusters to go deeper. Your
displacement will not change appreciably. Many subs,
including mine, use hard ballast tanks as well. These,
as the name suggests, are pressure-resistant tanks
with valves to control the air/water ratio. By
admitting or blowing out a certain quantity of water
and closing the valves, the weight of your vehicle can
be precisely adjusted without the air in the tank
changing volume with depth.
Hope this helps. Good luck.
Alan
--- "Dale A. Raby" <publisher@thegreenbayweb.com>
wrote:
> Now, that is good advice... I figured somebody would
> correct me. Mea
> culpa. So then as a sub dives and the air in its
> tanks becomes
> compressed... it really does start sinking? Gonna
> have to do some
> reading...
>
> On Mon, 2002-12-30 at 11:09, Herve wrote:
>
> > Hello there, and happy new year you all.
> > Again my advise would be for new comers in psusbs
> making, to get you
> > scuba certification.
> > Most of your questions would be answeared and your
> misconceptions
> > corrected.
> > You would also have fun and scuba is the logical
> start and a must for
> > any subman.
> > The rock weights the same whether you are at 1000
> ft or 10 ft.
> > everybody is positive buoyant at the surface,
> unless your body is all
> > screwed up and metal plate boned armored.
> > Although you have to kick to go down at first,
> after about 50 ft you
> > become negative as your body, not only the lungs
> are being compressed,
> > and you sink.
> > This is why bodies of seamen who went deep down
> with their ship never
> > resurface.
> > Herve
> >
> > ----- Original Message -----
> > From: Dale A. Raby
> > To: personal_submersibles@psubs.org
> > Sent: Monday, December 30, 2002 5:37 AM
> > Subject: Re: [PSUBS-MAILIST] Ballast
> Question
> >
> > Hello Pierre:
> >
> > I am far from expert... but if I screw up,
> I'm sure somebody
> > will correct me.
> >
> > I believe you to be laboring under some
> misconceptions. As
> > you descend, the air in a ballast tank
> that is open to the
> > outside does indeed compress... though I
> am not sure it
> > compresses by half at twice the distance.
> That does seem
> > logical, however.
> >
> > The thing is that with the air compressed
> to half size at
> > depth, the water it displaces is also
> "heavier", though it is
> > not compressible. The wight of the water
> is what causes the
> > air to compress. It has all the weight of
> the water above it
> > to add to its weight. Thus, though the
> compressed air is
> > denser (heavier) and theoretically not as
> buoyant, in water
> > that is also heavier by the same factor,
> it retains its
> > buoyancy... in fact it becomes even more
> buoyant, I believe.
> >
> > The reason you adjust ballast as you go
> down is because you
> > can't descend unless you do. In the
> example you are using,
> > trying to double your depth by using
> electric propulsion.
> > This might work if you had unlimited power
> because you would
> > have to maintain downward thrust in order
> to keep it down
> > there. It is kind of like a helicopter in
> reverse. A
> > helicopter opposes gravity by downward
> thrust, you would be
> > opposing buoyancy (or really just gravity
> again) by upward
> > thrust. As soon as you shut off your
> power, you would rise to
> > the level you had adjusted your buoyancy
> to.
> >
> > Now, if your ballast tanks are pressurized
> so that the air in
> > them cannot be compressed, you could have
> more buoyancy from
> > the same volume of air at sea level and
> this might allow you
> > to have smaller ballast tanks... but you'd
> pay for it in the
> > weight of the tanks themselves.
> >
> > Try this trick some day. Dive to the
> bottom of a pool and
> > pick up a rock... the biggest one you can
> iift and try to
> > bring it to the surface. You will find
> that while you can
> > lift it on the bottom, as you rise, you
> will need to use more
> > and more effort to keep rising. This is
> the same principle in
> > reverse.
> >
> > Now, for a better example. Go into the
> pool again and dive
> > down. You will find that unless you are
> one of those
> > unfortunate people who have negative
> buoyancy, you must
> > maintain muscular effort to remain below
> the surface. The air
> > in your lungs, your "ballast tanks",
> though compressed, is
> > still buoyant... and the deeper you dive,
> the more effort you
> > must expend to stay down. If you expell
> some air, you will
> > find that you can stay down with not
> nearly as much effort...
> > but don't go too deep, you also reduce the
> air reserve in your
> > lungs that you need to breath.
> >
> > Keep on asking questions. It is easier to
> answer your
> > questions than the questions your wife,
> children, and other
> > relatives will ask after your death.
> >
> > On Mon, 2002-12-30 at 09:30, Pierre Poulin
> wrote:
> >
> > > Hi there every body! It's the newbi
> again.
> > >
> > > I'm working on an a dry ambient sub. The
> hull is 60% finish.
> > >
> > > I tought that the ballast system was
> pretty simple but then I figured that
> > > if I have a main ballast system witch is
> opened at the bottom and I ajust
> > > the buyancy at 33 feet. Then, I go at 66
> feet using electric motors. The
> > > buyancy will need to be ajusted again
> since the air in the ballast will be 2
> > > times smaller. Right? So the deeper I
> go, the faster I sink?
> > >
> > > Would it be better if the ballast was
> able to support the pressure like an 1
> > > atm design and you close the bottom of
> the ballast witch is normaly open? So
> > > the volume of air will remain the same?
> I do realize that when I will try to
> > > put more air in the ballast, I will have
> to play with 2 valves at the same
> > > time to conpensate.
> > >
> > > Is that a variable ballast system? Is
> there a difference between those two
> > > systems (main and variable)?
> > >
> > > I'm a bit confused with that! I'm
> looking for something as simple as
> > > possible.
> > >
> > > I tought that some of you guys who are
> more experienced surely have the
> > > answer to my problem!
> > >
> > > Thanks for your help!
> > >
> > > Pierre Poulin
> > > >From cold Québec, Canada.
> > >
> > >
> > >
> > >
>
_________________________________________________________________
> > > MSN Messenger : discutez en direct avec
> vos amis !
> > > http://www.msn.fr/msger/default.asp
> >
> >
> >
>
=== message truncated ===
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