[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]
RE: [PSUBS-MAILIST] Stupid Question
James, please see comments below...
Rgds,
Alec
-----Original Message-----
From: owner-personal_submersibles@psubs.org
[mailto:owner-personal_submersibles@psubs.org] On Behalf Of James
Frankland
Sent: Tuesday, May 30, 2006 10:52 AM
To: personal_submersibles@psubs.org
Subject: [PSUBS-MAILIST] Stupid Question
Hi All.
I'm just trying to get my head around a few principles of physics that i
havent done since school! Just wondered if anyone can shed some light
for me. See what you think.
Ok, so ive got an idea/design for an ambient pressure sub. Pretty basic
design, just to get me started. Cabin will be open at the bottom, with
an automatic pressure compensator to pump air into the cabin. Thats ok
i think.
I can calculate the volume of my machine easy enough, to give me a
displacement. But i've no idea of the weight. Its to be a steel frame
with wood covered in fibreglass making up the hull. As its ambient
there is no need for a pressure proof hull.
What i was going to do is just get on and make it "about right", and
then take it to the tip where they can weigh it on their weighbridge. I
could then calculate the required ballast i would need to add to the
bottom of the keel to make it float at the right point.
AS: You really, really don't want to do that on a submarine project of
any sort. The weight MUST match the displacement for your sub to work,
and if you build without calculating, you will inevitably have to get it
to the right weight one way or another after completing the boat. If
your sub comes out too heavy, you'd have to add syntactic foam, which is
expensive. And if it comes out too light, you have to add lead, which is
expensive. Chances are with an ambient you'll err on the side of being
too light, but wouldn't you rather design the ballast into something
useful like extra batteries? Do the homework of calculating the weight
of every component before you build it, and leave yourself only a
reasonable margin to correct with dead weight. In addition to the weight
itself, there's the issue of where the weight is located. Without
calculations, you could end up not only with a buoyancy problem but with
trim and stability problems as well.
Of course, this doesnt allow me to alter the size of the tanks and i may
end up with them being far to big, or to small.
So, assuming i do all that and somehow its about right.
AS: Not an assumption I'd recommend making!
On the surface, the ballast tanks will be empty, with a water inlet
valve at the bottom, a vent at the top and a high pressure air inlet
somewhere.
AS: Main ballast tanks should not have any inlet valve on the bottom.
They are open on the bottom. Think of them as inverted cups that you can
fill with air to gain buoyancy. Just because they're called "tanks"
doesn't mean they have to look like a cylinder or even a closed
compartment.
But, this is where im getting confused.
On the surface, with tanks empty, there will still be a certain amount
of pressure crushing my fibreglass tanks? Ok, they will be only just
under the water, but still under pressure.
AS: No, there is no pressure differential per the above comment. If they
are open underneath, the tanks will not be subject to any pressure.
So, i open the inlet valve and water comes in until the pressure is
equal.
Open the vent to let air out (inlet still open) and the sub will begin
to sink. Shut off vent when neutral buoyancy is achieved and always
leave bottom inlet open. Adjust buoyancy with high pressure inlet air.
Pressure should stay equal.
AS: MBTs are not used in that way. What you seem to be describing is a
sub that operates with nothing but the variable ballast tanks. You
either have MBTs all full, or all empty. The reason is, if you left them
half full the air in them would compress as you dived (assuming you
start neutral). As it compressed, you would lose your neutral buoyancy.
I believe the source of your confusion is that you're overlooking the
need for two totally different sets of tanks. The MBTs are for surfacing
or submerging, and the much smaller VBT is for fine tuning buoyancy. The
VBT will have inlet and vent valves as you describe, but you would not
make it of fiberglass. They have to be capable of withstanding your
intended max dive depth (times a safety factor) so are heavy. I mean
that literally, on my little one person sub I have a small VBT, but
cannot lift it without a shop crane. A sub with large VBT capacity and
no MBT would work (though beware of stability problems caused by the
free surface inside the tank). However, since these tanks are so heavy
it is not a logical engineering compromise to build all your tankage
hard. All you need to be hard is enough tankage to adjust for the
difference between your lightest expected payload in salt water, and
your heaviest expected payload in sweet water.
Is that lot right?
Do i have to launch the sub with the inlet valve open and allow water
into the tank to keep the pressure equal? I think probably so. But
thats going to affect the buoyancy calculation.
hmmmm. Confused.
Anyway, i would appreciate any ideas suggestions. It might be quite
simple?
AS: I do not mean to discourage you, but based on the above questions
I'd emphatically recommend building to existing plans rather than
attempting a custom design.
Many thanks
James
________________________________________________________________________
__
Sent via Cable & Wireless Guernsey system at cwgsy.net
************************************************************************
************************************************************************
************************************************************************
The personal submersibles mailing list complies with the US Federal
CAN-SPAM Act of 2003. Your email address appears in our database
because either you, or someone you know, requested you receive messages
from our organization.
If you want to be removed from this mailing list simply click on the
link below or send a blank email message to:
removeme-personal_submersibles@psubs.org
Removal of your email address from this mailing list occurs by an
automated process and should be complete within five minutes of our
server receiving your request.
PSUBS.ORG
PO Box 311
Weare, NH 03281
603-529-1100
************************************************************************
************************************************************************
************************************************************************
The contents of this e-mail are intended for the named addressee only. It contains information that may be confidential. Unless you are the named addressee or an authorized designee, you may not copy or use it, or disclose it to anyone else. If you received it in error please notify us immediately and then destroy it.
************************************************************************
************************************************************************
************************************************************************
The personal submersibles mailing list complies with the US Federal
CAN-SPAM Act of 2003. Your email address appears in our database
because either you, or someone you know, requested you receive messages
from our organization.
If you want to be removed from this mailing list simply click on the
link below or send a blank email message to:
removeme-personal_submersibles@psubs.org
Removal of your email address from this mailing list occurs by an
automated process and should be complete within five minutes of
our server receiving your request.
PSUBS.ORG
PO Box 311
Weare, NH 03281
603-529-1100
************************************************************************
************************************************************************
************************************************************************