Re: [PSUBS-MAILIST] Actual water pressure

["John R. Farrington" <jrf@austin.ibm.com>]

Hi CARRO@wans.net,

> http://www.geocities.com/p_subman9/

I looked at your pictures, and just wanted to toss out a few
comments...

You might want to have some way of moving vertically rather than
using ballast tanks.  From your pictures, it looks like your
thrusters are for fore/aft movement only, plus, a bit of turning
action as well.

It might be a bit difficult when poking around on the bottom to
move up 2 feet, go over a rock, then back down 2 feet.  You don't
want to have to be messing with your buoyancy all of the time.

Also, your thrusters *may* be a bit under-sized for your design.
(I've never driven, or even ridden in a sub so I really don't know).
You've got 2 thrusters for a total of 72 lbs of thrust, trying to
move and turn a 5,000+ pound sub.  May or may not be adequate.

The thrusters look like they may be placed in a fairly vulnerable
position as well.  As you come down to touch bottom, you may be 
bashing a thruster into something.  The thrusters may also be
prone to sucking up any bits of debris (rope) that may be hanging
around the bottom, especially if you land on the bottom and sink
12 inches into silt.  That's a big concern for me because we've got
lots of 'stuff' hanging around the bottom of our lake.

Your battery pods look like that could easily accomodate more than
100Ah of battery power.  Looks like they could easily hold 300Ah or
so.  It might be wise to build them to hold a standard size battery,
perhaps a 'Group 24' or something like that, just for future convenience.

The battery pods, not being circular, may have a tough time handling
the depths that you're talking about (600+ feet).  Again, I'm not
an expert here.

25 cu.ft. of compressed air, stored at 1000 psi, in each pod, sounds
like it might be a little on the low side.  Your storage bottles are
approximately 0.4 cubic feet.

<approximate numbers here>

1000 psi / 15 psi (normal atmosphere pressure) = 67 (compression ratio)
25 cu.ft. / 67 = .37 cu.ft. (size of storage bottle)


At 600 feet, you're under approximately 20 times normal atmospheric
pressure.  (600psi/15psi=20X)  So if you were to entirely dump one
of your bottles, at 600 feet, you'd get approximately 1.25 cu.ft.
of air.  (25 cu.ft./20X = 1.25cu.ft.)

Of that 1.25cu.ft. you've got to subtract the volume of the storage
tank itself (since you're not pumping it down to 0 psi).

1.25 cu.ft. - .37 cu.ft = 0.88 cu.ft. of water displaced.  Water being
about 65 lbs / cu.ft. that gives:

0.88 x 65 = 57 lbs of buoyancy change from dumping 1 full tank, at
depth.  Not very much.

I'd definitely design around standard sized, 3000 psi (or 4500) scuba
tanks.  Much more air in them.  An 80 cu.ft., 3000 psi scuba tank has
an internal volume of .4 cu.ft. approximately, about the same as what
you're talking about.  So, rather than go with a 25 cu.ft. 1000 psi
tank, try 80 cu.ft., 3000 psi tanks.  Not much size difference.

I think the general consensus is that compressed air is the 'life-blood'
of any submarine.  So, the more you have, the better!

Also, you're going to need some emergency drop weights.  I'm not sure
what a good recommended amount is, but our sub will have approximately
6% of the total weight of the sub as droppable lead weights.  So, you
may be looking at 300+ pounds as a minimum.

You're going to need some oxygen tanks as well.  You don't have to
hammer out all of the details now, but just to keep that in mind.


So, those are some of my thoughts.  I'm not an expert by any means, and
my numbers are only approximate, just to make a point, but you get
the idea.  Sorry to be so long-winded, and for not also showing the
numbers in metric!

  ;-)

-John