Bill
I’m sure there is someone in the group who could
explain this more eloquently or correctly but I will have a go.
It is hard to throw out the rules you are familiar with but, it
is not like a plane prop where there is an area of low pressure in front of the
prop causing the air to be sucked in or pushed in and through so to speak. With the atmosphere as your altitude
increases you loose efficiency because the air is less “thick”.
This doesn’t happen in the big blue because you are immersed in a non compressible
fluid. If you move water it is replaced by more water without causing a low
pressure point or vacuum.
There is no change in density so the force causing movement
is applied mechanical pressure not suction so to speak. If you were to stop the
flow of air the prop would still spin just like when you block the vacuum
cleaner, (in fact by blocking the hose the motor spins more freely in the
cleaner) if you block the nozzle it should stop the pump (depending on the
type). The area of low pressure is the rest of the ocean.
Steve P
-----Original
Message-----
From:
owner-personal_submersibles@psubs.org
[mailto:owner-personal_submersibles@psubs.org] On Behalf Of Akins
Sent: Sunday, 9
April 2006 7:24 AM
To:
personal_submersibles@psubs.org
Subject: Re: [PSUBS-MAILIST] motor thought
Of
course I know that water molecules do not compress. Water is basically
uncompressable, but it can be pressurized.
I
should have said "pressurize" instead of "compress" when I
was talking about the pump exerting force to move the water.
It was
a symantic error, but hopefully you knew what I meant, just like I know when
you wrote "nozel" and "nozels" that
you
actually meant "nozzle" and "nozzles", and when you called
me "Atkins" you actually meant "Akins". I will try to
watch
my terms and keep them technically correct in the future.
You
explained that the pump can suck and blow just as efficiently at 1000 ft as at
10 ft. I'm still trying to wrap my head around that.
In
order for there to be an area of high pressure created by the impeller at the
pumps exit nozzle, doesn't there have to be an area of lower pressure
on the
other end of the impeller? If that is true, then wouldn't an
increased depth create more ambient water pressure on the low pressure side and
make it
harder for your pump to pressurize the water and result in less force coming
out the exit nozzle, and eventually equalize the pump's entire force
output
if you went deep enough? What am I missing here? As I said before, I
really haven't given this a lot of thought and just voiced the first questions
that came to my mind.
-----
Original Message -----
Sent: Saturday, April 08,
2006 1:42 PM
Subject: Re: [PSUBS-MAILIST]
motor thought
In response to Bill Atkins:
Water
doesn't compress. The pump simply sucks water in and increases it's velocity.
The nozel then decreases the apeture which creates pressure as the large volume
of water increases speed to get out of the smaller opening.
Since
water doesn't compress, the pump can suck and blow just as efficiently at
1,000ft as 10ft. Certain other fluids have been created that do not compress,
but also lubricate and protect. These hydraulic fluids would be the best thing
to seal an electric motor at depth. As long as the seal can keep the
fluid in at sea-level, it should be capable of doing the same when
submerged. Then a seal much like a dripless packing tube found on
inboard-powered boats may be an adequate seal.
I would
still rather have two pumps on board, in case one failed, and to give added
manueverabilty. Also I'd want to be able to aim the nozels.
My
question, however, is this- would a jet stream be as efficient as a propeller
AT SLOW SPEEDS? The benefit of a propeller is that it can be tuned to the work
it is doing. Angle, pitch, diameter- a psub like ours will have a lot of
resistance and won't be very fast.
That
being said, consider this: A hydraulic pump mounted inside the hull, driven by
an electric motor. This pump works on a closed-system, and recirculates
hydraulic fluid which drives propellers. Again, I'd choose two pumps (one for
each side, which provision to drive both props off just one pump), for added
safety and manueverabiltiy. You don't have to pressure-compensate anything, and
no moving parts are exiting the main hull, just a few pipes with cut-off
valves.