Akins, sorry for mispelling your name.
I think of it this way: The pump increases pressure over ambient by a set amount. It CREATES lower pressure on the suction side by sucking water into the pump. The volume of water it moves is the same regardless of depth. The water "pushes" into the vacancy, and that "pushing" force is equal to the resistance met on the other end. In shallow depth the water pushes and resists less, in deep depth the water pushes and resists more, but it always does it equally.
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.
Bill Akins.
----- 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.