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Old 02 December 2005, 09:04   #151
DGR
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Nice!!

I stand corrected on your constriction. I have to think more in 3D...

20 tons minute is a lot of water!! So axial impellers increase pressure at each stage, and the nozzle works to accelerate the flow of the water as it returns to atmospheric pressure.

Yes?
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Old 02 December 2005, 09:14   #152
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Quote:
Originally Posted by DGR
.. and the nozzle works to accelerate the flow of the water as it returns to atmospheric pressure.
Is it your understanding that the water in the nozzle it at atmospheric pressure?
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Old 02 December 2005, 09:39   #153
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Quote:
Originally Posted by DGR
Nice!!

So axial impellers increase pressure at each stage, and the nozzle works to accelerate the flow of the water as it returns to atmospheric pressure.

Yes?
Well, i'm assuming that the succesive impellers (sp?) are within a cone, getting smaller in diameter with each stage, but each having a higher pitch on the impeller blades. I would think it would have to work summat like that, as if the velocity is increasing along it's path, the hole must get smaller to keep it all in ballance, so to speak.
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Old 02 December 2005, 09:40   #154
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Think I’ve got this now one question still foxing me is why aim the jet back into the sea
Given that I’ve understood this correctly, the higher the velocity the better and the jet isn’t ‘pushing’ so anything in the way of the jet will reduce the velocity and have no advantage.
Sure aiming the jet down will facilitate steering and trim but for speed, ideally the jet should aim horizontally clear of the water. Des
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Old 02 December 2005, 09:42   #155
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aiming it down probably gives a little lift helping reduce drag from the hull.

And, I don't think velocity is the key word, I think it's the shere weight of water that you grab hold of, and throw back quickly (accelerate) that gives the thrust.
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Old 02 December 2005, 09:55   #156
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Also probaly helps reduce the bulk of the engineering of the thrust directing bucket(s) We have some awesome pumps at work for fluming whole potatos, the volute casing looking similar to an outboard jet unit. They can also pump whole eggs or lobsters or whatever un-broken. Not sure who the lady is maybe need a factory visit to Cornell USA.
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Old 02 December 2005, 09:55   #157
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OK.
I have just come been to the pub for a beer or 3 with a mate of mine who is a hydraulic engineer and quite bright despite that.
I have to admit that I understood s*d all of his explanation of how and why a water jet works.
One thing that did stick, was that the reason for the nozzle was to increase the velocity of the ejected water, and that is only to make sure it is traveling faster than the boat.
The expression "nozzle coefficient" was mentioned several times. Google time I think.

I'm off for a nap now before the missus comes home.
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Old 02 December 2005, 10:01   #158
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Quote:
Originally Posted by Mark Halliday
OK.
One thing that did stick, was that the reason for the nozzle was to increase the velocity of the ejected water, and that is only to make sure it is traveling faster than the boat.
That's kind'a what I was trying (and failing) to say.

PS. nice pump Simon B!
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Old 02 December 2005, 10:13   #159
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Quote:
Originally Posted by Jonny Fuller
aiming it down probably gives a little lift helping reduce drag from the hull.

And, I don't think velocity is the key word, I think it's the shere weight of water that you grab hold of, and throw back quickly (accelerate) that gives the thrust.
I think it's both. It's the change of velocity (acceleration) AND the mass that's important. Compared to a conventional propeller it moves less mass, but faster to compensate.

And I think that the water would need to be at atmospheric pressure as it comes out of the nozzle (or pretty close to it anyway). As I understand it, its the reduction in pressure through the nozzle that provides the increase in velocity.

D...
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Old 02 December 2005, 10:36   #160
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Quote:
Originally Posted by DGR
And I think that the water would need to be at atmospheric pressure as it comes out of the nozzle (or pretty close to it anyway). As I understand it, its the reduction in pressure through the nozzle that provides the increase in velocity.

D...
'Escaping to' the atmospheric pressure is probably more accurate isn't it.
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