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Old 01 December 2005, 04:49   #91
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Quote:
Originally Posted by codprawn
Well spotted - yes he IS right that Bernoulli's Equation is the one to use but for some reason he has posted Boyle's law instead - a slip of the old mouse perhaps???????
Indeed. Both apply. Bernoulli is more about the boundary layer - how liquid/gas flow will have a slower cusion when running against a solid surface. A lot to do with wind tunnels so pretty similar to jets. A very hazy memory from an engineering degree many many moons ago...
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Old 01 December 2005, 05:00   #92
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Quote:
Originally Posted by Richard B
One teeny little point, Mark - The water doesn't get squashed. Liquids, unlike gases, are incompressible. So the water jet has an impeller, not a compressor.
It was just a quote from the Illiad, or is that the wrong Homer?

OK. Read sqashed as pressurised, not compressed.
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Old 01 December 2005, 05:01   #93
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Quote:
Originally Posted by jwalker
That's not telling me how it works. That's telling me the result of it's action.
JW
How are you with letting go of ballons before tying the neck?
This might be a new line on understandin.
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Old 01 December 2005, 07:00   #94
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Originally Posted by kitten
Now you are just laying traps for people...

Caught.

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Old 01 December 2005, 07:05   #95
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Old 01 December 2005, 07:13   #96
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Quote:
Originally Posted by Mark Halliday
JW
How are you with letting go of ballons before tying the neck?
This might be a new line on understandin.
Funny you should say that cos I was reading the Rolls Royce website last night to try and get more understanding and they used that as an example but it brings us back to exactly the situation in the two wee diagrams I drew. Closed container, then open at one end.

However, I think I'm beginning to get an inkling now of various conditions inside the jet.
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Old 01 December 2005, 07:15   #97
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Originally Posted by Mollulnan
Good comment. You're so constructive.

If you find this thread boring, go away.
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Old 01 December 2005, 08:21   #98
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Quote:
Originally Posted by jwalker
Funny you should say that cos I was reading the Rolls Royce website last night to try and get more understanding and they used that as an example but it brings us back to exactly the situation in the two wee diagrams I drew. Closed container, then open at one end.
Great minds, eh?

I am not sure WHY it works either. Just trying to get my head around it too.

Firstly, thanks to DJL.

I am starting to think along the lines of Force = Mass x Velocity with Newtons 3rd. I dont think it has anything to do with any of the pressure laws.

You have a given Mass (the water sucked up) you Accelerate it with the impellor and nozzle which increases the Force applied. Apply Newtons 3rd and you have a reaction force in opposition and equal to the ejected Mass.
The force is transfered to the boat via the jet assembly because the impellor will not let the water return from whence it came.
I seems to have nothing to do with system pressure after all. In a baloon the force is applied the elastic nature of the baloon its self, not the pressure stored in it. (I think)

Comments?
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Old 01 December 2005, 08:22   #99
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Quote:
Originally Posted by Mollulnan
Several bottles of wine has that effect on me too.
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Old 01 December 2005, 10:07   #100
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can some one tell me and this may end up being a really stupid QUESTION but is the flow of water through the intake regulated and at what point because it seems to me the faster the boat travels the faster the water should be taken in and in that case bearing in mind that the way this thing works is a fast jet of water being expelled from the rear then at greater speed a lot of the work is already done and the engine should become more economical or am I just talking B*ll*cks

PS. being a diver we love jet drives no fear of props taking your head of and when on a boat with twin jet drives the diruption of water at the back of the boat is vast which means you can run and jump of the back into the water at 30 knots
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