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Old 01 December 2005, 18:52   #131
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Quote:
Originally Posted by kitten
At atmospheric on a 6" square grid, that could be up to 237Kg pushing the water in, that enough?
Kitten, don't understand your calculation, at the water surface the pressure can only be 1 bar absolute.

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Old 01 December 2005, 18:53   #132
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Daniel,

Not being funny, but how can you quote the Kutta condition and Bernoulli after posting that lift comes from the fact that all the air has gone from above the wing, so doesn't weigh on it?

Aerofoil wings are slightly more complicated than the single element models - if memory serves, Bernoulli is a 2 dimensional solution, and Kutta and the circulation models are a 3 dimensional extension of that.

And anyway - it's largely irrelevant to the water jet debate because they depend on compressible flows. Water isn't compressible.

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Old 01 December 2005, 18:59   #133
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Quote:
Originally Posted by Pete7
...Afterall the impellor can't generate a vacuum...
It doesn't need to create a vacuum, it just needs a reduction in pressure to encourage the water to go up the intake - I don't know how low it would need to be. It would be like a fuel pump drawing fuel up from an underdeck tank to feed the engine - but much faster!!

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Old 01 December 2005, 19:06   #134
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Agreed, would be interesting to stick a pressure gauge under a planning hull and another in the jet intake to see how the readings changed as you accelerated.

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Old 01 December 2005, 21:22   #135
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Quote:
Originally Posted by DJL
JW, youíre thinking about the reactive force in the wrong place.
No! I thought DGR was thinking about it in the wrong place.

Thanks to everyone who tried to explain that every action has an equal and opposite reaction...but I already understood that. What I'm unsure about is the forces which produce that condition. After all, it doesn't happen because it's one of Newton's laws. What is happening happens whether Newton observed it or not.

Thanks also to those who tried to explain the operation of the aerofoil of the impeller....but I already understood that too. (I'm gonna start a thread about how propellers work. )

Also, I can understand the water storing kinetic energy.

For now I'm simply going to accept the reaction principle without knowledge of the forces involved.

But, since we've accepted that this is a reactive jet, I reckon the forward component of the reaction must act upon all the forward internal surfaces on the downstream side of the impeller, including the impeller but not limited to it.

From what I can glean, some of the engine's energy is imparted to the water within the nozzle. This energy is stored as kinetic energy.

Now this is where I loose it and I'm going to use the analogy of a bullet in a gun.

To make it dead easy, let's say the area of the end of the bullet is 1sq inch and the area of the end of the shell case is also 1sq. inch. Lets say the pressure created by the burn is 1000lbs.

The powder ignites and produces a pressure of 1000lbs/sq". Therefore, because the bullet end has an area of 1sq" there is a force of 1000lbs pushing the bullet down the barrel. The reactive force on the gun is also 1000lbs since it is acting through the end of the shellcase in the opposite direction to the bullet. Fine, that's easy to understand. Now, lets imagine the gun is placed on a slippery surface and it is able to slide rearwards. When the bullet is fired there will be 1000lbs force driving the gun rearwards as well as driving the bullet forwards. This is more like the situation with the water jet and easy enough to understand. (The gun being the boat and the bullet the waterstream.)

Ok, now let's apply this to the waterjet. The engine drives the impeller which creates high pressure in the jet chamber, just like the burn within the bullet/shellcase. Now the tricky bit (for me anyway). There is a force pushing the boat forward but there has to be the equivalent of the bullet. It's got to be water but where in the chamber does the water cease to be the pressure source and begin to be the 'bullet'.

I guess the nozzle, water velocity and kinetic energy are significant but I don't understand how they generate the equivalent of the bullet.

I'm presuming once the water has left the nozzle it is no longer in the equation although its energy can be used against the bucket for steerage and braking.


Please help.
Yours, Bewildered of Scotland
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Old 02 December 2005, 04:52   #136
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JW

I think you have it, and don't know it.
Your gun produces a single event.
With the jet you have a continuing event. i.e. loads of bullets in a stream and a continual "burn" from the motor which keeps going until you shut the throttle, run out of fuel or bullets (airbourne?).
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Old 02 December 2005, 05:43   #137
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Old 02 December 2005, 05:55   #138
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JW,

Or a machine gun, but one that never runs out of bullets.

For the action vs reaction debate - imagine that there are 2 of you floating in space. One JW decides that the other one is too close, so pushes him away. With neither JW having anything to 'react' against except each other, the force applied by one pushes them both away from each other with the same force (and speed) in opposite directions. It doesn't matter which JW pushes, the effect is the same.

The same applies to you pushing your house - but you react against the floor with your feet, and the house reacts through it's foundations. Imagine what would happen if you stood on a patch of ice while pushing against your house?

The water ceases to be a pressure chamber and becomes the bullet as it goes through the nozzle. The job of the nozzle is to convert pressurised water into high speed water, i.e. Potential Engery to Kinetic Energy, where velocity (squared) has a big effect.

I think the nozzle is the most important bit of the water jet - without it, you don't get any increase in velocity through the unit (as water is incompressible), so there won't be any significant change in momentum. The impeller would act like a really crap propeller in a tube. As water goes through the nozzle, the pressure drops to atmospheric at the throat (narrowest bit), whilst accelerating the waterstream to its highest velocity at the point where the pressure returns to normal - making the momentum per second (i.e. force) the biggest at the point at which it leaves the jet. The reaction to that is what moves the boat forwards.

If you have a pressure washer, next time you use it, take the lance bit off the end and see what the flow is like. It's rubbish - better with a hose pipe - and you get no force pushing against you. Put the nozzle/lance bit back on, and although the water turns into a mist in less than 6 feet, you can feel it pushing against you. It's the nozzle that makes it work.

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Old 02 December 2005, 06:13   #139
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Surely, the constriction downstream of the impellor is mealy to increase the velocity of the water to a usable figure! Asking the water to enter the intake, make a couple of turns etc. without cavitating, must be far easier if the velocity is kept as low as possible, then squirt it out at a speed somewhat faster than the intended maximum speed of the boat through the constriction

All this talk of pressure and bullets n stuff, is based on the fact that the nozzle is constricted, but if there was no constriction, it would still push the boat fwd, it would just be much harder to go fast. Look at a bow thruster! An impellor in a tube, no constriction, but no requirement for high speed either.

If the boat had a straight, parallel tube (like the bow thruster) from intake on hull, to exit on transom are we agreed that it would still drive the boat fwd?

Moving the water through that tube quickly, say 50knts, wouldnít be too easy, but with the constriction, the business end of it can operate at far lower speed.

Is it not the case that supersonic aircraft have their air intakes designed in a similar way, so as to keep the air going into the motor sub-sonic.

Mebbe Iím looking at it too simplistically, but I reckon thatís the crux of it.

The water jet system is a highly tuned, sophisticated bow thruster. (with a nozzle)
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Old 02 December 2005, 07:06   #140
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Quote:
Originally Posted by Jonny Fuller
Surely, the constriction downstream of the impellor is mealy to increase the velocity of the water to a usable figure! Asking the water to enter the intake, make a couple of turns etc. without cavitating, must be far easier if the velocity is kept as low as possible, then squirt it out at a speed somewhat faster than the intended maximum speed of the boat through the constriction
Yes, but it's the velocity that provides the force in a waterjet, not just the mass of water being moved (which is constant through the waterjet).

Quote:
Originally Posted by Jonny Fuller
All this talk of pressure and bullets n stuff, is based on the fact that the nozzle is constricted, but if there was no constriction, it would still push the boat fwd, it would just be much harder to go fast. Look at a bow thruster! An impellor in a tube, no constriction, but no requirement for high speed either.
Jetski impellers seem to be centrifugal rather than the axial props in a bow thruster. A bow thruster moves more mass than velocity like a conventional propeller does.

Quote:
Originally Posted by Jonny Fuller
If the boat had a straight, parallel tube (like the bow thruster) from intake on hull, to exit on transom are we agreed that it would still drive the boat fwd?
Yes it would. It would be inefficient and v. slow.

Quote:
Originally Posted by Jonny Fuller
Moving the water through that tube quickly, say 50knts, wouldnít be too easy, but with the constriction, the business end of it can operate at far lower speed.
Yes, but it uses pressure to get the velocity, rather than mass flow past the prop.

Quote:
Originally Posted by Jonny Fuller
Is it not the case that supersonic aircraft have their air intakes designed in a similar way, so as to keep the air going into the motor sub-sonic.
Not really, supersonic intakes use shockwave systems to make the air in the engine sub-sonic, and then the nozzle on the back end accelerates the airflow back to sonic conditions (and beyond depending on the nozzle design). Water doesn't support a static shockwave in the same way.

Quote:
Originally Posted by Jonny Fuller
Mebbe Iím looking at it too simplistically, but I reckon thatís the crux of it.

The water jet system is a highly tuned, sophisticated bow thruster. (with a nozzle)
Kind of - but the impeller isn't axial like a prop.

Props and jets all use mV to produce force to drive a boat. Axial (conventional) props move both mass and increase velocity to provide the force. A jet moves less mass, but more velocity to achieve the same thing.

i.e. (simplistically) 30 kgs of water at 20 kts from a conventional prop,

vs

20 kgs of water at 30 kts from a jet.

the force, mV, is the same for both - although the kinetic energy of the jet is 50% higher for the jet though (1/2 x mV squared) - so maybe thats why they are less efficient than a prop?

The design of the conventional prop hasn't changed that much over the years - so maybe the design is as efficient as it gets?

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