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Old 01 December 2005, 13:58   #111
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Originally Posted by DGR
DJL,

Have a cold drink with your dinner, and drink it with a straw.
Ah, but will he be sucking it up or is he reducing the pressure at the top of the straw and the air pressure acting on the surface of his drink pushing it up the straw?

-----------------------------------

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JW - think of a rocket in space. You chuck stuff out the back, and rocket goes the other way. Theres nothing to react against there either, but it works. The thrust in a rocket is produced at the nozzle too.
There's got to be a problem with that because I don't think you can have a reaction against nothingness.
If I lean against the side of my house, it pushes me back with the same force. Now that's a reaction.

Of course, it could just be that me brain's fekked.
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Old 01 December 2005, 14:11   #112
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Of course, it could just be that me brain's fekked.
It's times like this WJ that I wish I was good at sarcasm!

would you like to borrow my Hamilton manual, I'll have to find it but If you get enthused the I have a unit that needs rebuilding.
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Old 01 December 2005, 14:12   #113
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I still haven't got me head around all this but I think it must also carry the forward propulsive load.
There is a pressure difference across the impeller, so there will be a very small thrust component forwards - but compared to the power of the water jet, it would be negligeable. If the impeller was the main thrust producer, you wouldn't be able to go backwards with the thrust reverser buckets - you would go forward, just slower!!

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Now that's vague bit; reacted against.
It's Codprawn skating with his 12 bore. Every action has an equal and opposite reaction, fire it one way, and you react in an equal and opposite way, and go in the other direction.

You push your house hard - it pushes back just as hard, so neither of you go anywhere. Same thing really......... Now if you could pick up your house and throw it......whilst skating......

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I've not been able to find a full explanation on the internet but I keep falling back to the necessity for the jet to entrain as large a mass (of water in this case) as possible, produce as high a velocity at the nozzle as is possible and the fact that there will be a low pressure within the nozzle because of the high velocity of the mass. If we consider the system, including and downstream of the impeller, the orifice at one end (the nozzle) will be at low pressure and at the opposite end, the impeller, is at high pressure. Could this be the pressure differential across the ends of the jet which causes the propulsive force?
I believe that you are right on the mass and velocity, but the pressure is a bit of a red herring. The pressure of the water will return to 1 atmosphere when it leaves the jet, but I think that the pressure in the jet unit after the impeller would be pretty constant up to the nozzle, and the velocity would increase as the pressure returns to normal as it goes through the nozzle. If Kinetic energy is 1/2 x Mass x Velocity squared, then upping velocity and mass flow rate increases the KE in all the right ways - but pressure wouldn't effect the KE.

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Perhaps I should have been making water rockets in the physics class instead of making steam engines in the metalwork class.
Nah - steam engines are well cool

D...
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Old 01 December 2005, 15:30   #114
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Ah, but will he be sucking it up or is he reducing the pressure at the top of the straw and the air pressure acting on the surface of his drink pushing it up the straw?

-----------------------------------
It's taking Daniel a long time to eat his dinner. I hope he's not trying to eat all of it through a straw - I doubt reduced pressure at the top and atmospheric pressure at the bottom would work with solids... ...!!!

Now THAT would suck!!
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Old 01 December 2005, 15:48   #115
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I have spent along time reading this thread and still stand by my origional post and think if you red it it does answer most of the questions, But if you want to know where the actual force is first aplied in a standard waterjet system I can say without risk of challenge.


















Between the piston and the cylinder head
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Old 01 December 2005, 16:08   #116
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Quote:
Originally Posted by jwalker
Ah, but will he be sucking it up or is he reducing the pressure at the top of the straw and the air pressure acting on the surface of his drink pushing it up the straw?
Spot on.

Quote:
Originally Posted by jwalker
There's got to be a problem with that because I don't think you can have a reaction against nothingness.
If I lean against the side of my house, it pushes me back with the same force. Now that's a reaction.

Of course, it could just be that me brain's fekked.
JW, you’re thinking about the reactive force in the wrong place.

The reactive force that is propelling you along is between the particle you shoot out the back and the device that shot it out the back. Not between the particle and the surrounding air/water etc.

eg - if you sit on a office chair with wheels and throw a brick, your chair will move in the opposite direction to that you throw the brick. As you accelerate the brick to throw it you're applying a force with your hand to the brick. The brick according to Newton’s 3rd Law is at the same time applying a force to your hand in the opposite direction - this is what makes you move - Not the reaction between the brick and the air - hence you could sit on a office chair in a vacuum, throw a brick, and you would move. Wouldn't like to clean up the mess afterwards though.
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Old 01 December 2005, 16:10   #117
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I have spent along time reading this thread and still stand by my origional post and think if you read it it does answer most of the questions.
I agree!
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Old 01 December 2005, 16:10   #118
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Originally Posted by DGR
It's taking Daniel a long time to eat his dinner. I hope he's not trying to eat all of it through a straw - I doubt reduced pressure at the top and atmospheric pressure at the bottom would work with solids... ...!!!
Now THAT would suck!!
had to cook it as well - then I got distracted by the TV...

Anyway...

Quote:
Originally Posted by DGR
A wing uses pressure differences to produce all of the lift force - but doesn't involve any substanial mass flow (the jet engines attached to the wing do that). A waterjet uses pressure to generate high mass flow rates of water through the nozzle to produce a motive force (like a rocket does).
All jets use mass flow rate to generate thrust - aero engines or water jet engines (and rockets too).
This is incorrect. How do you think the difference in air pressure is created? Remember Pressure = Force x Area and Force = Mass x Acceleration. By moving the wing through the air you're applying a force to the air (this is where drag comes from, as you move around you apply a force to the air to move it out the way and it pushes back) So by forcing the air downwards you make the air force you up. Then we come back to the area that you've moved all the air from above the wing. There's now nothing there - the weight of the air (~1.5kg per m^3 ) is no longer pushing down on the wing (air pressure - which is cause by gravity acceleration the mass of air towards the centre of the earth)

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Originally Posted by DGR
INTAKES......on a waterjet are self limiting. Water is sucked up to feed the impeller on demand - the intake is usually flush with the bottom, so if no more water can get into the intake, it just carries on along the bottom of the hull. You don't get any of the pressure effects (shockwave systems etc) that you get on an aircraft engine intake - which is where they get their increased efficiency at speeds and altitude from.
Water moves into the intake because the impeller has created an area of low pressure infront of it - like the wing - in the direction is wants to travel.


Quote:
Originally Posted by DGR
THE IMPELLER...
The only thing the impeller does is provide an increase in water pressure by accelerating the water and cramming it downstream. That's all.
The impeller increases the pressure behind it by moving the water from infront of it behind it, in the process it causes the area of low pressure behind it, which allows more water to move in. The impeller has to apply a force to the water to move it, the water then applys a force back onto the impeller - this is the force that moves the boat forward.

If for some reason there was a bend in the pipe leading from the impeller to the exit, the pipe at the bend would exert a force on the water to accelerate it - ie change its direction - the water at this point would push back on the pipe at the bend forcing the pipe in the opposite direction to the water.

Quote:
Originally Posted by DGR
THE NOZZLE...Provides an escape for the high pressure water, and steering. The mass of water that is fired out the back is reacted against by the rest of the boat, which moves off in the other direction.
The nozzle is bassically a bend in the pipe. When you turn the nozzle it applies a force to the water jet to change its direction (accerleration) - the water pushes back on the nozzle and hence the back of the boat moves in the opposite direction to the jet of water.
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Old 01 December 2005, 16:17   #119
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Ah, but will he be sucking it up or is he reducing the pressure at the top of the straw and the air pressure acting on the surface of his drink pushing it up the straw
Quote:
Originally Posted by DJL
Spot on
Nah,

Water will be forced into the jet because the water pressure under a planning hull is quite high (I think about 80 psi) I suspect although it will be the summer before I can prove it, that a jet will fill up quite quickly when a hull crashes back into the water after taking off a wave.

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Of course, it could just be that me brain's fekked.
That'ill be the Highland Park

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Old 01 December 2005, 16:58   #120
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blimey guys, interesting but lengthy discussion! I have just finished wading through it but got to the stage where i was just skimming so may have missed some cruicial stuff, so i may be repeating...

As has already been said water jets work on newtons 3rd law. If I stand on the back of my boat and throw a ball off the stern, the boat will move forward (ie opposite direction to which I throw the ball). Obviously the distance at the which the boat moves in this situation would be indetectable cos the ball is light and I couldn't throw it very hard...but the boat would in theory move an incy wincy bit. (codders gave a great analogy with the gun and ice rink thing)

So, its all to do with mass of substance being thrown and the acceleration of the substance. In the case of a water jet, the substance is obviously water and the energy to throw it is provided by gallons of motion lotion via a wacking great growler of an engine. The nozzle on the jet is there purely to dramatically increase the velocity of the fluid as the faster a given amount of water travels the more kinetic energy it contains. The rate of flow of the water through the intake is the same as at the exit, but because it is going through a narrow hole, it has to go much quicker.

When I throw the ball, the ball provides an equal and opposite force to my hand hence I (and the boat with me) would move the other way. if that equal and opposite force didn't exists, life would be a little tricky and I wouldn't be able to throw balls. So thats what happens in the jet, as the water shoots out the back, it "kicks" the boat away from it (bit of a mickey mouse explanation, but you catch my drift right?). What the water hits after it has come out of the nozzle has nothing to do with pushing the boat forwards (although anything that slows the water stream, ie...er...water, reduces the "stream's" kinetic energy [as daniel said...jeeez, that soton uni is a top notch educashun establishment isn't it?!]). Someone gave a good example of this with the pressure washer.

Now as far as i see it the impeller forces the water, therefore the water forces the impeller. The casing and nozzle serve only to change the direction of and speed up the water.

As has also been said before, the jet can only begin to work if it is primed with water (ie the impeller is submerged). The impeller cannot spin dry and suck the water up into it, its just not nearly efficient at sucking air to do this. So when the boat leaves the water, the fluid in the pump is ejected and it will then fail to work until the boat has landed and sunk sufficiently to prime the impeller again, thats why take up is not so quick....but i think someones already mentioned that. There will, as Pete 7.543 recurring mentioned be effects from the water being "forced" up into the intake by the baot landing again.


The only thing I'm slightly wolly on is why a jet boat can stop much more quickly. I originally thought it was because you could wack the reverse bucket down while still at full throttle, which on a beast like bugle billy you could do. However, before having a go on Jetski John's ski in Album bay he told me not to use the reverse bucket at speed as it would rip off. The jetski still accelerated negatively (no such word as decelerate ) very quickly just by releasing the throttle. My new theory is that the back of the boat/ski sits into the water more as soon as power is reduced thus turning the jet intake into a small parachute....am I about right here?

You've done well if you read all that crap.

Hope that helps, although I'm not totally sure its all correct cos my physics teacher was always a little nuts
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