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Old 01 December 2005, 10:30   #101
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Originally Posted by jwalker
Good comment. You're so constructive.

If you find this thread boring, go away.
Boring?! Who said anything about boring? I always find your contributions most interesting JW.
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Old 01 December 2005, 10:44   #102
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If you know so much about jet drives why not answer my Q instead of wasting your time and effort on idle banter with jwalker that way maybe we can all benefit from your knowledge
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Old 01 December 2005, 10:44   #103
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Quote:
Originally Posted by Mark Halliday


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?
Although you can't compress water, you can pressurise it. JW's original drawing (the box with the end missing) is right for the jet downstream of the impeller. The pressure change in the jet happens across the impeller blade (i.e. less before, more afterwards) - effectively that is all the impeller does.

The pressurised water is then accelerated by going through through a smaller hole, or nozzle, and (I think) that is where all the interesting physics occur, action and reaction etc, that make you whizz across the water with a smile on your face...

If the actions/forces happen at the nozzle, it explains why the structure to support a 'hole' is so big and chunky, and attached to the transom/hull in the way that they are.

Sound sensible?
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Old 01 December 2005, 11:53   #104
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Originally Posted by Milan
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 ...
I'm not sure what you are asking but the water is lifted into the casing by the impeller and this takes power from the system. I guess a certain amount of ram can be produced at the entry due to the motion of the boat and this will save some power.
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Old 01 December 2005, 12:03   #105
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Quote:
Originally Posted by Mark Halliday
Great minds, eh?

I dont think it has anything to do with any of the pressure laws.
Remember that pressure is just force per unit area. Pressure is kind of on the next level up in terms of thinking.

You need to break it down into smaller pieces. We've worked out that the boat moves forward because you chucking mass out the back and using the reactive force to push us forward.

So how do you apply the force to the water? Propellers work in a similar way to wings. The standard explanation is that the wing is shaped/angled so that the movement of air over the wing causes a high pressure area under the wing and low on the top. Therefore the wing moves into the low pressure area.

However, you can think about it in terms of force. Think about the wing in the diagram below moving in the direction of the black arrow. As it passes through the air its basically carving a slice of air and throwing it down (blue arrow). The wing is applying a force to the air in order to accelerate it downwards. The Newton’s 3rd law reactive force it then pushing the wing up. You also have the 'low pressure area' at the top of the wing, which in essence is a vacuum (blue scribble) - you've carved all the air out and chucked it downwards. Because the air is missing, the weight of the air is no longer pushing down on the wing.

So now we have reduced the force pushing down on the top of the wing and created more force pushing the wing up. Therefore the wing goes up

Or in terms of pressure, which is force per unit area, we have decreased the pressure on the top of the wing and increased it on the bottom.


So in the jet unit the prop/impellor is appying a force to the water, which in turn is applying a force to the prop blades. Think about it, if you undo the nut on a prop and reverse, the prop comes off.
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Old 01 December 2005, 12:12   #106
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As far as I know boat jet units won't suck water into the jet unit if they are full of air - the impellors are the wrong shape to shift the air out the back efficiently.

Therefore the jet unit needs to be full of water for it to work.


In rough water if the boat leaves the water and the jet unit empties, the water has to make its way back into the jet unit on its own (via gravity/mass/force or pressure - however you want to look at it), its not 'sucked' in by the impellor/engine. Hence you get a period without thrust while you wait for the water to make its way back in.


Props (on outboards etc) on the other hand are in instant contact with the water when the boat return from flight - no need to wait for air to clear.
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Old 01 December 2005, 12:40   #107
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DJL, I think you are talking cross purposes - what you say about wings and jets is true, but they are different in the way they work.

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).

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.

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 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.

BUCKETS...

...work like your diagram, but essentially divert where the water goes, and the boat reacts by going in the other direction.

Hope this helps!!

Dylan...
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Old 01 December 2005, 13:21   #108
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DGR I disagree with many of your points - however i'm off to eat so don't have enough time to rely properly.

However, can you explain to me how sucking works. No seriously.
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Old 01 December 2005, 13:25   #109
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Quote:
Originally Posted by DGR
The only thing the impeller does is provide an increase in water pressure by accelerating the water and cramming it downstream. That's all.
I still haven't got me head around all this but I think it must also carry the forward propulsive load.

Quote:
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.
Now that's vague bit; reacted against.

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?

Perhaps I should have been making water rockets in the physics class instead of making steam engines in the metalwork class.
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Old 01 December 2005, 13:36   #110
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DJL,

Have a cold drink with your dinner, and drink it with a straw.

When a jetboat is just sitting in the water, I would think that the jet unit is below the waterline, so it would be full of water, i.e. self primed etc.

Once on the plane, most of it will be above the waterline, and will need to draw water up into the unit to operate. The water drawn (or more controversially 'sucked') into the unit is drawn in to replace water that has gone through the impeller (much like a syphon or when you suck while drinking through a straw). If the intake gets into the air, then that buggers it up, and you lose power (same as when you run out of drink in your glass) - mainly because centrifugal impellers designed to move water are very inefficient air movers.

When the intake goes back into the water, water will force it's way back up the intake, but at steady state the effect of the impeller moving water through to the high pressure side would be a pressure reduction unless it is replaced by more water - so it sucks.

Seriously.



Dylan...

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.
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