Technical Question-Jetboats

[Mollers]

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.

[Milan]

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

[DGR]

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?

[jwalker]

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.

[DJL]

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.

[DJL]

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.

[DGR]

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...

[DJL]

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.

[jwalker]

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.

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.

[DGR]

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.

[jwalker]

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?

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

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.

[Rogue Wave]

Originally Posted by jwalker

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.

[DGR]

Originally Posted by jwalker

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!!

Originally Posted by jwalker

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......

Originally Posted by jwalker

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.

Originally Posted by jwalker

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...

[DGR]

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?

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

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!!

[kitten]

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

[DJL]

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.

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.

[DJL]

Originally Posted by kitten

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!

[DJL]

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...

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)

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.

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.

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.

[Pete7]

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

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.

Of course, it could just be that me brain's fekked.

That'ill be the Highland Park

Pete

[slimtim]

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