Technical Question-Jetboats

[slimtim]

Originally Posted by DJL

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)

daniel is right. A wing infact generates the majority of its lift from its angle of attack. The lift generated by the bernouilli's principle is not infact the major source of lift (i'm pretty sure of this but I'm prepared for stiff opposition ). Just incase anyone is unsure, bernouilli's principle (as far as wings are concerned) is based on the fact that faster moving air is at a lower pressure, so by pushing a wing profile through the air, you're forcing the fluid (air) to travel faster over the top edge because it has further to travel in the same amount of time than the fluid travelling along the bottom edge.

oooo, I love these discussions

[slimtim]

ok, what I wrote there is slightly, how shall I put it...wrong.

Increasing the angle of attack just increases the effect of bernoulli's principle, until you reach the stall angle...well, thats what a magic frog told me anyway.

[Hugh Jardon]

gosh the water dyson discussion is going at some pace here


how many of you learned chaps are going to see the rib ship on sunday to see all this in action then?

[DGR]

Originally Posted by DJL

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)

Slimtim - beg to differ, but Daniel isn't right. No air above the wing? Where does that go then? Lift has nothing to do with the lack of air (and it's weight) and gravity.

You are right that the lift does come from the pressure difference between the upper and lower surface - the air travels further over the top surface - and accelerates to accommodate that, so the pressure drops. The differential in pressure gives the lift.

ANYWAY - you can only go so far with the air and water analogy - 'cos it doesn't really work as water is incompressible, and air is very compressible until it goes supersonic, and then it gets even more complicated.


As far water jets go, as far as I can tell, for the most part we are agreeing on the same things.

Sucking = low pressure, so water travels towards the low pressure. Can we therefore agree that the impeller, which produces a low pressure area, draws water into the intake?

Where I disagree is on the nozzle. If the nozzle is only directional, why is the impeller not just a propeller in a tube? The jetski impellers I've seen are centrifugal - why is that? The only reason I can think of is that it needs to generate higher pressures, to give higher velocities in the outlet. To do that, it must accelerate through a nozzle - so surely it must be shaped with a constriction?

The force comes from Mass X Velocity. Mass is constant through out the jet system (otherwise it would accumulate somewhere) - so the only thing the jet drive can do is accelerate the water to the highest velocity it can - and so an impeller pressurises the water and then the nozzle constricts the water column, letting it return to atmospheric pressure as it accelerates at the nozzle throat.

If it didn't constrict in the nozzle, it would be as efficient as a propeller in a tube.

Yes? No? Burble?

[DGR]

My final thought on the matter tonight...

The engine provides the power to pressurise the water with the impeller, the nozzle provides the increase in velocity - and ultimately the change in momentum to produce the required force.

Still not 100% sure about the nozzle...

[Pete7]

Originally Posted by DGR

As far water jets go, as far as I can tell, for the most part we are agreeing on the same things. [Probably]

Sucking = low pressure, so water travels towards the low pressure. Can we therefore agree that the impeller, which produces a low pressure area, draws water into the intake?

Hmm not convinced, at the waters surface the water pressure is only just above 1 bar absolute (14.5 psi) so its hardly rushing into the jet unit but at speed the hull generates a force of say 80 psi (gauge pressure) or 6.5 bar absolute, now thats more like it.

[QUOTE Yes? No? Burble? [/QUOTE] no but interesting discusion for the long winters night. Makes you wonder about peeps like Bill Hamilton working away on his own in the early days trying to make it work.

Pete

[DJL]

The newtonian and bernoulli explainations are both correct - and say the same thing. Both require that the wing to deflect air downwards. Lookup the Kutta condition. A cambered wing acts like a tilted uncambered wing.

The speed differnce over a cambered wing is not caused by the air having to travel faster to travel the greater distance over the top of the wing. The air being deflected downwards by the wing causes the air to circulate around the wing - the direction of the circulation increases the speed of the air above the wing and slows it below. Hence the pressure difference

[DGR]

Pete,

I see what you mean, but by 'sucking' I meant that the impeller will reduce the pressure slightly at the impeller face from whatever it is under the hull - i.e. will encourage water go up the intake to the impeller.

I completely agree with you on the the airborne jetboat landing - the pressure under the hull would shove water into the intake replacing the air in there very quickly. I guess that any lag in power would be due to re-pressurising the water flow downstream of the impeller and getting the jet back up to full power?

D...

[kitten]

Originally Posted by Pete7

Hmm not convinced, at the waters surface the water pressure is only just above 1 bar absolute (14.5 psi) so its hardly rushing into the jet unit but at speed the hull generates a force of say 80 psi (gauge pressure) or 6.5 bar absolute, now thats more like it.

At atmospheric on a 6" square grid, that could be up to 237Kg pushing the water in, that enough?

[Pete7]

Originally Posted by DGR

Pete, I see what you mean, but by 'sucking' I meant that the impeller will reduce the pressure slightly at the impeller face from whatever it is under the hull - i.e. will encourage water go up the intake to the impeller.

the pressure forcing the water into the jet can't be anymore than the air pressure pressing down on the sounding water. Afterall the impellor can't generate a vacuum, the reason the water will flow into the jet is because the surounding water is subjected to an air pressure of 1 bar when the boat is stationary or lots of bars when the hull is on the plane, dianamic stability perhaps generating a large pressure, afterall if Newtons 3rd law applies then the whole weight of the boat is supported on a small running surface hence high pressure.

I completely agree with you on the the airborne jetboat landing - the pressure under the hull would shove water into the intake replacing the air in there very quickly. I guess that any lag in power would be due to re-pressurising the water flow downstream of the impeller and getting the jet back up to full power? D...

agreed, guess the folks will find out this weekend if Pressman can get her airborne

[Pete7]

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.

Pete

[DGR]

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.

D...

[DGR]

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!!

D...

[Pete7]

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.

Pete

[jwalker]

Originally Posted by DJL

JW, you’re 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

[Mark Halliday]

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?).

[Matt]

.

[DGR]

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.

D...

[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

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, wouldn’t 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 I’m looking at it too simplistically, but I reckon that’s the crux of it.

The water jet system is a highly tuned, sophisticated bow thruster. (with a nozzle)

[DGR]

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).

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.

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.

Originally Posted by Jonny Fuller

Moving the water through that tube quickly, say 50knts, wouldn’t 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.

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.

Originally Posted by Jonny Fuller

Mebbe I’m looking at it too simplistically, but I reckon that’s 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?

D...