Technical Question-Jetboats - Page 8

Matt · 02 December 2005, 11:12

No it's not, it's the acceleration of the mass

Quote:

Originally Posted by DGR

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).

Jonny Fuller · 02 December 2005, 11:46

Quote:

Originally Posted by DGR

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.

Realy? what makes this a centrifugal pump rather than axial?

looks like a regular axial impellor & stators to me (jet ski)

Matt · 02 December 2005, 11:56

Centrifugal, Axial - makes no difference.

Your diagram, as you note, is axial, but I have seen centrifugal pumps on jets too (had a weird jet outboard once that had one)

DGR · 02 December 2005, 11:57

JF,

So it does.

The jetski impeller I looked at yesterday wasn't axial, so I assumed they were all like that - ah well!!

That looks like its just a prop in a tube - there is no constriction in the system at all - it looks like it is a constant area duct.

That would need the water to enter the impeller and leave the unit at the same speed - so how would it stop the prop cavitating?

MadMat - it would make a difference if you were pressurising the water. I think a centrifugal impellor is less likely to cavitate than an axial impellor.

jwalker · 02 December 2005, 12:02

DGR thanks but you're mostly telling me what I know.

JF, you're at where I was a couple of days ago.

It's not a bow thruster cos that's just a propeller in a tube. However, waking up with a tad more brain power than I had last night has been a help and I think I'm getting closer.

This morning's puzzles. I was considering why there is a recoil when a bullet moves down a barrel but there would not be one if it got stuck and the pressure was simply turned to heat.
The bullet is absorbing energy because it is free to move and it is storing this as kinetic energy.
The energy will be released when the bullet hits something but that is only minorly significant cos that's where the waterjet is going to get it's manoeuvrability from.

My conclusion from this was that there needs to be something moving and in the waterjet is has to be water.

The waterjet needs to be able to hold water at a high pressure to provide the propulsive force and so it cannot simply be an open tube. Also we need to get the moving water (the bullet) to store as much energy as possible. The stored energy is going to be largely wasted out the back. Kinda like the bullet after it's left the barrel.

The way to get the energy into the water is to increase its velocity. Also, the more mass of water we can increase the velocity of, the better because it will store more energy. However, there will be a limit to the mass of water that can be got moving because if the hole out of which the water is flowing gets too big, the pressure inside the jet will begin to fall and it's this pressure which will make the boat move forward.

Enter, the nozzle. The nozzle and the taper before it is the device which is going to accelerate the water to increase its velocity and, hence, absorb energy at this end of the system. This is the bullet. So, now we've got something for the pressure at the forward end of the jet to react against. Of course, because the water jet is capable of drawing in water, it is able to constantly replenish the bullet. In turn, the boat will absorb the energy imparted to it and also store some of it.

I think......

Jonny Fuller · 02 December 2005, 12:03

Quote:

Originally Posted by DGR

JF,

That looks like its just a prop in a tube - there is no constriction in the system at all - it looks like it is a constant area duct.

I can see a constriction!

I agree that obviously, accelarating the water is what gives the reaction.

as I see it, if it has stators, it can't really be a centrifugal type pump, as maty says, I've also seen OB's with a centrifugal pump in place of the lower unit, and 'surf jets' (powered surf boards) have the same arangement, but they're wholely different from the type of pump we usually see in a jet boat.

the Jet Sprinting boats that David Manning, TD, Chrazyhorse and I had a go in, had what they described as two stage pumps, presumably the second stage impellor had a steeper pitch.

DGR · 02 December 2005, 12:03

Quote:

Originally Posted by MadMat

No it's not, it's the acceleration of the mass

It's both. In a pressure jet, more of it comes from the V than the m. In a conventional prop, more comes from the m than the V. Try the pressure wash experiment.

Acceleration of the m is just increasing the V!!

Whatever it does, it is the m x V that is important.

DGR · 02 December 2005, 12:11

Quote:

Originally Posted by Jonny Fuller

I can see a constriction!

Where? The outer casing just follows the profile of the gubbins on the inside of the tube. That is a propeller in a tube with a swivle on the end to direct the jet, but not a constriction in a nozzle.

Maybe there is more than one kind of waterjet? A 2-stage system would imply that it is pressurised. It would be interesting to see a cut away of one of those!!

D...

jwalker · 02 December 2005, 12:21

Quote:

Originally Posted by DGR

Where? The outer casing just follows the profile of the gubbins on the inside of the tube. That is a propeller in a tube with a swivle on the end to direct the jet, but not a constriction in a nozzle.

There is a constriction. You're maybe looking at it as 2D. The area around the stator will be considerably larger than the area of the nozzle.

It's interesting that they have left such a big space between the end of the inner nozzle and the steering nozzle. I wouldn't be surprised if this was a way of further increasing the mass of accelerated water by entraining it into the steering nozzle.

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

Edit on that last bit. I'm talking pish cos the jet won't be under the water when it's travelling.

Jonny Fuller · 02 December 2005, 12:23

Quote:

Originally Posted by DGR

Where? The outer casing just follows the profile of the gubbins on the inside of the tube. That is a propeller in a tube with a swivle on the end to direct the jet, but not a constriction in a nozzle.
D...

I'm sure if you were to calculate the area of the nozzle, and the area tween pump body and impellor hub, the latter would be greater.

here's some interesting stuff from a jetsprinting site, including talk of a three stage pump:

The development of the Jet Pump has had a great influence on performance

The first production unit appeared in 1954. The Hamilton "Quinnat" was a vertical shaft centrifugal pump friven through a right-angle gearbox, and was followed two years later by the simpler "Rainbow" which had a direct drive The most significant early designs were the "Chinook" that was produced between 1957 and 1963 and its successsor, the much simplified "Colorado" - both multi-stage axial-flow units that were very similar to the units used in jetsprinting today.

In the early years of the sport the Hamilton "Colorado" was the popular choice, but the most widely used and dominant unit in jetsprinting today is the Scott Jet 2-stage pump, designed and manufactured in the Geraldine (South Canterbury) workshop of Brian and Konrad Scott.

The main components of the jet unit are:-

Inlet Housing - rear mounted over the water intake in the bottom of the hull. The first impellor blade is mounted in the rear of the inlet housing in a closely machined tunnel.
Grille - mounted below the water intake to prevent debris entering the unit.
Stator Unit - mounted behind each impellor blade, the stator is a stationary set of blades that remove the "twist" from the water as it leaves the impellor. In a single-stage unit the water exits directly through the steering nozzle, in a 2-stage unit there is a second impellor and stator, in a 3-stage unit there is a third impellor and stator.
Steering Nozzle - directs the outlet water stream that steers the boat.
Reverse Bucket - rotates out behind the steering nozzle to direct the water back under the boat to provide reverse thrust.
The jet unit must operate by direct drive from the engine.

Performance gains in the jet unit come from several sources:-

Different styles of intake grills.
Altering the length, pitch angle and number of impellor blades
The length and number of vanes in the stator.
Nozzle size - a smaller nozzle diameter gives high velocity and less water volume, a larger nozzle diamter gives low velocity and bigger water volume
Vernier Wedges between the tailpipe and nozzle to raise or lower the jetstream.
Jacking Screws in the nozzle insert for finer control of the jetstream.
The evolution "big-bore" Scott Jet 2-stage unit has an increased diameter from 8 1/4 to 8 1/2 inches - even such a small increase in size equates to a significant increase in boat speed and cornering ability with a water flow rate of more than 20 tonnes a minute passing through the jet unit.

The torque factor of the engine is critical in determining what impellor/stator/nozzle set-up to run at a particular track.

also, here's a couple of pics of the business end of the jet-sprinters we saw. ...loads'a bits inside!

DGR · 02 December 2005, 13:04

Nice!!

I stand corrected on your constriction. I have to think more in 3D...

20 tons minute is a lot of water!! So axial impellers increase pressure at each stage, and the nozzle works to accelerate the flow of the water as it returns to atmospheric pressure.

Yes?

jwalker · 02 December 2005, 13:14

Quote:

Originally Posted by DGR

.. and the nozzle works to accelerate the flow of the water as it returns to atmospheric pressure.

Is it your understanding that the water in the nozzle it at atmospheric pressure?

Jonny Fuller · 02 December 2005, 13:39

Quote:

Originally Posted by DGR

Nice!!

So axial impellers increase pressure at each stage, and the nozzle works to accelerate the flow of the water as it returns to atmospheric pressure.

Yes?

Well, i'm assuming that the succesive impellers (sp?) are within a cone, getting smaller in diameter with each stage, but each having a higher pitch on the impeller blades. I would think it would have to work summat like that, as if the velocity is increasing along it's path, the hole must get smaller to keep it all in ballance, so to speak.

Scary Des · 02 December 2005, 13:40

Think I’ve got this now

one question still foxing me is why aim the jet back into the sea

Given that I’ve understood this correctly, the higher the velocity the better and the jet isn’t ‘pushing’ so anything in the way of the jet will reduce the velocity and have no advantage.
Sure aiming the jet down will facilitate steering and trim but for speed, ideally the jet should aim horizontally clear of the water.

Des

Jonny Fuller · 02 December 2005, 13:42

aiming it down probably gives a little lift helping reduce drag from the hull.

And, I don't think velocity is the key word, I think it's the shere weight of water that you grab hold of, and throw back quickly (accelerate) that gives the thrust.

Simon B · 02 December 2005, 13:55

Also probaly helps reduce the bulk of the engineering of the thrust directing bucket(s) We have some awesome pumps at work for fluming whole potatos, the volute casing looking similar to an outboard jet unit. They can also pump whole eggs or lobsters or whatever un-broken. Not sure who the lady is maybe need a factory visit to Cornell USA.

Mark Halliday · 02 December 2005, 13:55

OK.
I have just come been to the pub for a beer or 3 with a mate of mine who is a hydraulic engineer and quite bright despite that.
I have to admit that I understood s*d all of his explanation of how and why a water jet works.
One thing that did stick, was that the reason for the nozzle was to increase the velocity of the ejected water, and that is only to make sure it is traveling faster than the boat.
The expression "nozzle coefficient" was mentioned several times. Google time I think.

I'm off for a nap now before the missus comes home.

Jonny Fuller · 02 December 2005, 14:01

Quote:

Originally Posted by Mark Halliday

OK.
One thing that did stick, was that the reason for the nozzle was to increase the velocity of the ejected water, and that is only to make sure it is traveling faster than the boat.

That's kind'a what I was trying (and failing) to say.

PS. nice pump Simon B!

DGR · 02 December 2005, 14:13

Quote:

Originally Posted by Jonny Fuller

aiming it down probably gives a little lift helping reduce drag from the hull.

And, I don't think velocity is the key word, I think it's the shere weight of water that you grab hold of, and throw back quickly (accelerate) that gives the thrust.

I think it's both. It's the change of velocity (acceleration) AND the mass that's important. Compared to a conventional propeller it moves less mass, but faster to compensate.

And I think that the water would need to be at atmospheric pressure as it comes out of the nozzle (or pretty close to it anyway). As I understand it, its the reduction in pressure through the nozzle that provides the increase in velocity.

D...

Jonny Fuller · 02 December 2005, 14:36

Quote:

Originally Posted by DGR

And I think that the water would need to be at atmospheric pressure as it comes out of the nozzle (or pretty close to it anyway). As I understand it, its the reduction in pressure through the nozzle that provides the increase in velocity.

D...

'Escaping to' the atmospheric pressure is probably more accurate isn't it.