Technical Question-Jetboats

[hard1]

Blimey.

[Hugh Jardon]

not sure my argos micrometer set will be up to that but could give it a go i guess

[Richard B]

I expect you to use nothing inferior to a Mitutoyo!

[DJL]

Originally Posted by codprawn

Newtons 3rd law - for every action there is an equal and opposite reaction.

Way a rocket works in space - chucks mass out the back so it goes forward.

That’s exactly it.

You’re accelerating a mass, in this case water, out the back of the boat. To accelerate a mass you need force acting upon it. The engine/jet system is applying a force to the water to accelerate it out the back of the boat. Now according to Newton’s 3rd law there is reactive force acting upon the boat in the opposite direction, which moves it forward.

The reason it’s more efficient to discharge the jet into air is that air applies a much smaller force onto the jet of water than water. Think about moving your hand through air and through water - its much harder to accelerate you hand in water than air. The same thing applies to the engine - it has to work harder to accelerate the water jet when discharging into water.

[jwalker]

Originally Posted by codprawn

Stand on an ice rink and fire a gun - the bullet will go forwards - you will go backwards.

It is all down to the mass and the force it is propelled with. Fire a small calibre air rifle and you won't move much - fire an elephant gun and you will.....

That's not telling me how it works. That's telling me the result of it's action. Anyway, I understand that bit.

Draw a diagram to explain to me where in a waterjet the forces are being applied. Where is the force against which there is the reaction? Where is the propulsive reactive force applied, to the blades of the impeller and thence through a thrust bearing and on into the structure of the boat? To the inside of the casing?

I still don't understand it properly and I'm sure the notion of the fast stream of water squirting out the back somehow pushing the boat along is nonsense.

To Moll, you are being glib and flippant but you haven't taken the trouble to explain to us how the system works. If this thread is below you, I suggest you don't read it.

I'm Mr. Grumpy tonight.

[jwalker]

DJL, thanks for that.

[jwalker]

Originally Posted by jwalker

.. I'm sure the notion of the fast stream of water squirting out the back somehow pushing the boat along is nonsense.

I take that back cos there's going to be a fair amount of kinetic energy stored in that water.

[The Jackeens]

Originally Posted by jwalker

Draw a diagram to explain to me where in a waterjet the forces are being applied.

Jeff, do you think the force being applied is the mechanical one of the impellor/stator? If that's what's accelerating the water (mass) out the back & through the smaller opening then that must be causing the initial action (like pulling the trigger or snapping an elastic band.) (Thus the action/reaction Newton bit).

DJL found your explanation about operation of the jet in air (as distinct from water) clear & helpful.

Where is the force against which there is the reaction? Where is the propulsive reactive force applied, to the blades of the impeller and thence through a thrust bearing and on into the structure of the boat? To the inside of the casing?

Oh OK I think I get it. You want the exact mechanics of where the impellor acts and where the energy transfer actually happens ? Down to the nuts and bolts ?

This link posted by gArf is pretty helpful but I haven't checked it all to see whether it is demonstarting the specific mechanics of the thing.

Hamiltons here (again)http://www.hamjet.co.nz/index.cfm/Th...jet_Works.html

(Next build project? Pete 7 seems to be working on a Fareham version in the back garden so perhaps he will keep us posted )

BTW thanks again for all helpful contributions

missus

[eupa]

just come back . Wow ,what knowledge!!, what writing, what
prowess, Really been missing something!!!!!!!!!!!!.

jonathan

[kitten]

Originally Posted by jwalker

I take that back cos there's going to be a fair amount of kinetic energy stored in that water.

Now you are just laying traps for people. smack your wristies

[Rodan]

Originally Posted by codprawn

Well spotted - yes he IS right that Bernoulli's Equation is the one to use but for some reason he has posted Boyle's law instead - a slip of the old mouse perhaps???????

Indeed. Both apply. Bernoulli is more about the boundary layer - how liquid/gas flow will have a slower cusion when running against a solid surface. A lot to do with wind tunnels so pretty similar to jets. A very hazy memory from an engineering degree many many moons ago...

[Mark Halliday]

Originally Posted by Richard B

One teeny little point, Mark - The water doesn't get squashed. Liquids, unlike gases, are incompressible. So the water jet has an impeller, not a compressor.

It was just a quote from the Illiad, or is that the wrong Homer?

OK. Read sqashed as pressurised, not compressed.

[Mark Halliday]

Originally Posted by jwalker

That's not telling me how it works. That's telling me the result of it's action.

JW
How are you with letting go of ballons before tying the neck?
This might be a new line on understandin.

[jwalker]

Originally Posted by kitten

Now you are just laying traps for people...

Caught.

[Mollers]

[jwalker]

Originally Posted by Mark Halliday

JW
How are you with letting go of ballons before tying the neck?
This might be a new line on understandin.

Funny you should say that cos I was reading the Rolls Royce website last night to try and get more understanding and they used that as an example but it brings us back to exactly the situation in the two wee diagrams I drew. Closed container, then open at one end.

However, I think I'm beginning to get an inkling now of various conditions inside the jet.

[jwalker]

Originally Posted by Mollulnan

Good comment. You're so constructive.

If you find this thread boring, go away.

[Mark Halliday]

Originally Posted by jwalker

Funny you should say that cos I was reading the Rolls Royce website last night to try and get more understanding and they used that as an example but it brings us back to exactly the situation in the two wee diagrams I drew. Closed container, then open at one end.

Great minds, eh?

I am not sure WHY it works either. Just trying to get my head around it too.

Firstly, thanks to DJL.

I am starting to think along the lines of Force = Mass x Velocity with Newtons 3rd. I dont think it has anything to do with any of the pressure laws.

You have a given Mass (the water sucked up) you Accelerate it with the impellor and nozzle which increases the Force applied. Apply Newtons 3rd and you have a reaction force in opposition and equal to the ejected Mass.
The force is transfered to the boat via the jet assembly because the impellor will not let the water return from whence it came.
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?

[Mark Halliday]

Originally Posted by Mollulnan

Several bottles of wine has that effect on me too.

[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 or am I just talking B*ll*cks

PS. being a diver we love jet drives no fear of props taking your head of and when on a boat with twin jet drives the diruption of water at the back of the boat is vast which means you can run and jump of the back into the water at 30 knots