Technical Question-Jetboats

[jwalker]

I think me brain's just come back on line. It'll be the reaction to accelerating the water not the reaction to the accelerated water.

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

Doesn't have to thrust against anything - ask Newton!!!

Well Cod, since Newton is not here, could you please explain it?

[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.

[jwalker]

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.

You don't fekkin know.

[Pete7]

My jet unit has a large flat plate underneath about 12 x 12". If at planning speed this is level with the water surface then the jet exit water will be above the surface however it is angled down which suggests that the exit water will contact the surface about 6 - 12" further aft. Well thats what I hope it will do if I can re build it.

Pete

[codprawn]

Originally Posted by jwalker

You don't fekkin know.

Know what??? Thought it was SO obvious myself!!!!

Could make the joke about it hardly being rocket science but to be honest it IS - and there's nothing complicated about that either!!!

[Mark Halliday]

Pete.

Is there anything you have not got lying around your garden?



There must be a tea-chest full of used tenners somewhere.

[codprawn]

Originally Posted by Mark Halliday

Pete.

Is there anything you have not got lying around your garden?



There must be a tea-chest full of used tenners somewhere.

Sounds as bad as me - doubt I have a tea chest full of tenners though!!!

[Erin]

Just a thought... but I understood the most effective jet-drive was when the jet was just above or partially above the water line and pointing down/at it. I thought that if the jet was totally submerged it wasn't as effective.

I may be wrong of course, but I did pick that nugget of info up somewhere in the dim and distant, but didn't think it would ever come in handy

[jwalker]

Originally Posted by codprawn

Know what??? Thought it was SO obvious myself!!!!
.. - and there's nothing complicated about that either!!!

Well I'm a numpty, explain it to me.

[DGR]

Originally Posted by Erin

Just a thought... but I understood the most effective jet-drive was when the jet was just above or partially above the water line and pointing down/at it. I thought that if the jet was totally submerged it wasn't as effective.

I think that you are right, but it's because a jet of water won't accelerate as much if going straight into water, whereas if it goes into air you get a bigger/better acceleration effect (Force = Mass x Acceleration) before it hits the water.

If you use a pressure washer to clean the car, it doesn't push back harder the closer you are to the car - the force all comes from the water coming out of the nozzle, not what it hits.

It's the same as the hosepipe into a bucket that Kitten mentioned earlier.

...I think...

[codprawn]

Originally Posted by jwalker

Well I'm a numpty, explain it to me.

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.....

[Pete7]

Originally Posted by Mark Halliday

Pete. Is there anything you have not got lying around your garden? There must be a tea-chest full of used tenners somewhere

Hmm, yes collection is growing however its mainly scrap, but your a fine one to talk, you have a different 4x4 for each day of the week don't you

Pete

[Shaggy]

How a Waterjet Works

A waterjet generates propulsive thrust from the reaction created when water is forced in a rearward direction. It works in relation to Newton's Third Law of Motion - "every action has an equal and opposite reaction". A good example of this is the recoil felt on the shoulder when firing a rifle, or the thrust felt when holding a powerful fire hose.

Put simply, the discharge of a high velocity jet stream generates a reaction force in the opposite direction, which is transferred through the body of the jet unit to the craft's hull, propelling it forward (see diagram below).

In a boat hull the jet unit is mounted inboard in the aft section. Water enters the jet unit intake on the bottom of the boat, at boat speed, and is accelerated through the jet unit and discharged through the transom at a high velocity.

The picture below shows where water enters the jet unit via the Intake (A). The pumping unit, which includes the Impeller (B) and Stator (C), increases the pressure, or "head", of the flow. This high pressure flow is discharged at the nozzle (D) as a high velocity jet stream. The driveshaft attaches at the coupling (F) to turn the impeller.

Steering is achieved by changing the direction of the stream of water as it leaves the jet unit. Pointing the jet stream one way forces the stern of the boat in the opposite direction which puts the vessel into a turn.

Reverse is achieved by lowering an astern deflector (E) into the jetstream after it leaves the nozzle. This reverses the direction of the force generated by the jet stream, forward and down, to keep the boat stationary or propel it in the astern direction.

Shaggy

[Mollers]

Don't you start Shaggy!! This has turned into the " Look how clever I am!" thread. Lets face it a waterjet aint very complicated. It's bloody harder to drive one than understand how it works! You chill out and have a glass of wine mate, there's a bit of swell due later in the week.

[Mark Halliday]

Kathleen, I hope you have read this far.


So..........

Water goes in, water gets squashed, water goes out, boat moves...........




Donuts MMMMMMMMMMMMmmmmmmmmmmm

(Homer Simpson c2005)

[Pete7]

Mark, she might be more familiar with the idea if you mention one of these

TAXI

[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.

[The Jackeens]

Originally Posted by Mark Halliday

Kathleen, I hope you have read this far.

(Behave Cap'n Pete 7.5 )

I've certainly had a go at figuring it out Mark

Jeff am I right in thinking your diagram does not take into account the extra forces applied (impellor etc.?)

Thanks for all very helpful replies. Here are a few misconceptions I have now formed as a result ! Please correct!

• Water is pumped by an impellor through a large intake and forced though not necessarily compressed in volume through a smaller outlet = an increase in speed and pressure
• Stator 'aligns' the water
• 'Homemade jetwash effect' (with hosepipe) or 'fireman's hose effect'
• Newtonian effect : action produces equal & opposite reaction
• Greater force apparent near outlet than near intake (more about blowing)

advantages

• fewer moving parts
• no chopper proper (external prop)
• stop on a sixpence
• good fun for passengers
• potentially good for slow speed manoeuvres

disadvantages

• can suck in debris
• fuel guzzling
• might lose some drive in the rough ? However jets used by RN (not fairweather boyos but need jets for other reasons too)
• need to be well set up to avoid 'slide'

(Oh and for good measure Kitten explained why one gets that sinking feeling (literally ) if prop cavitation occurs)

OK....

(Does a jetwash recoil when closer to the car? )

Perhaps the jet may operate better (i.e. propel) in the opposing pressure of water (like feet pushing off from blocks in a race) than in air (less dense?). (The likelihood of hitting air might be more likely in rough conditions so not mutually exclusive? )

Then again, does the effect of submergence mean less power to the boat ? Less acceleration?

What's the effect of the stator/impellor on JW's diagram of 'pressure'?

Some very helpful posts above and links too! (Cheers gArf that's a good 'un).

Many thanks

missus

[Hugh Jardon]

Originally Posted by Richard B

One teeny little point, Mark - The water doesn't get squashed. Liquids, unlike gases, are incompressible.

not strictly true but i would say for the purposes of considering how a water jet works then true enough

all that happens is the prop speeds up the water and the converging jet funnel speeds up the water still further causing it to exit the tube at high velocity giving thrust

as kitten said...spot on....hopefully

[Richard B]

OK shiney-bottom, I'm going to insist that you measure your RIB with a callibrated micrometer, to the nearest thousandth of a millimetre!