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Old 07 December 2005, 17:11   #181
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Sorry Paul
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Old 07 December 2005, 17:17   #182
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I think the engine/drive combination might work well as the jet will be possibly more robust than a bravo 3x

what's the weight of the boat please I imagine the Power to weight ratio is quite good
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Old 07 December 2005, 17:19   #183
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It's 3 tonne with no fuel, carries 1300 litres
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Old 07 December 2005, 17:25   #184
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heres the best i got of the jet drive itself



and it on the go



and another

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Old 07 December 2005, 17:29   #185
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Looks cool Steve & Caroline

Especially slicing throught the water like that !

missus
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Old 07 December 2005, 17:32   #186
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Next time Kathleen
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Old 07 December 2005, 17:36   #187
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from the stern

how many ltr a second would each engine be pushing through at full blast





paul
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Old 07 December 2005, 17:39   #188
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Cool Pics guys, tried to send you a PM But ............
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Old 07 December 2005, 17:39   #189
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Quote:
Originally Posted by pressman
Next time Kathleen
Excellent.

Mind you, looks like himself had fun at the helm.

Many thanks Steve

K & P

Will get round to pms soon.ta
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Old 23 May 2006, 11:17   #190
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More Jet Drive info

Quote:
Originally Posted by Mollulnan
Lets face it a waterjet aint very complicated. It's bloody harder to drive one than understand how it works!
Well not quite so. To really get the most out of them there remains some new work to attend to.


Quote:
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....
First part you got right....need some sort of regulation of inlet. Second part wrong


Quote:
Originally Posted by 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.
You got a lot of it right.


Quote:
Originally Posted by DGR
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 jet pump does suck water in, even when it is 'out of the water' And the nozzel is not just a directional item, but rather important to the operation of the jet pump.


Quote:
Originally Posted by DGR
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.
YES


Quote:
Originally Posted by DGR
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.
Actually the impeller is more axial than a prop


Quote:
Originally Posted by jwalker
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.
YES

I found a number of these discussions interesting. I thought some of you might find these additional discussions on the YachtForums.com site interesting as well, Jet Drive vs Prop . There are additional discussions on varible inlets and advanced nozzles from the webmaster who is extremely knowledgeable about the subject from both military and commerical experience. There is also some as rim-drive impeller suggestions I introduce.

(for some reason I couldn't make the links, as that tool bar was not available to me??)
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