blimey guys, interesting but lengthy discussion! I have just finished wading through it but got to the stage where i was just skimming so may have missed some cruicial stuff, so i may be repeating...
As has already been said water jets work on newtons 3rd law. If I stand on the back of my boat and throw a ball off the stern, the boat will move forward (ie opposite direction to which I throw the ball). Obviously the distance at the which the boat moves in this situation would be indetectable cos the ball is light and I couldn't throw it very hard...but the boat would in theory move an incy wincy bit. (codders gave a great analogy with the gun and ice rink thing)
So, its all to do with mass of substance being thrown and the acceleration of the substance. In the case of a water jet, the substance is obviously water and the energy to throw it is provided by gallons of motion lotion via a wacking great growler of an engine. The nozzle on the jet is there purely to dramatically increase the velocity of the fluid as the faster a given amount of water travels the more kinetic energy it contains. The rate of flow of the water through the intake is the same as at the exit, but because it is going through a narrow hole, it has to go much quicker.
When I throw the ball, the ball provides an equal and opposite force to my hand hence I (and the boat with me) would move the other way. if that equal and opposite force didn't exists, life would be a little tricky and I wouldn't be able to throw balls. So thats what happens in the jet, as the water shoots out the back, it "kicks" the boat away from it (bit of a mickey mouse explanation, but you catch my drift right?). What the water hits after it has come out of the nozzle has nothing to do with pushing the boat forwards (although anything that slows the water stream, ie...er...water, reduces the "stream's" kinetic energy [as daniel said...jeeez, that soton uni is a top notch educashun establishment isn't it?!]). Someone gave a good example of this with the pressure washer.
Now as far as i see it the impeller forces the water, therefore the water forces the impeller. The casing and nozzle serve only to change the direction of and speed up the water.
As has also been said before, the jet can only begin to work if it is primed with water (ie the impeller is submerged). The impeller cannot spin dry and suck the water up into it, its just not nearly efficient at sucking air to do this. So when the boat leaves the water, the fluid in the pump is ejected and it will then fail to work until the boat has landed and sunk sufficiently to prime the impeller again, thats why take up is not so quick....but i think someones already mentioned that. There will, as Pete 7.543 recurring mentioned be effects from the water being "forced" up into the intake by the baot landing again.
The only thing I'm slightly wolly on is why a jet boat can stop much more quickly. I originally thought it was because you could wack the reverse bucket down while still at full throttle, which on a beast like bugle billy you could do. However, before having a go on Jetski John's ski in Album bay he told me not to use the reverse bucket at speed as it would rip off. The jetski still accelerated negatively (no such word as decelerate
) very quickly just by releasing the throttle. My new theory is that the back of the boat/ski sits into the water more as soon as power is reduced thus turning the jet intake into a small parachute....am I about right here?
You've done well if you read all that crap.
Hope that helps, although I'm not totally sure its all correct cos my physics teacher was always a little nuts