i have a question regarding jet skis...

hi all, i need some help here...
any of you have any write ups or links that explains how jet ski propulsion works? best if it comes with a diagram??

ya thx alot! ^_^

yours,
n[o]j

 

The drive is an axial fan (propeller). Typically the engine is turning ~6000 rpm (Sea Doo, Kawasaki is trademark holder of the name "JetSki"). The fan has mutiple blades. steering is by directing the jet (exhaust), thence to turn even at low speeds the engine needs to "goosed" to just get a steering kick. ANY damage to the blades of the fan will seriously effect preformance as cavaition develops quickly at the high RPMs. Basically, do not run these in shallow water. By the time you hear the gravel in the fan the damage is done!! Replacement props are ~$100+ plus labor.

 

Reply seemed a little negative towards the jet drive system. Although valid points, they also carry big advantages. the main one being no prop protruding under the hull.
their main disadvantage, as mentioned above, is at low speeds, steering is effected.

I havnt done a write up on jet drives, other than touching on them in rescue craft applications, but in basic terms, water is drawn into a tunnel, via an inlet in the bottom of the hull, drawn in by a propeller mounted inside the tunnel. It is then forced out of a nozzel in the transom, just below the waterline. This produces a large thrust, by narrowing the tunnel towards the exit, forcing the water out with greater driving force.
The propeller blades can be damaged if allowed to suck up gravel etc, but the inlets are usually protects by a grate, or mesh of some form, which prevents items large enough to cause damage from being sucked up, usually, and this is not always the case.
Steering, as mentioned earlier, is done by directing the jet left or right, kind of like an outboard motor, just directing the driving force.
reverse is accumplished by a cup, or "bucket" that comes down over the jet, which means the water is forced out against this bucket, in effect pushing the boat, jetski, backwards. This design works fine for normal reversing operations, but for emergency stopping, or controlled reversing, it lacks some control and effectiveness, but as yet, is the only method i have seen to achieve reverse thrust.

Main reason they are used in jetskis is A, lack of prop means they are safe to use in shallower water. a prop hitting a large rock at speeds in a craft as small as a jetski could be very dangerous and very easily throw the rider off or worse.
B)ease of use. you can just drive a jetski onto your trailer and go, without having to mess around lifting props out of the water.
C) they generally take up less space. Jet skis are light, and small, and often use small engines, a jetdrive takes up most of the space which normally is wasted, under the engine, etc, where as using outboards or outdrives would not only require gearboxes and such, but also take up valuable room, requiring the craft to be bigger, and heavier.
D) they are designed mainly for fun. a safe (yes, they are meant to be safe) boat to play in. If you fall off, it stops, you get back on, and carry on. with a prop this would be a problem, for fear of being struck by the prop.

basic way it works. I dont have any diagrams, but hopefully that gives you a general idea