PWC normal pump rpm

[inventing_man]

Greetings ,
Does anyone know what the normal rpm range is on a PWC with direct drive ? When I say "normal" I'm speaking of the range of best performance. I also guess that "most" modern PWC's are infact direct drive ?
Thanks!

[marshmat]

Depends on the PWC, and yes, they do tend to be direct drive. A PWC's engine will often run 5000-6000 rpm, if not more. Manufacturers will thus tune the pump to be at its best performance at the engine's best-power RPM (usually near its top end on these high-strung motors), and will set the impeller to absorb the engine output power at that speed without cavitating. So you end up with a very small impeller turning very fast. Check the max RPM and power rating on the engine a jet-ski pump was designed for, and that's probably the pump's optimal input.

[inventing_man]

Thanks marshmat. So then its possable to have a pump tuned to be more low end torque than speed . Thats kinda what I'm looking for .
Who does this sort of tuning? Solas ?

[gcunagin]

Power absorbed by a water jet is very predictable. It works on a cube function. Basically, If you half the rpm's, horsepower absorption is reduced to 1/8th. I.E. a pump that absorbs(turns to thrust)100 hp@6000 will absorb 12.5 hp @ 3000 rpm's. That is why the 14' river jon boat I am building is using a large high pitch Yamaha pump only using 25HP@3600 rpm. I am hoping the pump won't mind a little sand and gravel so much at the lower rpm's.

This site has good basic design information.:

http://www.ultradynamics.com/section...nal/design.asp

See:
Waterjet Impeller Nomenclature (Impellers Demystified)

Have fun,

Gary C.