Impeller theory

[plantchairman]

LV:
Ive got a cottage on an island in Lake St.Clair (one of the lesser known Great lakes) I do a lot of motoring back and forth to the marina ferrying company and family. I save the speed runs for the swimming hole and blowing away the prop boats in the lake.
I have heard of over propping a boat (dogging it) i guess you could compare it to driving your car around in 4 th gear all the time. Fine on the hiway, crap in the city.
But a jet over propped?? Ive allways thought of a jet being a dyno on the back of your boat. It will hold your motor at its hp/rpm at what the impeller is designed for. Every Impeller/volute (bowl) made has a torque curve graphted for it. that way an end user can set different gpm and flow rate to a pump by applying a different hp for a given rpm to the same pump.
In my line of business (makeing salt out of brine) we will sometimes put in a smaller impeller behind a given volute, but all this will do for us is reduce our efficency ( same amount of power to move less brine). So we allways try to reduce/increase hp aplyed to a pump to change volume.
hence my idea of a big impeller and increase/decrease hp (throttle position) to change volumes, again i never really had anybody to talk to about this, not to many hot boat jetters around these parts.

[UBFJ #454]

In all the impeller test data that I've seen, the maximum nozzle output is, within a % or 2, the same for all impeller cuts ... the only difference is the RPM at which the maximum occurs.
This is why in jetboat pumps different cuts are used to match to different motor Tq/Hp curves ... Different impeller cuts, impeller blueprinting, tweeking, etc. are all done to Match a Given Pump to a Particular Motor's Output Characteristics ... One wants the pump efficiency characteristics (Pump's Thrust RPM Spectrum) to match the motor's output characteristics (Motor's Tq/Hp PRM Amplitude Spectra).

[Squirtcha?]

Off topic, but bringing back some memories. As a kid my family used to boat on Lake St. Clair. Used to hang out on Strawberry island and swim/mess around. Had some good Walleye fishing in the river back in the day as well.
Man, that was like 35 years ago.

[LVjetboy]

Plantchairman, alway glad to talk tech stuff. BTW, just my thoughts I'm no pump/prop expert.
Pump as a dyno: I've also heard this phrase before. Seems to make sense as a "single point" dyno. Or you could map your engine's power curve by swapping out impellers and recording each full throttle rpm point. With accurate pump curves figure a rough power/torque curve by connecting the dots. But then if you had a prop boat with accurate propeller curves** you could do the same. So does that make a prop just a dyno too? I'm not sure the distinquishing trait making a pump any more of a dyno than anything else you hook to an engine to pull it down. Except with a pump, the power absorbed may be a bit more independent of vehicle speed?
OverPropping: The wacka's not my thing but from what I understand their performance matching is very similar to ours. They just have more things to monkey with like pitch and number of blades. For example, they get best performance by propping for full throttle rpm at the top of the engine's operating range. For them, too much pitch means less full throttle rpms and less performance...overpropping. In this test...
Prop test (http://www.iboats.com/sites/trailerboats/site_page_1480/article_page_246.html)
they conclude over-propping does little to improve economy at best cruise speeds, but hurts performance. I'm not saying the same applies to trimming impellers or choosing an overly aggressive profile (pitch?), but could be parallels here if an engine's dragged below it's most efficient operating point. If you could imagine the extreme, maybe an Aggressor 4A, both the engine and the pump could be pulled way below best efficiency point.
Without performance curves or impellers test like Bear mentioned it's hard to know much. BTW bear, can you hook me up with that data?
jer
**Pump curves follow a cubic power law, typical prop curves I think also close to that, maybe 2.8 power?

[UBFJ #454]

Jer -
The data available to me is proprietary so I am bound by that.
I have been told that Don Bowers (Don's Pump Service & patent holder for the "cavitation reducer"/inducer) has done a lot of testing of the kind that your interested in ... perhaps you could talk with him about his results and get some hard copy ??? ... His shop # in Ohio is (614) 488-1335. He also has a website that you should be able to contact him through.
Note, that while I know a number of his customers, I do not know him myself.
Jak

[LVjetboy]

I'll give him a call. He did one of my pumps a long time ago and we talked a bit in his shop.
jer