HP, RPM, and Speed Question

[TX19]

Utilizing optimum set ups (impeller, shoe, pick up, nozzle, etc.) for the combination, will an engine developing 650 peak horsepower at 6,500 RPM have a higher top speed than an engine developing 650 peak horsepower at 6,000 RPM?
The boat is a TX19 / Dominator Jet.

[jim lee]

Good luck getting a straight answer on -that- one. :-)
-jim lee

[jweeks123]

No

[wsm9808]

I dont know about the top speed but I'll venture to say you could go a lot faster if you dont bring the pick up along-- a truck skipping along behind the boat will cause a lot of drag. http://free.***boat.net/ubb/smile.gif J/K
sorry, I know better than to stay up this late
http://wsphotofews.excite.com/017/UX/yI/zU/PU70832.jpg
gotta love those TX-19s
[This message has been edited by wsm9808 (edited December 10, 2001).]

[HBjet]

I've never seen a thru transom exhaust like that before...
HBjet

[LVjetboy]

My guess...speed near identical. Assuming the hull and pump setup the same...just the impeller sized down two notches for the 500 rpm match. Cutting the blades reduces loading, which spins impeller faster for the same hp.
Now the next is pure conjecture on my part, but I'm thinking the flow benefit of higher blade speed is partly offset by flow losses from eddies or turbulance caused by increasing the vane-blade spacing. This making the pump somewhat less efficient as impeller size decreases. May only be noticeable with large changes though, as in going from an "A2" to a "D" or maybe "E"
It is an interesting question though. Something I've wondered before too. Maybe tough to verify without true back-to-back or rig testing.
jer

[wsm9808]

OK, on a more serious note, I actually do know a little about pumps. Spent 3yrs mechanicing in an oil field centrifical pump shop and a little of it did rub off on me.
When ever we wanted to lower the pressure and/or volume of a pump slightly to better suite an application, we would cut the impeller to a smaller OD size, but, these pumps were driven at a constant pre set speed, therefor the smaller impeller at the same speed was less efficient in the same volute. Dont take this as gospel, but the way it was explained to me was that when you move the exit blades of the impeller farther away from the bowl vanes the efficienty drops because of the increased turbulance and volume in the bowl and the loss of centrifical energy of the water as it travels farther and less directly to the discharge vanes. Now if you throw in more rpm, the centrifical energy increases, but so does the turbulance and it amplifies the inefficientcy of the smaller impeller and you end up right back where you started before you cut the impeller, only now you are useing more rpm to do the same amount of work. The amount will vary over differant rpm ranges but at the rpm varances you where asking about, I would suspect that you would be hard pressed to tell that there was any advantage of more rpm with the same HP in the same bowl.
I think that is pretty much the same thing LVjet said, just worded a little differantly.
HBjet, the exhaust on my old TX-19 was just some PVC elbows I slipped over the tips to make the exhaust exit below the water line so it would run quite in the no wake zones.
[This message has been edited by wsm9808 (edited December 10, 2001).]