[Question] Does more RPM = more speed?
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Does more RPM = more speed?

  1. #1
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    Default Does more RPM = more speed?

    Hello,
    Im new to the forum and looking for some advice. I have a Jetboat with a berkeley pump. lets say hypothetically my peek engine HP on a dyno is at 4700 RPM even though the engine is capable of turning 5500-6000 safely, would it be better to set my pump/boat up to top out at around 4700 RMP or would I gain more speed by setting up the pump/boat to turn more RPM say 5500?
    Thanks for reading.

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    Senior Member larryfknrocks's Avatar
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    if you can turn a larger impeller at a higher rpm then you'll gain more mph. You want the impeller to match your engine rpm

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    Thanks for the insight Larryfknrocks. If Im understanding you correctly the faster I can turn the impeller the more water it will push giving me more MPH?

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    Quote Originally Posted by aquaholic530 View Post
    Thanks for the insight Larryfknrocks. If Im understanding you correctly the faster I can turn the impeller the more water it will push giving me more MPH?
    Yes,

    All things equal. If you can spin an A impeller 4500, then make some engine improvements and spin the same A impeller 5500 you will go faster

    Though, changing the impeller to a smaller cut like a B or a C you may spin higher RPM but not go faster because the impeller is moving less water.

    Also, keep in mind the pump needs to be efficient. If you have no engine changes and are now spoinning 500 more rpm, you might have cavitation issues or a worn out pump.

    But bottom line, same impeller spinning faster means more thrust/water/speed

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    Senior Member bp298's Avatar
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    Quote Originally Posted by aquaholic530 View Post
    Hello,
    Im new to the forum and looking for some advice. I have a Jetboat with a berkeley pump. lets say hypothetically my peek engine HP on a dyno is at 4700 RPM even though the engine is capable of turning 5500-6000 safely, would it be better to set my pump/boat up to top out at around 4700 RMP or would I gain more speed by setting up the pump/boat to turn more RPM say 5500?
    Thanks for reading.
    the objective is to size the impeller to absorb the most hp available from the engine, or as close to it as is "reasonably" achievable. using your hypothetical, if hp peaks at 4700, that implies that hp will start to descend after rpm passes peak. so you are making less hp at 5500 than 4700, and even less at 6000. so going through an exercise to try and spin the engine at higher rpm beyond peak hp would be a waste of gas.

    keep in mind that if you are at peak hp at 4700, and the engine tops out at 4400, you are somewhere on the hp curve going up. how much less hp does the engine make on the dyno at 4400 than it does at 4700??? i.e., how much hp is left on the table to go chase after??? a person can go nuts and go through a lot of impeller changes trying to chase right up to peak hp. and then the weather changes, screwing up that chase.

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    Senior Member Futs's Avatar
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    Quote Originally Posted by larryfknrocks View Post
    if you can turn a larger impeller at a higher rpm then you'll gain more mph. You want TO MATCH THE PEAK HP RPM TO the impeller
    Fixed it for ya

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    Or Seth, either one Budweiser's Avatar
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    Quote Originally Posted by Futs View Post
    if you can turn a larger impeller at a higher rpm then you'll gain more mph. You want TO MATCH THE PEAK HP RPM TO the impeller
    Fixed it for ya
    Match Impeller to Max HP RPM or build the engine to match an existing impeller... Two sides of the same coin.

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    Senior Member bc18's Avatar
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    Quote Originally Posted by Budweiser View Post
    Match Impeller to Max HP RPM or build the engine to match an existing impeller... Two sides of the same coin.
    Wouldnt spend a whole lot of time building an engine around an impeller. There is a lot of adjustments that can be done to an impeller to meet a desired rpm after the fact

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    Senior Member bp298's Avatar
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    Quote Originally Posted by Budweiser View Post
    Match Impeller to Max HP RPM or build the engine to match an existing impeller... Two sides of the same coin.
    yes, but....

    it's not always that simple, desirable, or feasible to try and "match" an impeller to max hp rpm. most engines designed specifically for jetboats end up with relatively flat hp curves over the last 500-1000rpm before peak. i.e., hp increases maybe 5-7hp per 100rpm over the last 500-800 rpm on the curve up. if a person is somewhere in that ball park, they can trim the impeller to get closer to peak. but the closer you get, with such a small comparative hp gain, may not result in an observable performance improvement.

    many many years ago, the engine I had at the time peaked around 6700 rpm. the pump had been blueprinted by mpd. I was running an ss "brand a" B cut impeller and would routinely see 6230-6250 on a pass. at the end of that year, we installed 2 new carbs and a vacuum pump and went back to the dyno. the engine had the same peak, same hp curve, but made 40hp more across the board. if you overlaid the curve's, they looked identical. at the same time, I installed a "brand b" B cut ss impeller blueprinted by mpd.

    so after that, the first pass of the next season the boat picked up .3 in et while losing 200 rpm. i.e., instead of seeing 6250, it was down to 6050. but .3 is quite a bit of improvement, and at the time I didn't need any more. the only problem I would have was when I got to bad weather or bad water. if the engine run under 6000, it got too close to peak torque and engine performance would drop like a rock. so for those conditions, I had him make a C that was very similar to the "brand b" design, so I was back up over 6000 in bad conditions.

    the points are, there are differences between impellers besides just the cut. there is more to performance than just trying to "match to peakhp". it's up to the individual how far they want to go to chase down that last 20-30hp. and it may not matter at all if it's solely a lake/river use. you're just not going to notice it that much unless you're looking at timeslips. and even then it may not be what was anticipated. my engines have always been happiest running somewhere between peak torque and peak hp. they like it there...

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    Or Seth, either one Budweiser's Avatar
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    Quote Originally Posted by bc18 View Post
    Wouldnt spend a whole lot of time building an engine around an impeller. There is a lot of adjustments that can be done to an impeller to meet a desired rpm after the fact
    I agree.

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    Senior Member larryfknrocks's Avatar
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    Quote Originally Posted by Futs View Post
    Fixed it for ya
    Thanks, I was a few beers in

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