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  1. #1
    Senior Member Havasunuts's Avatar
    Join Date
    Jun 2010
    Lake Havasu

    Default RPM vs MPH

    I'm having a hard time gauging the correct RPM vs MPH. Cruising at 40 mph the rpm is reading 4000, it just seems the rpm is running high at that speed. My friend is running 55 mph at 4300 rpm?

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  3. #2
    C Ya on da lake throttle's Avatar
    Join Date
    Dec 2007

    Default from another thread...

    Are you using a gps? or the boat speedo?

    raising trim allowing the boat to get lift (air under the boat)... Speed increases with more air and lift...

    water conditions also play a factor... cat on flat water is not so good. add some chop on the water and you get... again more air under boat increasing lift...

    just sayin... its not much, but...

    take the boat out with your girlfriend... try to do this without your wife knowing (hint).

    okay, seriously...

    take the boat out and drive it... part of getting to know and understand your new partner... get the boat up on its running surface, like you said maybe around 4k rpm. raise and lower the drive, watch your speedo.

    you could have a happy or not so happy speedo

    had to cut myself off earlier...

    lets see if I can give another point or two...

    when cruzin at say 4k rpm, raise the trim (drive) slowly. Watch the speedo, you will witness the boat speed go up as the boat finds its sweet spot. The boat will drop speed again when over trimmed.

    Dont get wrapped up in how fast the boat goes, just get comfortable with her...

    then, once you have learned this and are comforatable adding some more throttle, you will again find that you will trim up even more.

    if you want real numbers, you should use a handheld gps, your boat will perform better in some chop and driving into the wind is gonna make a difference too.

    later, decide your needs... I am not that familiar with HO's numbers and that 24" prop. But I am gonna take a shot at it, you might run in the low to mid 50's around 4,000 rpm.

    The top rpm range for your motor is 4800-5200. Now, add some havasu heat and start deducting some of those rpm's because of the heat... maybe 2-3 or even 400rpm in extreme heat.

    the best time to really run your boat to the test will be in the cooler water and months after summer is over.

  4. #3
    JWM is offline
    Boat Test GURU JWM's Avatar
    Join Date
    Dec 2007
    Santa Ana, CA


    Throttle is right. Increase bottom drag will slow the mph reduse the drag and the speed should increase but the rpm will
    also increase slightly.
    Give us more information on your boat speed, rpm, prop size, tell us everything and I will see if we can solve your problem.

    Sales & Service! V-Drives, Outdrives, Jet-Drives, MerCruiser, and Mercury Racing Parts.
    -Performance Engine Building-714-540-8908

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