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  1. #1
    RonJon's Avatar
    Join Date
    Sep 2007
    Sarasota, FL

    Impeller Confusion

    Quote Originally Posted by njrealtor55 View Post
    strizki's turbo 15f was gps'd about 3 weeks ago in the high 70's @ 11psi and 7300rpm. his next step is to up the boost to 15-16psi & hope to be well into the 80's with a re-pitch on the impeller to get rpms back to 7100.
    My friend over at Yamaha was telling me about people who call in asking what is the steepest prop they make for the 150F four-stroke outboard so they can order one for thier bay boat or whatever. This motor is rated at 150 @ 5500 and designed to run safely between 5000-6000rpm WOT. He says extensive testing has shown that the best setup for both acceleration AND highest top speed was a prop in which the motor would rev to the full 6000 wot... He said the steeper props that brought max rpm to 5000, 5500, 5700...ect were not only slower on acceleration, but also had a lower top speed. This is with various loads, conditions, and boats. He said this also applies to all the outboards they sell, not just the 150. You'd think a little more pitch even with a little lower rpm would bring better speed, but not so. I know propellers are different than impellers, but the concepts are similar.

    My question is this: Wouldn't stirzki's ski accelerate a little better and/or have slightly better top speed if he went to say 7600rpm? Here's my reasoning: (*impellers piches are hypothetical* an impeller with a leading edge pitch of 15" @ 7300rpm, would theoretically pump water 1824 inches every second. However, with a leading edge pitch of 14.5" @ 7600rpm it would theoretically pump water 1836 inches every second. >>>Turn a few extra rpm because of lighter pitch. Accelerate faster because of lighter pitch. Have rpm where it should be (still efficient at 7600).

    Any thoughts?

  2. #2
    Formerly Ultra250 TN Aquaholic6801's Avatar
    Join Date
    Feb 2007
    Old Hickory,Tn
    In any given set up an engine has a peak horsepower output range, you always want to have the prop(impellar) pitched to the RPM where it it has the best horse power output...........

  3. #3

    Join Date
    Sep 2008
    BUT you MUST understand that PROPELLER pitch and diameter can be manipulated to make a certain horsepower outboard perform properly throughout all rpm ranges without exceeding maximum operating rpm range on a certain length/weight boat. Now an IMPELLER's pitch can be changed and it will affect the take off vs. top end performance in a positive/negative way. BUT, in order to change an IMPELLER's diameter the housing must make the same diameter change. {If you don't what you end up with of course is too much clearance or a blade that wont go into the housing.} That's what ultra250tn is saying with more technical mumbo thrown in.

  4. #4
    I have gone through testing of five props - I have yet to find anything better than the stock prop

  5. #5
    giguchan's Avatar
    Join Date
    Sep 2007
    LI I$land New york


    Thats what i heard too. stock prop is best!
    I even heard that if the stock prop has been damaged it can rob you of MPH... even slightly... I hope they sell the stock prop.. as a few small stones have nicked mine

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