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Boat Prop Pitch Help

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  • Boat Prop Pitch Help

    Hello everyone, I have a '96 Procraft V150-B bass boat with a '96 90hp Mercury (2 stroke). My current prop is a stainless 21pitch Quicksilver. Speedometer on my depth finder at full throttle reads 42mph. I have clocked it one time at 46mph with a gps headed down river. I am looking to get more speed out of it while keeping a decent holeshot. A local boat shop has a used 24 pitch laser ii prop for a decent price and I'm really considering buying it! At the same time, my tachometer has not been consistent in the past and I'm not really sure what rpm the engine runs with the current 21 pitch prop. The engine rating is for max 5500rpm, that's all I know. I have read in some older forums that people are getting 50+ out of the same boat. I guess I need to know if a 24 pitch prop is too much, and also how to know what rpm my engine should be at full throttle. Thanks in advance for any tips or input!

  • #2
    Re: Boat Prop Pitch Help

    Without knowing the wide open throttle (WOT) RPM, I'd say no one should guess what prop to use.

    You need a reliable tach, run it at WOT and note the RPM. Make passes in different directions, and trim the motor out for max performance if you can, then post the WOT rpm.

    But yeah... you need a tach.
    Checkmate 2400BRX/Optimax 300xs
    Alumacraft 17 w/ Merc 40 4S

    Date an I/O, but marry an outboard!


    • #3
      Re: Boat Prop Pitch Help

      Seconded ;without a reliable tach reading it is just guess work.Also the only really accurate everytime speed is gps.
      In the meantime check your setup.
      The antivent plate just abover the prop should be about even with the bottom or slightly above.


      • #4
        Re: Boat Prop Pitch Help

        I was gonna run the boat this weekend but didn't get a chance. I will try and run it Tuesday and post what the tach says, thanks!


        • #5
          Re: Boat Prop Pitch Help

          Ok, so I think I caught my tach on a good day, it seemed to be reasonable accurate. First I ran the boat with my current prop(21p) wot at about 5600 rpm and it gps'ed at 43mph. I then changed to the 24p prop and rpm lowered to 5200 rpm at 45mph. To me the loss in hole shot isn't worth 2mph. This prop is significantly bigger than my original. Should I leave it alone or try to find a 22p prop that's the same style as my current prop in order to try and get my rpm less than 5500?


          • #6
            Re: Boat Prop Pitch Help

            Also, I wasn't sure about the antivent plate, is this the trim tab?


            • #7
              Re: Boat Prop Pitch Help

              I think your tach is in the ball park but I think it is reading high the 21 was at 11% slip the 24 at 12%.
              I think you still need to varify the tach. If the tach actually should be reading 5300 at 43 slip would be 6%.
              The antivent plate (anticavitation plate) Is the plate just above the prop that is intended to limit venting
              sometimes called "cavitation" in error.
              If the plate is too low you are dragging too much motor through the water.
              Usually ends up even with or slightly above the bottom.
              In the extreme sometimes you can gain a couple of mph just by raising the motor.


              • #8
                Re: Boat Prop Pitch Help

                Ok I'm with you now. I checked and that plate is even with the bottom of the hull.