prop slip calculation

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  • prop slip calculation

    hoping someone can clear this up for me, when using a slip calculator do you add anything to the pitch if the blades are cupped or if it is a 4 blade?



  • #2
    I don't.
    You will see people say add or subtract this or that. I use the real numbers.
    If they say its a 17 pitch, they they are saying it goes forward 17 inches per rotation. period, doesn't matter how many blades, or cupped or not cupped, whatever.

    However, different brands and models of props run differently.
    There are certain props I expect to have basically zero slip, or sometimes negative numbers.
    Some props I expect 15-20% slip

    Solas brand props for instance normally have really low slip numbers.
    Mercury, it depends on which model prop your dealing with.
    Powertech, also depends on which model prop.

    Just like clothing sizes. you can have two pair of pants that are the same size, one is too big, and the other is too small. doesnt' mean you gained or lost weight, is just the two brands are different.
    Also, just like clothing, you can have two of THE SAME pair of pants, or props, and they can be different. different slip ratio and RPM.

    Basically, "prop slip" is one thing to look for. Its helpful, but by no means is it the only thing to look at. just because one prop has lower slip then the next, doesn't mean it gets better economy or goes faster, its just a number.

    Ken Reeves
    [email protected]


    • #3
      Here is an interesting one.
      Yamaha F350 props changed their pitch designation because they didn't like the slip numbers. (on paper)

      When the original XL props came out for the F350 gearcase, the slip was high, 20ish % most of the time.
      They came in pitches from 17 to 25.
      When they started putting the "SDS" hub in the props, they also changed the pitch they were called.
      They were now 15 to 23 pitch. Props are the same, but different number stamped on them. 2 inches less pitch then before.

      What use to be a 19 pitch was now called a 17. So instead of 20% slip, now they are about 10% slip, Great, looks better on paper. but in reality, its the same exact prop.
      nothing changed expect our expectations of what the slip should be.
      If you have the original XL I expect around 20,
      if you have the XL "SDS" I expect around 10.

      Hard to find any mention of it anymore, but I saw it on Crowleys Marine
      Also available with the SDS Shift Dampening System. (NOTE: deduct two inches of pitch for same or similar performance when converting non-SDS XL propellers to SDS-equipped Saltwater Series XL propellers)

      On that link, you can see the diameters and pitches of the orignal XL series.
      The diameter of the "XL" 17 will be the same as the diameter of the "XL, SDS" 19

      Ken Reeves
      [email protected]


      • #4
        Thanks Ken
        great information clears things up for me