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  1. #1
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    Prop Slippage Calculator

    Occasionally, someone will post a question about the "prop slip calculator".

    Here it is with an in-depth explanation that follows. ( for you guys that need to know how it works?)

    "RPM" divided by "Gear Ratio" times "Pitch" divided by "1056" = "Theoretical Speed"
    "Actual Speed" divided by "Theoretical Speed" = "Efficiency"
    "100"
    minus "Efficiency" = "Slip"

    There are on-line calculators where you just fill in the data & it gives you the result.

    Example: Engine RPM of 6000 / 1.87 Gear Ratio = 3208.56 prop RPM
    3208.56 prop rpm times 26" pitch = 83422.46 inches per minute

    Now you have to convert
    "inches per minute" into "miles per hour".
    12" to a foot & 5280 feet in a mile
    60 minutes in one hour
    So ........ 12 times 5280 divided by 60 = a factor of 1056

    Back to Example: 83422.46 inches per minute divided by factor of 1056 = 78.998 Theoretical Speed in MPH with NO SLIP!!!!
    Say actual speed (GPS?) was 72 mph? 72 divided by 79 mph = 0.9114 expressed as 91.14%
    100 minus 91.14 = 8.86% slip

    But here's where the problems begin........ What if your tach is off? What if the
    "effective pitch" of the prop is different from what is stamped on the prop or what you think it is?...... then your results are going to be off. Back in the day, before GPS, the measured speed with a speedometer was another variable that could throw your results off.

    So, in conclusion, don't get
    "hung-up" on slippage numbers! Use them for what they're worth.... as a comparison tool & realize that your results may not be as accurate as you think?
    Thanks,
    Tim Bostic

    Hoss Marine Propellers
    www.hossprops.com
    918-479-5167 (shop)




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  2. #2
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    What if your tach is off? What if the "effective pitch" of the prop is different from what is stamped on the prop
    I've tried to explain that exact same concept to some guys who try to compare slip between different props down to the gnat's hair. And I've always wondered how accurate the stated prop pitch is. Within 1%, within 10%?

    OH, I've also actually measured the accuracy of a couple of tachs before I put them in the boat (using a laboratory signal source where I work). One was only off by about 50 rpm at 6000, the other was off by about 200 rpm at 6000.


    1979 Checkmate Eluder
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    1993 Rude 175
    1990 Johnson 200 - died
    Where NOT to buy an engine in Savannah, GA http://forums.iboats.com/showthread.php?t=537238

  3. #3
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    How many 28p props lay flat on a pitch block?
    Quartershot T-3R 15" 3.5L E-Tec 1.62 Sportmaster


  4. #4
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    Right on, Tim.
    '89 Hydrostream Vegas XT, '90 Merc 2.4 Bridgeport PCU EFI
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  5. #5
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    I was always glad to be born stupid. Never figured out all that scientific stuff. If a prop is fast, I call it "good", if it's slow I call it "bad prop" and make it sit in the corner.
    Living in the Freedom provided by Bud Conner and his fellow warriors.
    R.I.P. my Heathen Brother






  6. #6
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    Quote Originally Posted by groundloop View Post





    OH, I've also actually measured the accuracy of a couple of tachs before I put them in the boat (using a laboratory signal source where I work). One was only off by about 50 rpm at 6000, the other was off by about 200 rpm at 6000.


    I know my Gaffrig tach is off close to 200RPM. That said what do I need to check how off it is? what type of signal source do I need? Can I build one from radio shack parts (LOL)?
    Checkmate 2002 Convincor 270 496 MAG-HO

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  7. #7
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    Quote Originally Posted by transomstand View Post
    I was always glad to be born stupid. Never figured out all that scientific stuff. If a prop is fast, I call it "good", if it's slow I call it "bad prop" and make it sit in the corner.
    For how long? Lmao
    Laser 1550 ...under construction with a serious weight loss
    Motor 2.4....undecided on hp yet

  8. #8
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    Quote Originally Posted by 1FASTLASER View Post
    For how long? Lmao
    Some have been there for years, they just won't learn
    Living in the Freedom provided by Bud Conner and his fellow warriors.
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  9. #9
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    Quote Originally Posted by Euroski View Post
    I know my Gaffrig tach is off close to 200RPM. That said what do I need to check how off it is? what type of signal source do I need? Can I build one from radio shack parts (LOL)?
    It takes a function generator that can put out a square wave. I just did a quick search and saw this one: http://www.ebay.com/itm/New-DDS-Func...-/251334144274

    I have no experience with this unit (I have a much more expensive laboratory one at work), but it might be ok - especially if you have a good multi-meter with the frequency counter function (like a Fluke) so you could verify the frequency.
    1979 Checkmate Eluder
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  10. #10
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    Is the tach on a smartcraft monitor accurate? I see a diference in it and my analog tach.

  11. #11
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    Quote Originally Posted by ssv1761982 View Post
    Is the tach on a smartcraft monitor accurate? I see a difference in it and my analog tach.

    On mine the SM is 200rpm lower than the Gaffrig. I would say the SC is correct. But would like to check the tach...
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  12. #12
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    HUUUmmmmm... I suppose if you could input gps speed, enter prop pitch and gear ratio and input tach signal, a slippage Gage could be marketed
    The GPS speed and Rpm would be the only variable inputs.
    L.T.


  13. #13
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    Quote Originally Posted by LaveyT View Post
    HUUUmmmmm... I suppose if you could input gps speed, enter prop pitch and gear ratio and input tach signal, a slippage Gage could be marketed
    The GPS speed and Rpm would be the only variable inputs.
    Not much of a market (small slice of a small pie). If you could market a guide that would help people to understand slip and it's function in the equation, that might be more helpful. Wait, Mercury did that for years ("Everything You Always Wanted to Know About Propellers" but it still doesn't stick.

  14. #14
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    John,Is that information on the web? I was able to copy the stuff below but I don't think I have seen the pamphlet that you speak of.Thanks. IPSlip is not a measurement of prop efficiency, rather slip is the difference between actual and theoretical travel resulting from the necessary prop angle of attack. A 10 " prop moves 8 1/2" in one revolution leaving a slip of 15%. If the blade had no angle of attack there would be no slip The objective of prop design is to achieve the right amount of slip or angle of attack which is around 4 degrees. This is accomplished by matching the right amount of blade diameter and blade area to the existing hp and prop shaft rpm. Too much diameter or blade area will lower slip but will also lower prop efficiency. resulting in reduced performance
    L.T.


  15. #15
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    If you have a smart phone, you can download a musical instrument tuner app with a frequency counter readout, and use audio of the running engine to measure RPM. Just multiply the Hz by 60 to get RPM. Overtones may sometimes cause the resulting number to be half or twice the actual RPM, but it will still be accurate.

    My neighbor's Johnson 15 hummed along at 98.5 Hz, or 5910 RPM.

    I use "Pano Tuner" on my iPhone. There's a free version with banner ads, but it works just fine.
    '89 Hydrostream Vegas XT, '90 Merc 2.4 Bridgeport PCU EFI
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