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Thread: Prop Slip? How much is typical?

  1. #1
    Essex502
    In trying to understand my boat's performance and reaction to various props I always come upon the "prop slip" in speed calculations. Now I pretty well understand what it is but don't know what is normal. Granted different props probably have different slippage but what range is considered normal? Or efficient? Inefficient?

  2. #2
    kevnmcd
    Essex502:
    In trying to understand my boat's performance and reaction to various props I always come upon the "prop slip" in speed calculations. Now I pretty well understand what it is but don't know what is normal. Granted different props probably have different slippage but what range is considered normal? Or efficient? Inefficient? 8-10%

  3. #3
    Essex502
    And you expect me to believe YOU!

  4. #4
    Essex502
    Actually, by the prop calculators on the web I have only 6% slip. So...either the GPS is off or the Gaffrig Tach is off if 8 - 10% is the correct range.

  5. #5
    Trash
    Essex,
    6% is a perfectly valid number for slip. 8-10% is only a ballpark average range for good slip numbers. The fact that you have 6% just means you're doing better than average. Keep in mind that slip is an ever changing number throughout the rpm and speed range for a particular drive, prop, hull combo. I like to check slip at 3000, 4000, and 5000 (if I could get there )rpm. If you're getting 6% at 3000 rpm that's great for cruising, but slip might be higher at higher speeds. If you're getting 6% at WOT its probably set up great for top speed. Take a look at some of the higher speed boats with elevated x-dimensions. Their slip at 3000 often isn't that great, but improves as the speed increases. Conversely, a boat with stock (deep) x-dimenstion may have a good slip number at cruise while it deteriorates with speed.
    I've got similar slip numbers as you in certain rpm ranges if it makes you feel any better. wink

  6. #6
    Essex502
    The 6% was at WOT (5100 and bumping the rev limiter).

  7. #7
    kevnmcd
    Essex502:
    The 6% was at WOT (5100 and bumping the rev limiter). 6% seems on the low side...but what would I know.

  8. #8
    Havasu Cig
    I just calculated mine from last weekend at Havasu at it was 9% at WOT. That was with about 100 gallons of fuel on board, and just myself.

  9. #9
    Essex502
    6% does seem to be very low. Other figures I have heard are always higher. It leads me to believe maybe the tach isn't accurate. I wonder what would be a good way to test that?

  10. #10
    Steamin' Rice
    E502,
    From what I have heard, 6% is very low slip for a stock V-bottom, so I wonder if either your tach is off, your prop pitch is different from what it should be, or your speed readings were off. If you were bumping the rev limiter and the tach was reading 5100 RPM, you can use that to check your tach. Do you know where the rev limiter is set at on your motor? If it's set at 5100 RPM, then it sounds like your tach is correct. I think the last time I checked, I had about 12-14% slip on my boat..
    [ August 07, 2003, 09:08 AM: Message edited by: Steamin' Rice ]

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