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  1. #1
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    Prop calculation chart

    Does anybody know if there is a chart for calculating props with RPM and slippage. Or is there just a formula?



  2. #2
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    This is my favorite:

    http://www.rbbi.com/folders/prop/propcalc.htm

    I have it saved in my favorites. It sure beats gettin' out the calculator. For gear ratio, just type in the first number. Example, for 1.87:1 as in Mercs, just type in 1.87



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  3. #3
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    Thanks it could become a favorite of mine also.



  4. #4
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    I have that in my favorites too. 1 thing I noticed though, if you want to figure out your slip at anything over 100mph, it cant calculate it for some reason.
    Helmut
    slow boats

  5. #5
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    I use this one

    But it has the same problem as what stvhelm was saying.
    http://hillpropellers.com/propcalc.html

    upon further review. They look very similar to each other. HEHEHE. Oh well you have more boat sites you can go to now.
    Lata
    Travis
    Last edited by outboards4life; 05-10-2003 at 01:07 AM.

  6. #6
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    This is the one I've always used-- Gear ratio, If you've got 1.87-----1 divided by 1.87=.535 x .000946 x pitch x rpm = mph with no slip. you can take 10% off for slip normally. An example is my yellow flamer: .535 x .000946 x 32 x 8300 = 134.34 mph. My actual speed on GPS was 124 mph. to calculate slip, divide 134 by 124 = 1.08. The slip is 8%
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  7. #7
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    thanks everyone...

    Thanks for the links and formula, that is really helpful.

    96' STV Euro

    Smarta$$ of the
    Sunshine Syndicate,

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  8. #8
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    If you want a real good one http://www.aeromarineresearch.com has an excellent one. You have to buy it but does all the crap without back and forth stuff. Change pitch or rpm or slip ratio what ever without refilling the boxes. Change anyone and all the others change to the new speed.


    There is one thing about the calculators, they generally have one peice of garbage info (garbage in garbage out.) The prop is a guestimate of it's pitch, also the style changes the amount of slip that is "good".
    Don't get carried away with them just use em as a rough reference, and forget about slip. High slip is sometimes the fastest props. Figured from emperical evidence.
    Any prop comparision, same boat many props and styles will result in a very wide unreasonable slew of results. The props are a variable that can't be predicted with a number. The slip that results changes the speed that you get so it aint exact.
    Slip can be ~14% - ~5 % Try both extremes and you'll see a big change.

    The formula is actually a short hand version of the real thing. Rpms are measured in minutes and MPH in miles and hours, prop in inches. Convert the thing to the same units (miles and hours) and you can do it without the .000946 number- which is a conversion of those.
    '90 STV
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    for far too long

  9. #9
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    -----------------------
    93 STV Mod VP/MERC 2.5 200
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  10. #10
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    damn, that one stops at 31pitch and 120mph
    Helmut
    slow boats

  11. #11
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    Hydrostream Registry

    That board has a download for the best I've seen.

    http://www.hydrostream.org/Prop.htm

    David

  12. #12
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    That is the one

    The one on the Hydrostream site is the one I like too

  13. #13
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    I want to thank all you guys for your help. All these sites are really cool. Thanks again Dave



  14. #14
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    prop slippage

    can any 1 tell me if the same formula applies when twin motors are used
    thanks
    paul

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