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  1. #31
    Join Date
    Apr 2006
    Location
    , Langkawi, Malaysia
    Posts
    5,750

    Default "Propeller calculations are no

    "Propeller calculations are not that difficult. The mix of metric and inches, mph/kmh/knots messes it a bit up, but here is a general formula.

    rpm x pitch" x 0.0254 x 60
    -------------------------- = speed (theoretical)
    gear ratio x factor

    The factor is:
    knots: 1852
    mph: 1650
    kmh: 1000
    This will then give you the theoretical boat speed, no slippage.
    Then deduct about 10 - 15% slippage, and you should get close to your real speed.
    If your slippage on a planning hull gets above 15%, you are most likely either under powered or running wrong prop type. Normally too little blade area/too small dia.
    But be sure you get the recommended rpm!"

  2. #32
    Join Date
    Jun 2008
    Location
    Island Heights, NJ,
    Posts
    2,605

    Default "RE: "Could it be possible

    "RE: "Could it be possible that there is a ratio other than 2.15:1 causing such a low rpm with 30 mph and the prop I currently have? How can I tell exactly what the ratio is?"

    That is probably the first thing to check.
    If you pull the prop and the zinc ring, you will find the actual reduction ratio stamped on the housing next to the prop shaft... (usually, unless it was replaced with an aftermarket part)

    Once the boat leaves the factory and meets it's first mechanic, there is no gaurantee that the actual ratio is the one that is indicated on the tag at the top of the drive.

    If you do not have the correct ratio, it will be unlikely that you will ever be able to correctly prop the boat... I know, I got annoyed once and tried!"
    Capt Bob
    1969 23ft ChrisCraft Lancer
    Merc 5.7L 260HP/ Volvo 280
    1972 22ft Bristol
    Honda 7.5HP 4 stroke Long shaft

  3. #33
    Join Date
    Oct 2008
    Location
    Lakewood, NY USA
    Posts
    317

    Default "Tim, I have a 17.5 ft boat

    "Tim,

    I have a 17.5 ft boat with a aq 125a/270 outdrive.my propeller is a 14x17 cupped and my top end speed is 32MPH GPS with the tilt pin in the rear most position. I get 29MPH with the tilt pin in the middle position but with better plane time. Thats on a 17.5 foot boat. Not a speed demon however very efficient and reliable"

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