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  1. #21
    Platinum Member Platinum Member mcollinstn's Avatar
    My Boats:
    1991 F311SR1
    Join Date
    Sep 2001
    I always thought that numerically-higher gear ratios were used in outboards to give adequate life to the shift dogs. Outboards and Alpha-style drives have typically used higher ratios thru the years.

    It wasn't until the advent of upper-gearset clutch shifting (volvo/bravo) that "taller" or numerically lower ratios were feasible.

    If 2:1 or 3:1 ratios were inherently "better" we would be seeing more of them.

    There HAS to be a point where a prop has "too much" pitch to be efficient. If we look at extremes, it is easy to see that a 100" pitch propellor COULDN'T work very well. On the flip side, a 1/2" pitch propellor would be wasting energy just churning water with very little usefulness.

    Any thoughts?
    I see London, I see France...

  2. #22
    Platinum Member Platinum Member
    My Boats:
    fuel injection, superchargers
    Join Date
    Jan 2001
    PA and MD
    Exactally. I've heard props in the 26-28" range are the most efficient. Ask SteveDavid- they can set their trans to any ratio and set it up for a ? 26" prop. No, don't have any efficiency curves to back this up.

  3. #23
    Platinum Member Platinum Member Steve 1's Avatar
    My Boats:
    Slippery when wet!
    Join Date
    Apr 2001
    Beautiful Fort Lauderdale
    The outboard gear ratios are for the simple fact that the little 2 strokes do not have much useful torque to begin with Unless you have a very very light boat then numerically high ratios are needed IE 2:1 1.75:1 1.86:1
    As the propellers go beyond the 30 inch pitch range and the “helix” angle approaches 45 degrees ! Efficiency starts its march into the amazing and wonderful world of diminishing return.

    Oh by the way on the propellers since the US is not a signatory to the ISO 484/2 classification system look out (there are standards 1-3 and a fourth “S” being the finest) in a class 3 propeller there can be as much as 1.5” of drift in the Pitch !! between a pair, If your shop does not have Prop Scan or something equivalent, Run ! Pitch blocks and Hammers LOL for stone masons or the village blacksmith but not my propellers.

    Best Regards
    Last edited by Steve 1; 06-15-2002 at 07:42 AM.

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