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  1. #1

    Default 1997 force mercury made 75hp prop help

    Well have a 97 force 75hp on a 18' stingray 181rs. Have a 13" 19pitch prop on now turning 4600 rpm according to boat tach and 31mph. I'm less than impressed will changing to a 17 pitch prop get me more speed..the boat is kind of a disappointment as far as speed goes. I come from an automotive background and hp to pitch to me translates hp how fast you wanna get there and pitch to gear ratio..any helps appreciated. I wanna tube and pull skiers but don't know if this package is gonna do it

  2. #2
    Join Date
    May 2007
    Location
    Inverary, Ontario, Canada - The Great White North Eh!
    Posts
    9,113

    Default Re: 1997 force mercury made 75hp prop help

    Short answer is NO, you won't get more speed.

    Your motor will be turning a little closer to it designed/engineered "wide open throttle range" which will cause less build-up (carbon) and your hole shot/time to plane (may) improve somewhat, but you are only making 31 mph because that's all that combo is likely capable of.

    If you want more speed you are going to need another motor.

    Pitch is roughly translated into "how many inches of forward motion you will get for each rotation of the prop" - but that's in it's purest sense.

    If your motor is turning at 4600 rpms and (to keep it simple) you have 2:1 gears, then the prop is turning at 2300 rpms.

    2300 X your 19" prop, without slippage or resistance to forward motion would give you 43,700 inches (forward) per minute, or 3641 feet per minute or 41.3 mph - since you are only making 31 mph, you are "slipping" about 25% (actually more than that because I believe your gears are closer to 1.8:1) - a little high which leads me to suspect that a 75 horse motor on your particular boat is leaving it a little "under powered" - 15-17% "slippage" is more typical.

    Now, if you went to a 17 pitch, your rpms (at the crank) would increase to 4800. That would mean "about" 2400 rpms at the prop.

    So 2400 X 17" - 40,800" per minute - 3400 feet per minute - 38.6 mph - so less then the "theoretical" 19 pitch, but should slip less (just don't know to what degree) - if your slip rate dropped to 20% (7.2 mph loss) your "top speed" would be 38.6 - 7.2 OR the SAME 31 mph you are making with your 19 pitch.

    Looking at the specs for your boat it's rated for 120 horses max.

    Typically you would fit a motor that is "minimally" 80% of the max to get the boat to operate "as designed" - so 100 horses would better suited to your boat, and a 115 would actually make it run the way it sounds like you want it to.

    That 75 horse "Force", which while it would have kept the cost down when new. But with a boat tipping the scales at 1600 pounds (without passengers, gas, equipment, stuff etc), you ARE under powered - in light of that, 31 mph sounds actually "pretty good" for the way you are rigged....
    Graham

    A "professional" is someone who gets paid for their work - it doesn't necessarily mean they are good at it :)

  3. #3
    Join Date
    Nov 2011
    Location
    tennessee
    Posts
    2,581

    Default Re: 1997 force mercury made 75hp prop help

    That engine has 2.3 gear ratio and those calculations are waaay off. The speed is correct with about 12% slip with the 19 pitch prop.Switching to a 17 pitch will help holeshot and gain about 300rpm but speed will still be within 1mph of old prop. It is underpowered for a 18ft boat
    Pappys Sales and Service
    Owner and Factory Certified Technician
    Former racer KDBA (Kentucky Drag Boat Association)

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