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Author Topic:   PROPS
FISHNFF posted 04-08-2001 04:09 PM ET (US)   Profile for FISHNFF   Send Email to FISHNFF  
I have gone insane trying to figure out props. Now I must throw gear ratios into the formula! My previous Merc 75 2 stroke(2.31:1 gear ratio, 4750-5250RPM) ran a Ballistic 19"SS prop for 5600RPM light, 5200RPM loaded). My new Merc 90 4-stroke(2.07:1 ratio, 5000-6000RPM)runs a Ballistic 16"(repitched 19") at 5950RPM light, 5400RPM loaded. Speed went from 35knots to 38 knots. I tries a Yamaha black stainless 19" and got 5800RPM and 39 knots with good holeshot. The Ballistic 19" touched 41knots, RPM?.
Here are my questions:
1. Would a % change in gear ratio and a corresponding % change of pitch yield similar results all other factors being equal?
FISHNFF posted 04-08-2001 04:20 PM ET (US)     Profile for FISHNFF  Send Email to FISHNFF     
2. Why are only pitch and diameter the only variables listed on a prop when cupping and rake also have a big impact on performance?
3. How much does blade shape affect things?
4. I am to believe a 4-stroke has a broader powerband than a 2 stroke. Does this mean it handles a load better?
5. Why isn't torque listed on outboards? I figure it has more meaning in boating due to pushing through a denser medium with a single gear.

I also tried a Yamaha Pro Series prop in a 13.5"x19" (which is for sale)and got poor holeshot and lost over 1000RPM and speed. Maybe for a 115.

I know there is a prop reference section, but these answers still elude me.

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