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Need assistance with Prop and RPM....

R

RIBS

New member
I have a 15.5 fiberglass bow rider. Total loaded weight with me in water is around 1400-1600 pounds.
It’s got a 50 HP Mercury Bigfoot with 2.31-1 ratio? Prop is a 3 blade 13.25x15p. Also tried a 13.25x19p which is too strong a pitch.

The 13.25x15 idles/trolls at about 2.5-3 mph, and top speed is around 30 mph via gps.
however the engine only turns 4000 rpm, not the recommended 5000-6000 at WOT.

the 19p trolls more like 4.5-5 mph, and has identical top end.

Mercury seems to recommend 13.25x16p, stainless steel, which only they make, and cost is high..

so questions- What prevents the engine getting to top rpm? I don’t think 15p is too high. Engine Mount height is good, power trim works and has minor impact on top speed, throttle operates correctly and goes to full open, boat hull is smooth and clean, keel is flat. Boat planes and runs well.

is a 4 blade a good option. Should I expect any higher performance or am I in the ballpark for boat size and weight and engine combo?
thanks!
 
Determine motor health.-----Not by looking at or the paint job.----Verify accuracy of tachometer.-----Find a shop with a dynamometer or test prop.-----If motor is producing proper power try a 13" pitch.
 
There are 3 ways to tune a prop you can reduce or increase the diameter, change the pitch (each inch changes the rpm by about 400 rpm), if the prop's pitch is too low you can have it cupped that will reduce the rpm 200-400.
That 4000 rpm is too low - something is not right, to prop it as it is would require a 9" pitch prop to run in the proper rpm range.
 
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