First thing you need is a tach, can't tell anything without one. If you don't know how many rpm's you are turning, how can you make any adjustments???
I can assure you, a 15 is waaaay to much prop on a tunnel boat for that motor. Once you add enough cup to that 15 to stop cavitation, the average pitch goes to about 17 or 18 pitch, a 60 can't turn that much pitch. My guess is you are turning right around 5000 rpm, if that. That motor makes the most torque at 6000 rpm. Torque moves the load, not horsepower.
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