Posts: 3508
Location: Elk River, Minnesota | Hiya,
In your case, if you went to a 17 aluminum in a 3 blade, you would see about the same RPM's as what you have now. Adding the 4th blade would reduce those rpms about 200 or so, but then reducing diamter to 13 would increase by 300 (1/4" diameter is roughly equivalent to 100 rpm). So overall, you would see a rough decrease in 100 or so rpms, give or take some as these are general guidelines and not hard and fast numbers. Hole shot would probably decrease as well since you are trying to get the motor to turn a higher pitch.
I think if you reduce your diamter to 13-1/4 x 15, you would gain roughly 200 rpm there, then by moving the motor up 2 holes, you will gain another 200 rpm. This would get you to roughly 5500 given the same brand and design of prop.
When checking your numbers from original, I too am wondering if your tach might be misreading, or the speed is not a gps speed but a pitot tube speed. Theoretical speed calculation on your motor with the given numbers would put you at roughly 37mph without any sort of slip. Given a 12% slip would give you a rig only going roughly 33...
If we can check the tach and make sure it is on, then get numbers from there, we can get a better idea of what to do...
Steve |
