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If you are fine tuning your prop selection for performance, you can't select an aluminum prop. Just not going to cut it.

I surmise you are trying to improve your time to plane but not necessarily WOT speed?

Given the low horsepower to weight ratio, and the shallow power curve of the 4 stroke motor, I would agree with Tim, you need to increase the RPM's in the lower end.

When testing props, you need to test in the same make and model and then change size. Prop performance is NOT universal across models or even brands or materials.

Said another way, a 13.25x17 X brand aluminum prop is different than 13.25x17 X brand stainless steel and different than a Y brand 13.25x17 stainless steel prop.