34.5 mph at 4500 RPMs equates to a 17% slip. That's pretty high.
Did you move the engine up and down on the jackplate with that heavy load?
If memory serves me correctly my 21p Rev4 likes to be ran with the centerline of the prop shaft about 5.4" below the pad. I need to remeasure and send my prop to Mark Croxton so he can add some cup and clean it up.
I'd like to try an OFX4 next.
If you were able to get to a 7% slip at 4500 RPM your speed would be around 38mph.
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