I am turning a Ron Hill 11 x 20 cleaver through hub @5700rpms and 48.9 mph,at 6400 rpms 50.4 mph. It gets up quick,then goes to slipping then bites again.Would I need to add some cup or more pitch to keep the bite?

I also have a Mercury Chopper 13-3/8 x 18p with extra cup @ 5600 rpms and 52mph, but still learning on how to drive the boat.

Do you think if I drilled some 1" holes on the houseing on the 2" wide section about 3" above where orginal exhaust is coming out this would help some?