Ok hears form what I was told. shorter the tune pipe more topend rpms,but you can also lose the tork power too=have to run smaller props. Most guys on a marine motor at 9" and start cutting them down1/4 at a time until you find that sweet spot. Buggys some start at 8-8 1/2 then go from there.A 37mm prop might be a good starting point? The one prop that was asperating/cavating, sounds close,but you might need to bump up one prop bigger than that one,that might be your ticket prop. I'm running a X437/3 and man my boat moves faster,then my 40mm prop. The 437/3 is a 37.6mm prop size.