I have a Tahiti 18' with 350 Chevy/OMC 800. Motor is mostly stock, Edelbrock performer RPM, mild cam, O/T pipes on the factory risers. I GPS'd at 59.3 Sunday at 4400 RPM, and turned a second pass at 4600 later in the day without GPS, which would be 62.0 MPH. I currently run a 21+ s/s tri-blade prop, but want to try a 22+ or 23+.
Here is the link to the ADM Prop/Speed calculator: http://www.admboatworks.com/pitch.htm . If you have a know GPS run plug in everything but the slippage, get your slip rate, then play with RPM and prop pitch to get an idea where you need to be to get your 60 MPH.
I am hoping for 70 by next summer, if money is available.