My 1957 Crestliner Voyager 18 with 50hp Merc classic runs at about 27mph and 5500rpm at WOT when lightly loaded. When carrying a typical load, it runs at 5000rpm and just over 20mph at WOT. This calculates to a lot of slip (about 35%) and the boat "feels" like the prop is not hooked up very well. The prop is a stock Mercury 13 pitch (about 10" Diameter and somewhat beat-up).
This hull is easy to get on plane, but has a lot of wetted area and isn't a go fast kind of boat. Hull, motor and gear weigh about 700lbs, a "light load" would bring total weight to about 950lbs. Loading the whole family brings the total to about 1250lbs (about 30% increase!) so the sensitivity to loading makes sense.
I typically cruise at 3800-4000 rpm, which translates to 15-18 mph depending on loading, and I'm often asked to pull the kids on a tube. Also, even trimmed down fully, the current prop will ventilate in a sharp turn.
I'm interested in improving performance and economy at cruise (and pulling a tube), and don't care so much about top end speed. I'm considering a 4-blade Solas prop and dropping the pitch to either 10" pitch (11.4" Dia.) or 11" pitch (11.1" Dia.). Any thoughts?