I'm running the recently purchased 2002 1750 Fish Hawk dual console with a 115 Merc 2 stroke.....boat max rating on the engine and way more power than this boat needs for lively performance...Power Trim standard and you have to get it right to get the speed which is all the way up when running at WOT......gotta get the hull out of the water and the reverse chines on the Crestliner do a super job of getting the hull up and out. You can go to the Crestliner www and pull up the year by year boat specs and compare boat specks on yours and mine. I run alone and 20ish gallons of fuel and maybe 300" of me and gear. Engine is about 350-375#.
Currently choosing between a Turning Point Hustler aluminum 4 blade at 17 and 19 pitch. The 19 would be the choice if I were running calm water but seldom is it calm here and I want a soft ride so I have the 17 currently installed. On WOT, the 17 busts 40 MPH and the 19 busts 45 mph with rpms running at the 5250 limit of that engine...has a rev limiter as OEM equipment. Go-fast.com, prop slip calculator can help you understand your equipment. Variable in those calculations is prop slip and that varies as a function of where you are on hull position....IN the water, HALF-IN, or OUT......like mine is around 15% in the 20-25 mph area and around 5% at WOT. Boat planes out at around 12-15....reverse chines working for you again.
I think your current 15P is the right starting point. If you are only getting 30 out of it, something is wrong. Plug your numbers into the calculator and see where you are. If I knew your gear ratio and top rpms I'd do it for you. Speed can be GPS checked. Engine RPMs can be checked with a digital tach from HF or ebay for less than 20 bucks.....pull the cowl and stick a furnished reflective tape on the flywheel, get somebody to run the boat and you take the measurement at the engine. I just bought a Farina Tach, same as on your boat, for around 75 bucks off ebay, new. Ensure you set the dial on the rear to match your engine before installing.
Just did a quick check on the www and the 115 Honda 4 stroke gear ratio is 2:1 with a max range of 5k to 6k RPM. Taking 6k on the PS calculator and your 15P and tossing in a 10% prop slip (sloppy slip for that boat) comes up with a WOT speed:
15P prop, 2:1 gearbox, 10% slip, 6k RPM = 39 MPH
Moving things around a bit:
......................................15% @ 6k = 37
.......................................15% @ 5.5k = 33
Prop type/condition will have a significant effect on these numbers when combined with the right trim position and engine height.
So either your engine is not up to rpms, prop is failing you, your instruments are lying to you, or you have too much hull drag....could be trim position tucked in too far, or dirty bottom, or hook in the hull, or poor weight distribution.......
