Re: Do I need to go down in pitch?
If this is a Mercruiser, there's probably a max hp RPM range printed right on your flame arrestor for that motor (my guess is it will say 4,400 to 4,800), so I agree your 4,500 estimate is probably pretty close. However, I'm a little surprised you can't get more than 4,000 out of it with a 17 pitch prop on a 17.5 ft boat. That's an aluminum boat, right? For comparison sake my 115 hp 3.0 L I/O spins 4,600 on my 18.5 foot fiberglass runabout with a 14 1/4 X 17 prop with 3 people on board. That's about 37 mph on GPS. And I'm guessing my setup is a bit heavier than yours. So before you switch props I'd check for a possible mechanical problem. Are you sure your carb opening all the way at WOT? Are you getting full fuel flow? When was your last tune-up? Might need a throttle cable adjustment, fuel filter, plugs, cap/rotor (if equipped), timing adjustment, etc. Or do you have a huge weight load on the boat? Theres a long list of things that could cause this...<br /><br />Absent of any such problems, then you're left with dropping pitch to 15, or even less than that (if you can find one) to get yourself into your proper powerband. You could try a smaller diameter (14 is common), but I think that will only gain you maybe 100 or 200 rpm at best. Pitch has a bigger effect.<br /><br />But my honest first guess...your throttle cable isn't opening the throttle plate on the carb all the way and you're not truly at WOT when your controls are jammed all the way forward. Mercruiser controls are well-known for this (not sure if that's what you have.) A simple throttle cable adjustment might cure your prob. Good luck.
