WOT Only 4000 RPM

[mulliganmagic]

I have a 1974 Starcraft 15' runabout trihull with a 1977 70hp Mercury. I am using the prop that came on the engine, and I do not believe it is the correct one for my boat. I bought a Mercury tacometer harness, a universal tach from bass pro, and had the engine rewired. The tach only shows 4000 RPM at wide open if that. I am almost certain that the tach is set correctly because it shows somewhere between 800-1000 RPM at idle. Any other tach setting showed up to 1500-2000 RPM at idle. When started on the water hose the engine does sound a little loud, but when put in the water it sounds as if it is idling correctly, and if idled down any more it dies when you put it gear. I had this same engine and prop on a 15' bass boat and I got more speed and better engine, but no tach. Also, I do not know the exact numbers on the prop, but I took another one and held it next to my prop and they look identical. That other prop was a 10-3/8 x 13p. Sorry if this is confusing, but thank you for the help in advance.

 

[WillyBWright]

Re: WOT Only 4000 RPM

I would expect a 13 to be appropriate for that setup. It's hard to know for sure by looking. They make up to a 17 prop which is way too much pitch for just about everything. Make sure the motor is running on all three cylinders.

 

[mulliganmagic]

Re: WOT Only 4000 RPM

I broke down and went to the local prop shop and bought a 10-5/8 x 12p quicksilver prop. This bumped me up to 5000 RPM at WOT. Turns out my old prop was a 15p. The engine now has more all around power, better hold shot, and a little more speed. Thank you for the advice.