Prop pitch vs. RPM

[StanMan]

My prop is marked 13.75 x 15 and as far as I know came with the 90 hp, 2001 Mercury when new. I recently bought the boat and motor and at WOT the tach reads 5400 rpm (spec is 5000 to 5500)and GPS reads 39.1 mph (speedo says 40). Total weight is about 1900 pounds, gear ratio is 2.3:1. The boat seems to plane OK and all I need is good all around performance.<br /> <br />However, with everything else being accurate, a 15" pitch would have 7200 plus rpm which of course is too high. The Mercury web site prop selector recommends 22" pitch. Which of the following are most likely? A bad tach? (idle reading appears accurate) Could someone have re pitched the prop that much? <br /><br />Have I been unknowingly over reving the engine with the 15" pitch prop since I've had it? Or does this motor have a rev limiter?<br /><br />How can I measure the prop to determine if it's really a 15? <br /><br />Please advise me of best direction I should take before I go buy a new prop. Thanks.

 

[tommays]

Re: Prop pitch vs. RPM

more info about the boat would be helpfull<br /><br /><br />tommays

 

[StanMan]

Re: Prop pitch vs. RPM

The boat is a 17' Crestliner Fish Hawk. Pretty sure the boat, motor and prop came together. Thanks.

 

[Silvertip]

Re: Prop pitch vs. RPM

If GPS and your speedo indicate roughly 39 MPH, and we know the gearing is 2.33 on that motor, if one uses a slip figure of 12%, the engine would indeed need to spin 7400 RPM with a 15 pitch prop. The only two variables left are pitch and RPM. Use a known good tach to verify the tach. If that's ok, you do not own a 15 pitch prop but likely a 20 or 21 pitch which would agree with the Merc prop selector. Any prop shop can check pitch. Don't know if a 15 can be repitched to 20 or not but it may have been damaged at one time and repitched when repaired.

 

[Solittle]

Re: Prop pitch vs. RPM

I think the max on a repitch is 2".