correct prop for a mercruiser 120 i/o?

[airborne84]

just bought the boat, a 17' Invader open bow it has a mercruiser 120 inboard it has a 13.5" - 17" prop thats beat up. Im looking for a new one and was thinking a 15 - 15 thats on this site was just wondering if anyone has had a merc 120 with a prop of that size and how it worked, I heated,bent and filed my old prop back and painted it then took it out and it ran well but not much over 4000 rpm but then the trim tab fell off and finished off the old prop.... its hard to judge from the old prop because it looks as tho some one added to it. any suggestions appreciated thanks

 

[QC]

Re: correct prop for a mercruiser 120 i/o?

Welcome board airborne!!

There is no correct prop for an engine and drive combo (I/O). It is the combination of engine, drive and boat that determines the correct pitch. So the best thing we have is your 4000 RPM reading. If the engine is running right, and your tach is right, then yes a 15 sounds like the ticket. Take a read of the sticky at the top of this section and this will make more sense to you.

 

[steelespike]

Re: correct prop for a mercruiser 120 i/o?

welcome,sure would help to know the year of the setup.
My guess is the boat has a 1.98 gear ratio max rpm of 4600.
In my opinion it should be able to easily get max rpm with a 17" prop & 10% slip
and make about 34 mph.the 17 at 4100 about 30, A 15 at 4500 (the likely increase in rpm)
29. Do you have your present speed at 4100? gps is best.

 

[jayhanig]

Re: correct prop for a mercruiser 120 i/o?

My boat has a similar setup to yours but has a 14.25" aluminum prop with a 19" pitch. It runs at about 3600 rpm at WOT while it ought to run somewhere in the 4400-4600 rpm range. I have strongly been considering switching to a stainless prop of the same diameter but a 15" pitch. Something to do this winter....