23 Pitch Prop Too Much ?

I have an 18' Larson, 3.0L MPI. Came with 21 Pitch prop which is OK, but I can easily hit 5K rpm with more lever to go. Anyone try a 23 pitch prop with a similar application? Before I purchase one. I realize it will take a little longer to get on plane.

Re: 23 Pitch Prop Too Much ?

Can you get us your gear ratio and GPS speed? I question your tach. You don't hear of 3.0s pulling 23s ever and rarely 21s, so I am concerned about the tach's accuracy . . .

Re: 23 Pitch Prop Too Much ?

Gear ratio 2.0. GPS speed at 4500 rpm, with 2 on board and full tank was 30. Thought this would do more than that

Re: 23 Pitch Prop Too Much ?

3.0 MPI? New technology guinea pig, eh? How do you like it so far?

Using a prop slip calculator with your figures above, you have a theoretical speed of 45 mph at 4,500 rpms with that prop. So if you're going only 30 mph, then that's 33% slip. That's not really possible with a planed hull(shoud be high single digits to upper teens). So unless you are defying the basic laws of physics and math, one of the four numbers you provided is simply incorrect...so lets look at each number:

Ratio? 2.0...very typical for a 3.0. HOWEVER...a 3.0 MPI? I do not know for certain if the 3.0 MPI's are running 2.0 ratio...where did you obtain this number and are you confident in it? I do see on Mercruiser's website, they offer an optional 2.4 ratio outdrive for the 3.0 MPI...and this would give you 19% slip...realisitic at 30 mph. Are you absolutely POSITIVE you don't have a 2.4 ratio? If you do, then indeed you need to go up in pitch. If youre POSITIVE it's a 2.0, then read on...

Speed? GPS. Very accurate, not suspect.

Prop pitch? Assuming you read the stamp on the prop correctly, a 21 is a 21...unless a prop shop has worked it over and changed the pitch, or it is severely chewed up and beat up...both very doubtful on a brand new boat..so not suspect.

That leaves us with RPM. #1 suspect.

I agree entirely with QC. I believe your tach is quite inaccurate. But again this takes me back to the MPI thing...a brand new boat with a bad tach? Sounds like you'll need to take it back to the dealer...presumably under warranty. I doubt many people on this forum are familiar with that brand new technology...I certainly haven't the foggiest where a 3.0 MPI gets its tach signal or pickup from...or what kind of tach it has on your new Larson. But I highly suggest you somehow check it with another good tach sometime and re-run your numbers before considering another prop.

I am quite certain that you do NOT want to purchase a 23 for that rig (unless you have the 2.4 ratio, in which case a 23 might actually be perfect)...and there is no question that one of your four numbers provided above is incorrect. Only you are able to figure out which of those numbers it is....

Re: 23 Pitch Prop Too Much ?

It's a 2010. 3.0L MPI WEC (with emission control,(catalyst), Alpha with ratio of 2.0(just rechecked it on drive housing). Original & backup prop 14.25 X 21. Thanks QC and craze1cars for insight. I'll see if I can check the tach.