Prop & RPM Question

Hi guys.

My boat has a Mercruiser 140 3L I/O engine. The maximum RPM for this engine is 4200-4600.


I just want to confirm that my understanding is correct:

1. The prop on my boat should be one that produces a max RPM at WOT that falls between these numbers...right?


The boat is still relatively new to me, and the prop that came with it WILL allow it to exceed 4600RPM. When I first got the boat, not knowing any better, I did take it up to WOT and it was hitting 5000RPM (eesh! glad I didn't blow anything out.) So now I'm thinking about getting a coarser pitched prop.


2. I like the idea to have a finer pitched prop handy as well for water sports, where acceleration is important. Is there any downside to keeping the finer pitched prop for use with watersports as long I keep RPMs below 4600?


Thx guys.

Re: Prop & RPM Question

Was it just you in the boat when you got those numbers? Or loaded with more people and gear and gasoline? If the 5000rpm was with lighter load and there was not some RPM's left on the table then you will probably be ok just watch the engine RPM's. If it could rev to 5000rpm loaded down then I think you need to go up one size in pitch.

Re: Prop & RPM Question

If you give us the info required in "read this first" we can tell if your setup right and get an idea if the tach is accurate.

Re: Prop & RPM Question

There is nothing wrong with being underpropped, provided you are the human rev limiter. There is a lot wrong with being overpropped.

If you are at the helm and you limit RPM (although most modern engines should be doing this), all is good.

Re: Prop & RPM Question

@Maclin, I don't think I was anywhere near max weight when I hit those numbers. Myself plus one other guy, gas tank was likely full. Compliance notice says I'm good for 900 lbs payload, and all we really had was a bit of fishing gear.

I think my questions are for the mostpart answered then. I'll hang on to this prop for sporty days at the lake, carefully watching RPMs. And maybe get another slightly coarser pitched prop to throw on for cruising days out on the salt.

Thx guys.

Re: Prop & RPM Question

Before making a decision on the right prop we should know what size prop and the actual wot rpm and speed buy gps.
its intirely possible the prop has nothing to do with the high rpm.Prop size is usually under the prop nut.