Prop size vs RPM question

[jporter5333]

I had a new prop put on my boat, not 100% sure but I think the one they put on is slightly bigger than the old one. I think the old one was a 10-1/8" and the new one is a 10-1/2". I took it out first time it ran smoother (rebuilt carb and fuel pump also), it was also a lot quieter but it didn't seem like the RPM's were as high as they used to be. It seemed to plane fine and it was a little faster. I guess my question is should the seemingly lower RPM's concern me or could it be that it used to be louder and slower because the old prop was just so bad that it was over revving and this is just the first time I've actually seen it act right?

 

[Bondo]

Re: Prop size vs RPM question

I guess my question is should the seemingly lower RPM's concern me or could it be that it used to be louder and slower because the old prop was just so bad that it was over revving and this is just the first time I've actually seen it act right?

Ayuh,.... What are We talkin' about anyways,..??

 

[jporter5333]

Re: Prop size vs RPM question

Ayuh,.... What are We talkin' about anyways,..??

LOL. Let me explain. With the new prop the engine runs quieter and it seems like the rpm's are lower ,too. I am assuming that the engine is now quieter because the rpm's are lower. But could the rpm's be too low because the new prop is too big or could it be that the new prop is correct and the rpm's are where they should have been all along? The old prop was realy bad, so I sdidn't know if maybe it was letting the rpm's run too high. I don't have a tach, I'm just playin it by ear, but it seems to be running awful good. Alot better than with the old prop.

 

[junior1113]

Re: Prop size vs RPM question

a tach is very important for any outboard. being within recommended rpm range is essential for optimum performance and engine life. even a mini tach

 

[jporter5333]

Re: Prop size vs RPM question

Thanks junior 113. That brings up another question. When checking rpm, is it best to check it on the water or isrunning it in a tank or with flushers good enough? Also, where can I get a tach that plugs into a mercury control box? It's either a 4 or 5 pin connection.

 

[sho3boater]

Re: Prop size vs RPM question

You have to check the rpm at WOT on the lake, it must be in the max rpm range of the given motor. What motor/hull is it? A damaged prop might not work right hard to say, either way damaged props will degrade performance in near every way.

 

[jporter5333]

Re: Prop size vs RPM question

It is a 1981 40hp merc. on a 1981 16' bass tracker sho3boater

 

[sho3boater]

Re: Prop size vs RPM question

If that motor can charge a battery you should be able to put a tach on it, or you can get a tiny tach. I'd get a tach and hook it up even a used universal outboard tach. Then run the boat with a light load, get it going as fast as you can. If you are near the top rpm for that motor then you have the right prop. If you haul stuff all the time, then you could even be a little over on rpm and ok. But if it runs under the top rpm range, that is not good for the motor.

 

[jporter5333]

Re: Prop size vs RPM question

Thanks sho3boater. I'm gonna order a tiny tach and check it the next time I take the boat out. I appreciate the help.