50 Merc prop diameter question

[reelhappy]

Hi Everyone, I have a 2001 50hp 2 stroke on a boat I just put together and it has a 10.5 x13p prop and the WOT is about 400 rpm to high. I can get a good deal on a 11.5 x13 prop and was wondering what RPM reduction I would see. The boat is a 17" Scout.

Thanks

 

[Silvertip]

Re: 50 Merc prop diameter question

Probably won't slow it up much. Pitch is the characteristic you want to change -- not diameter. 13P is pretty standard for that motor. A 15P should bring the motor down about 400 RPM. BUT -- if the recommended WOT rpm is 5500 and you are spinning it at 5900 and you are alone in the boat, I'd leave it alone. Putting two or more people in the boat with the 13P will bring the rpm down. With the 15P and two or more people in the boat, the engine would then have a hole-shot problem and would likely be lugging across the rpm band. In other words, prop the engine for average load, not minimum load. You can always throttle back on an over rev condition, but you can't throttle up when the engine is lugging.

 

[reelhappy]

Re: 50 Merc prop diameter question

Thank you for the information. I will put the larger diameter out of the picture and consider a pitch change. When I tested the boat I had 2 people on board but no gear so I was probably still on the light side. The prop on the engine is a little chewed up so maybe between that and being a little on the light side a step up to a 14p instead of 15p would be a happy medium. Once again Thanks for the insight.