prop pitch?

[keiron49]

hi all, i have a 13" pitch prop on my merc 50hp and i have finally got it all running , now i am after more , are we never happy with what we've got? Anyway back to topic, i reached a speed of 32mph on my gps and was doing just under 6000 rpm at WOT . can i change the prop to get more speed or is that the best its gonna get? thanks folks.

 

[Plainsman]

Re: prop pitch?

6,000 rpm? I wouldn't go past that. Be happy with what you got.

 

[keiron49]

Re: prop pitch?

thanks for that, but would a higher/lower pitch increase speed and lower rpms at wot? i only went upto 6000rpms to see how fast she would go, i have been told to stay below 5500rpms.

 

[River - Runner]

Re: prop pitch?

Know for sure your motor rpm range and that your rpm's are reading correct. The basics... A higher pitch prop gives more speed and less rpm's and a lower pitch prop gives less speed and more rpm's of the same prop kind. With proper setup (many things considered), prop your motor to the max. rpm. You'll then have all hp delivered with top speed.

Your 12' Shakespeare is a fast, performance boat and these forums have a wealth of knowledge to get the most out of it.

 

[Silvertip]

Re: prop pitch?

While increasing prop pitch "theoretically" increases speed, that only applies until you reach the point where the engine does not have enough power to twist that prop at its recommended WOT. At that point you start losing speed because the engine is now lugging and can't get into its power band. If your engine is indeed running at 6000 RPM with a 13P (verify the tach) then you apparently have a relatively light boat and could "theoretically" go to a 15P. Now follow this carefully: What will happen is that rpm will drop about 400 RPM to 5600. Speed will not increase because your new WOT rpms is 5600 not 6000. With the 15P prop the engine will now run out of breath at 5600 just like it did at 6000 with the 13P. If by chance the engine does still rev to 6000 RPM, then yes, speed will increase but again, you would need to increase pitch again but that is highly unlikely. Lastly, if you are doing your testing with just you in the boat, you probably have the best setup because when you add load, the rpm will come down. If, however, you run alone most of the time, then a prop change is certainly something to look at.

 

[keiron49]

Re: prop pitch?

Thanks silvertip. the tacho is brand new teleflex one and is working. speed assessed by gps @ 32mph. I always go on the boat with my co pilot who is 16 stone ( 224 lbs). If i read your reply correctly then a 15p at 5600rpm will be the same speed as a 13 at 6000 ? but even if that is the case surely lower rpms would not do as much harm to the engine? My mechanic man in the outboard shop reckons a 14 pitch would do but if a 15p only drop the revs 400rpm's maybe i should go for that. But as they are quite expensive i would only be buying one ,its just whether to risk a 15 or stay safe with a 14 or even not rev the hell out of it and stick with 13.