Engine RPM vs. prop RPM and impact on prop

[tony_cliffy]

If I run my 115 4-stroke (Johnson) at 4500 RPMS, does a higher pitch prop suggest higher speed? For example, would a 14x23 prop propell the boat faster than a 14x21?<br /><br />I ask because my 14x23 gives WOT @5600 and 14x21 gives WOT @5900, but I plan to run engine mostly at cruising RPM of around 4500. Wondering (for my stated situation) if sticking with 14x23 might give a faster boat at cruising speed. (I only tested 14x21 at WOT and didn't note speed at 4500 and don't have 14x21 again to test with).<br /><br />Thanks for advice!

 

[Dhadley]

Re: Engine RPM vs. prop RPM and impact on prop

Theoritically a higher pitch will give higher speed. However, you want the load on the engine to be in the correct rpm range at wot so you dont lug or "overwork" the motor at ANY throttle setting.

 

[tony_cliffy]

Re: Engine RPM vs. prop RPM and impact on prop

Agreed, but I'm wondering if that matters given where my RPM's are for the two props listed. Both are within the vague 5000-6000 recommended by Johnson. Someone suggested I could possibly get more WOT speed with the 14x21 (getting into higher RPM's) but I might want to stick with the 14x23 if I truly planned on cruising at less than WOT.

 

[JB]

Re: Engine RPM vs. prop RPM and impact on prop

Use the 21, Tony.<br /><br />At a given rpm below WOT the 23 will give more speed but it is loaded more and will use more fuel. Therefore there is no advantage.<br /><br />Your decider is that the 23 cannot reach the upper ranges of the operating window.<br /><br />That's just my opinion. I think it is compatible with DHadley's advice, just more specific.