prop question

[medic2051]

Motor is 1975 johnson 50, (50esl75)currently has 12 1/4 x 15 prop. My manual, which is with the boat out of state, gives other prop sizes you can use. Does anyone know what they are,and how the different sizes/pitch will affect operation?<br /><br />Thanks!!

 

[rickdb1boat]

Re: prop question

13 X 11<br />12-1/2 X 13<br />12-1/4 X 15<br />11-3/4 X 17<br />11-1/2 X 19<br /><br />Higher the pitch, lower the RPM's. Each inch in pitch equals 200 RPM's (Approx). You need a prop that will let you turn 5500-5800 RPM's at wide open throttle.....

 

[medic2051]

Re: prop question

So if I were to get a smaller prop/steeper pitch, say an 11-3/4x 17, the boat would travel at WOT at lower RPM? Bear with me, I'm fairly new at this. I also do not have an RPM guage on the boat. Should I? How difficult are they to install??<br />Thanks.

 

[Paul Moir]

Re: prop question

Yes, that's correct. Normally we don't worry about the propeller's diameter - that's really the domain of the propeller manufacturer. We're just concerned about the pitch and whether it fits your outboard.<br /><br />You need a tachometer (RPM gauge) before you pick a propeller. Without the WOT RPMs, you're just guessing.<br /><br />A tach is pretty easy to add to your boat. Cost is about $100. The wiring should already be in place in the wiring harness. An alternative is to use a tinytach (tinytach.com) or a laser tachometer.

 

[JB]

Re: prop question

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