Prop pitch and diameter. Confused, please help.

[steviecops]

I have a 11 3/4 X 17 prop on my Johnson 55, but I want to get my revs up a bit so I'm looking for a 15 pitch prop.<br /><br />So far I've only found 12 1/4 X 15 props and I've been told by local outboard mechanic that dropping down in pitch won't make the difference I need if I increase diameter.<br /><br />My manual gives alternative props which decrease in pitch as they increase in diameter.<br /><br />Any suggestions as to what prop I should get? I'm turning 4500 rpm at WOT. Oh yes, the local mechanic s says that one inch in pitch will result in about 1000 rpm difference Is that right? I've read on here that one inch = about 200 rpm.<br /><br />Thanks <br />Steve

 

[Dhadley]

Re: Prop pitch and diameter. Confused, please help.

Some prop companys go down in diameter as pitch goes up. Today most prop builders are going to larger and consistant diameters even as pitch changes. <br /><br />Tell us more about your rig and set up. To reach your rpm target of 5800-6000 you'd have to lose a lot of pitch.

 

[rickdb1boat]

Re: Prop pitch and diameter. Confused, please help.

What kind of boat are you running?? As, Dhadley <br />said, you would have to drop pitch WAY down to get in the RPM range you need..12 1/4 X 15 will not do it.. An 11" would get you close...1" drop in pitch will get you close to 200 RPM's..<br /><br />BTW- What year is your motor??

 

[ezeke]

Re: Prop pitch and diameter. Confused, please help.

Try this site, it might help: web page<br />That would lead you to this page: OMC Props<br />If you find an OMC # and want the Michigan counterpart, go here: Michigan props for OMC

 

[steviecops]

Re: Prop pitch and diameter. Confused, please help.

My motor is a 1980 2 cylinder.<br /><br />I'm running a 17ft cathederal hull fishing boat which is quite heavy when loaded up for fishing. I'm just trying to improve the revs so as not to put as much strain on my engine.<br /><br />Thanks<br />Steve

 

[Dhadley]

Re: Prop pitch and diameter. Confused, please help.

Are we sure the tach is right?<br /><br />Are we sure the motor is running properly?<br /><br />Is the linkage getting to full advance?

 

[Jack Shellac]

Re: Prop pitch and diameter. Confused, please help.

You're headed the right way, but I agree with the other posts that you're going to have to go further down in pitch than 15" to get your RPM's up to where they need to be. I don't know about your mechanic-- to say a 1" change in pitch will change 1000 RPM is absurd. 200 to 250 is more like it. Your motor is small for that kind of high-friction boat. I'm guessing a pitch of 11-13" is going to be more in the ballpark. Unfortunately, prop pitch is far from an exact science, so try to find a prop shop that will let you try out different props before buying. Good luck.

 

[G DANE]

Re: Prop pitch and diameter. Confused, please help.

If you give me your email adress I can mail you a copy of the OMC propselection chart, suggesting prop pitch and size related to boat size and speed.

 

[rickdb1boat]

Re: Prop pitch and diameter. Confused, please help.

Or, you can go here:<br /><br /> web page

 

[steviecops]

Re: Prop pitch and diameter. Confused, please help.

Thanks again for the advice.<br /><br />I'm as sure as I can be that the tach is reading right, but I'm not 100% sure. I suppose I'll have to try it with a different tach to find out.<br />The motor seems to be running properly. Although the idle isn't very smooth, at anything over idle, the motor has plenty of power and sounds sweet. I've done a link and synch, and the linkage is getting to full advance, (ie, coming up against stop), but I haven't checked the WOT timing since I got the engine.<br /><br />G DANE, my email address is steviecops@aol.com<br /><br />As you say the local mechanic is way off with his calculations o pitch and RPM, is he also wrong about pitch and diameter? He says that dropping pitch and increasing diameter cancel each other out and I won't gain much in the way of RPM's by choosing a prop with less pitch but more diameter.<br /><br />Thanks again.<br />Steve