Method for determining prop pitch?

[tcreech]

I have a spare prop that came with my used Johnson 115 (1970). It has NO discernible markings. I know it fits, but I don't know the pitch.<br />How can I determine the pitch?<br /><br />Thanks,<br /><br />TC

 

[Texasmark]

Re: Method for determining prop pitch?

Merc prop web page does a good job of explaining it.<br /><br />Webster's dictionary defines it as: The distance the propeller would travel forward in a solid if you turn it 1 revolution, measured parallel to the propshaft.<br /><br />--------------------- <br /> Tilt your engine so that the prop shaft is parallel to the floor.....assume it (and the boat) is sitting on the trailer. <br />---------------------<br /><br />Get a couple of sticks (devices) of some sort that are straight and a measuring tape capable of measuring at least 4'. Get a square (carpenters square, speed square, piece of square writing paper) of some sort to allow you to obtain a 90 degree angle and get some help. An additional set of hands would make this real easy.<br /><br />Take the circumference of the prop (to be measured) for the length of a vertical line (which you will scribe on one stick).<br /><br />Assuming a 13 1/2 diameter prop, that would be pi x d = 3.1417 x 13.5 = 42.4". Take one stick and scribe 0 on it and measure out 42.4" and scribe another mark. This stick you will place vertical to the floor (90 degrees) with the 0 on the floor, out behind the propshaft a convienent distance....not important how far. <br /><br />Take the other stick and while holding one end at the 42.4 mark (of the first stick), which is up in the air vertically, move the other end of the stick so that it's parallel to one blade of the prop measured at the hub (since some blades have variable pitch across the blade this is a good place to "guess"). <br /><br />If you can line it up so that your line of sight is from the stick thru the blade (at the hub) to the propshaft (imagine where it is as the prop is covering it up) your measurement will be more accurate. Easy to do, just rotate the prop while holding the stick till it all lines up.<br /><br />Measure across the floor, the distance, along the floor, between both sticks. The answer is your pitch.<br /><br />For a 10" pitch, you will be 10" forward of the first stick. For a 24" pitch, you'll be 24 inches forward.<br /><br />This should be easy to do and be accurate. Not responsible for any errors. Grin.<br /><br />HTH,<br /><br />Mark

 

[Texasmark]

Re: Method for determining prop pitch?

Hey Mr. T. I lied. <br /><br />So totally disregard what I said other than what I said about the Merc website and the Michigan Wheel (MWC) site (I didn't mention) has similar things.<br />-------------<br /><br />This morning when I shot my mouth off, I spent about an hour working over numbers as I was blowing off, hopefully to provide you with accurate information. But I was bothered by the fact that we are talking about recriprocal motion and dealing with it in linear terms which didn't seem right.<br /><br />So, this afternoon I went out to my boat, and pulled 2 prior Ballistics off the shelf, and applied (attempted to apply) what I said to you. Really didn't like the answers I was getting.<br /><br />Then I got out my machine screw thread book, a 3/4 diameter coarse threaded bolt and nut (10/inch), my trigonometry math book, logic and 3 Ballistic props and after spending several hours I am totally frustrated.<br />---------------------<br /><br />Conclusion: I am not your authority on pitch determination.<br />--------------------<br />The guys that do it for a living must apply "Black Magic" or something cause my logical (illogical) mind can't come up with the recipe.<br /><br />Sorry for the false info.<br /><br />Might send an email to one of the prop biggies and see if they can give you some real, hard, accurate, facts.<br /><br />In remorse.....but I did try to help you.<br /><br />In retrospect. If we were dealing with standard props, maybe all this would make sense as I did learn a few things about ASME Coarse screw threads today. But in applying it to high performance props, with high rakes, variable pitch, cupping, pitch scooping, and all the whoopie that the mind can conceive, it's a ghostland....for me anyway.<br /><br />Mark

 

[tcreech]

Re: Method for determining prop pitch?

Thanks for trying!<br /><br />TC

 

[walleyehed]

Re: Method for determining prop pitch?

Are you sure there are no numbers on the rear of the hub where the nut goes against??? Or maybe between 2 of the blades...<br />If you can't find any numbers, a prop shop can tell you what it is...

 

[tcreech]

Re: Method for determining prop pitch?

Ok, stamped on hub is "Michigan PJ 81"<br /><br />I didn't think those were part numbers, tho.<br />Do they mean anything?<br /><br />Thanks,<br /><br />TC

 

[walleyehed]

Re: Method for determining prop pitch?

Yes, it does, and I'll grab my books tomorrow and tell you what it is...

 

[tcreech]

Re: Method for determining prop pitch?

Walleyehead, you didn't forget about me, did ya? <br /><br />TC

 

[walleyehed]

Re: Method for determining prop pitch?

No...I haven't forgot, but you're going to need to take this prop to a shop..My books skip over the PJ81...from PJ77 to PJ83.<br />Unless someone has a very old MWC book to look at, a prop shop may be the only way to determine the size...sorry I couldn't help you.