Finding a prop pitch (no numbers in prop)

I have a prop that does not have any markins on it. Believe me. I have looked everywhere and have let other people familiar with boating look at it and no cigar...

I do have another prop and they look very similar... except for the paint and minor design differences. The prop place is a long 3 hours + round trip and I dont want to do the trip unless is absolutely necessary.

I was thinking I could use the prop with the known pitch number to find out if it is the same as the unknown one. I can apply some expanding foam to the face or back side of the blade all the way until it gets to the body of the prop (will use some sort of release agent), pull that cast away and use it to measure the other prop. I will think that if the blade and body of the unknown prop fits that mold then both props are the same pitch... Will that be true? or Should i just cut the mold at the point that it touches the prop body and use that as a template? Would this work at all? I think that is similar to the process that the prop shop will use to find the pitch...

The 2 props do look the same to me and I dont recall any change in speed when I changed them. I dont know about the RPM since I dont have one installed (bought it, just working in the installation now). The known prop is 13 1/4 x 17, should a 13 1/4 x 19 or 13 1/4 x 15 be easy to distinguish from a 13 1/4 x 17?

Thanks!!!

Re: Finding a prop pitch (no numbers in prop)

Why don't you just put it on a string stretched the length of your bath tub. Fill your bathtub with hot water and add a few packs of Jello. Then submerse it, When the jello cools, turn the prop and see how far it travels.

Seriously, put the tach in and that will tell you what you need to know. Its roughly 200 RPM for each inch of pitch. Put the known 17 on and check the WOT RPMs. Then put the next prop on and check it. If you drop by 400 RPM its a bigger prop (19) if you increase by 400 rpm it's a smaller prop (15) and so on.

Re: Finding a prop pitch (no numbers in prop)

You can do some testing without a tach and you may be able to detect differences. Any changes are dependent on the prop designs material and brands.Speaking broadly changes over the known 17:; the 19 would have a slower hole shot and possibly higher top end and lower max rpm.A 15 would have a better hole shot slower top end and higher max rpm.Again these are generalizations and subject to the points I mentioned.I wouldn't make any permanent changes without a tach and known max rpm and speed with the identified prop.What boat/motor are you running the props on?
By the way "Mercruiser" is an I/O model.You have a Merc 500 outboard.I
have the 65 version.

Re: Finding a prop pitch (no numbers in prop)

Tried it today and both props performed the same so I will take it that they are the same. I was dong 5400 rmp at wot. Looks like this boat was prop for water sports which I dont do much. I probably will benefit more with a cruising setup. I will look into a 17 or 19 pitch prop. What do you guys think?