Cupped prop change 1973 50hp

[FillupD]

I have a 1973 50hp evinrude that I put on my 16' Lund fishing boat. I had a 13-3/4X15 prop on it and was running 4800 RPM alone in the boat. When my son and the dog got in we ran 3500.
I thought a 13" pitch would help and I put it on. It is a cupped slightly on the fins and the 15" prop wasn't. We are still running 3500 even with the pitch change. Is the cupped fins giving the prop more pitch than 13. I am wondering if I didn't just take off a 15" pitch prop and put on a 13" pitch prop that was cupped or flared out a tiny it on the ends, and didn't achieve anything. The evidence of the rpm's make it look like it.
Shall I try to find a 12.75x13 or 13"x13 prop to put on it. Iboats calls for a 13-3/4 and a 14" diameter prop. If I put on a 14", the cavitation plate would only clear by 1/8".

 

[towtruck1]

Re: Cupped prop change 1973 50hp

Hello FillupD

First of all... what is the diameter of the new prop? Since you didn't say, I'll assume its the same as the old prop.

Check out this link.
http://www.performanceprops.com/faq.html
Assuming everything else stays the same, cupping will add performance... but it's got to come from somewhere (we engineers call it an 'energy equation'). If the prop is working harder, the engine is working harder. When you reduce prop slip, the prop behaves like a prop with more pitch.

Have you verified that you haven't lost some engine performance, like making sure the boat still does 4800 with the old prop & just you on the boat? And what is the engine -manufacturer rated max rpm?

ALso:
You may want to post the make and model of both props. Prop technology has changed a lot in the last few years. If the original prop is old, and the new one is new design, it makes to hard to compare props just based on diameter & pitch, cause they're all different.

Finally...
Personally, I would avoid a prop that gets that close to the anti-cav plate. I am concerned about hydrodynamic interferences between the tip of the prop and the cav plate.

Hope this helps a bit.

 

[FillupD]

Re: Cupped prop change 1973 50hp

I put the 15" pitch prop on and got 4800RPM versus the 3500 with the 13". That seems backwards. It must be the 15 is old and the 13 is newer cupped. My motor is supposed to run 5500 RPM. How would I gain 1000RPM with a prop change?