View Full Version : RPM change with new Prop (same pitch and diameter as old prop)

11-10-2008, 08:03 PM
So about 3 weeks ago I was pulling my boat on to the trailer and it did not make it all the way up the bunks. I backed up to take another shot at it, and as I backed it did its usual move toward the starboard side. I did not think it had moved that far, but as I pulled back on I took out the prop on the trailer frame since the boat was not centered. I sent the prop in to OJ and they said it was beyond repair. As luck would have it, the exact same prop (12.5 X 12 4 blade) was on ebay as dealer overstock so I was able to get a good deal on the a brand new replacement.

So here is my question. I got the prop over the weekend and took the boat out today to see if anything else was damaged. It is as smooth as it always was, so I was really happy. Then I tried using Perfect Pass and when it locked in at my 32 mph setting, the boat was only going 30 mph according to both the analog speedometer and the PP digital display. I realize PP operates based on RPM's, but I would not expect there to be a 2 MPH drop with the same prop diameter and pitch. Then I turned PP off and went wide open throttle to see what my top end was. It topped out at 41 mph and the RPM's sort of oscillated between 4900 RPM and 4800 RPM almost like a rev limiter was causing it to mis-fire. I am not certain what the top RPM's used to be, but I am nearly positive the top end used to be 43 MPH. The same 2 MPH drop as when I tested Perfect Pass.

Water temp was 47 degrees so that may have been an issue to, but the engine temp was right where I expected it to be. Boat is winterized now, so I will not be able to really look at much until spring, but I thought I would post to see if anyone had any thoughts.



11-10-2008, 10:28 PM
Two props may very well have a different cup. I'm trying to find the perfect prop for my boat, but that search will have to start up again in the spring.

OJ XMP 3 blade---I've tried 13x12 standard cup and 13x12 with less cup to increase the rpms. The one with less cup turned almost 300 rpms more, but top speed dropped off and skiing speeds 34 and 36 mph also turned the extra 300 rpms. Also tried a 13x11.5 with standard cup and it performed just like the 13x12 with less cup.

11-10-2008, 11:33 PM
Steve, it was COLD today....you are dedicated!

11-11-2008, 08:52 PM
Steve, it was COLD today....you are dedicated!

I was not in the water Eric. Only on the water inside the boat. Really was not too bad when in the sun, but I had a winter coat on. :)

Really wanted to confirm if I was in for more parts in spring or if the prop was going to take care of it.

Still not sure about the top end though. :o Do not remember it ever dropping out at high RPM's before, but then again, I am not sure how many times I have actually run at wide open throttle for any period of time. If you or Will have any thoughts, let me know.



11-11-2008, 10:44 PM
I would not be surprised if your ebay prop was repaired

11-12-2008, 10:21 PM
I would not be surprised if your ebay prop was repaired

I wondered that too, but it sure looks new. It is a CNC prop and you can still see all the machine marks from manufacturing. Would that be part of the "repair" process?

I suppose I could ship it off to OJ and have them confirm if it matches the manufacturing specs.

11-14-2008, 03:43 PM
a reheated prop will definitley have some discoloration...

04-19-2009, 04:21 PM
I thought I would post an update to my original question. I was in Tennessee on vacation last week, and was able to stop in to OJ to look at the prop. The prop came from ebay from a former MC dealer in California who lost the MC sales rights. Eric was kind enough to look at it, and was immediately able to tell that although it was sold as a new prop someone had taken all of the cup out of the prop. He was also kind enough to recup and rebalance the prop for me. THANKS ERIC! I had the boat out yesterday and the speed is almost EXACTLY where it was with the original prop. Perfect pass is off by about 0.1 mph. My top speed is back to where it was, and it did not feel like I was hitting a rev limiter anymore. I will need to confirm the actual Perfect Pass setup when I can get into a course, but it appears the prop cup was the issue.