View Full Version : Top RPM

07-06-2006, 12:05 PM
I have a 2005 197 with MCX 1:1 trans. When new my max RPM (WOT)was 4700 and MC recommended 48-5200. (with 4700 WOT the RPM's are equal to MPH in the skiing speed range) They sent me a new prop and got it 4900. I had a run in with a stick and had to install my spare. That too only pushed 4700 RPM. Then when my prop came back from repair at OJ it had dropped to 4700. Spoke to OJ, with CNC props they are very tight on tolerance. He said he could take cup out to get back the RPM's. Wonder if prop really does not reach recommended RPM's as machined to factory specs.

Is anyone else seeing the top RPM just below the range? Experiancing issues with performance or speed? Even when barefooting I don't hit WOT so the only time I open it up is to test the top end. Not sure if it is worth playing with the props some more or not. Seems it would only increase fuel consumption.

07-06-2006, 12:38 PM
I will help keep you at the top
pendo is concerned about top end so I am sure he will chime in here
my oj cnc was smooth as silk when I bought the boat (same boat, motor and trans as yours), idled into a sandy shallow a month or so ago and I feel a slight vibration now that I dont like
I want to get a spare prop anyway so I am interested in your findings.
if OJ can repair my cnc prop back to brand new balance, then I am shipping it there for sure.
I dont know my top rpm as I never remember it and of course now it wont be accurate due to the vibration

07-06-2006, 01:41 PM
pendo is concerned about top end so I am sure he will chime in here

I have not seen above 4950 rpms with my 06 MCX 1.26:1 ... I think part of it is the use of a 4 blade prop ... think of it as a comparison between mountain bike tires and road bike tires ... the mtn. bike tires are wider and get more traction when you start going, but when you get up to speed the "traction" inhibits your ability to generate more speed.

With that said, when you watch the tach from 4000 on up you notice that an increase in 1 mph takes more than 1000 rpms ... I charted mine out last year, but do not have the data here ... using the OJ prop model #438 (13.7 X 15)

Ideally I would like 5125 to be my top end on the tach (but I doubt I would gain even 1 mph between 4950 and 5125)... with the OJ on there I dont think I will get there with the stock pitch, but I really like the pull out of the water (and across the water) with the 4 blade OJ ... actually I still want to try the #446 (13.7 X 15.5) just for the heck of to see how it performs ... here is the OJ application guide:


If you really want to see closer to 5200 rpms I think you may have to let them take some of the cup out, or try a 3 blade from OJ or Acme.

My obsession with mph/rpms has been put to rest with summer and smooth water ... I'm sure in the fall when it is too cold to ski I will get back on my soapbox:)

07-06-2006, 06:08 PM
I would think that the slot would not translate 1mph per 1 rpm anyway would it?

07-06-2006, 07:16 PM
If you really want to tune the prop, supposedly these guys can do it www.accutechmarine.com . They specialize in tuning props for specialty applications. Very sophisticated system that is supposed to be far more accurate than CNC machining. Apparently, they aren't just adding or removing cup. I haven't used them personally, but they are highly recommended.

07-06-2006, 07:24 PM
My boat's a V-drive with MCX. With the stock 14x18 prop, it would turn 5100 RPM which was 46-47 mph. I put a new Acme 4-blade CNC prop on it and it now turns 5200 RPM at WOT, which is 48-49 mph. Killer holeshot, too...

07-06-2006, 10:07 PM
I would think that the slot would not translate 1mph per 1 rpm anyway would it?

our mph and rpm curves if graphed on an X and Y axis will look different, but both should have a point where they cross and then diverge... mine (slot) will spin the shaft slower, but since the prop is a larger pitch I think the 1:1 and 1.26:1 would both be at the same speed at 5000 rpms (just picking that number) ... I belive that I am at 3700 rpms at 34.4 mph FWIW.

07-07-2006, 12:32 PM
The repaired prop I installed (Had a major bend in one blade) ran fine when I installed, from a smoothness perspective. Just the issue of the lower RPMs