Posts: 3508
Location: Elk River, Minnesota | Hiya,
Just looking at the numbers: rig weight, gear ratio, prop, etc...I have a feeling you are about 2 inches too much in pitch. If you drop the 2 inches in pitch to a 19, you'd gain roughly 300 to 400 rpms...possibly even more. Then, if you can move the motor up two holes on the transom (which I am guessing you probably can..most are mounted one bolt hole up), you will gain another 150 or so... Thus...with a gain of 550 rpms, you are getting real close to an optimized setup (although you could go to an 18 pitch and gain even more rpms)
Theoretically, using your numbers as a starting point you should see anywhere from 45 to 49 mph and your rpm range is right where it should be. Hole shot will be much better than where I am guessing it is now (I'm guessing pretty slow coming out if you punch it and extremely slow if you have a heavy load). If you happen to gain 600 or so RPMs with a change to a 19 pitch, and you move the motor up two hole (if possible) you'd be right at 5750 for the motor... And..figuring around a 10% slip factor or so if the prop holds well, you could be almost touching the 50mph mark (being generous here a bit...nothing I would expect, but it may be possible...)
Motors are funny things....at one point somewhere along the line, one will exceed the power of the motor and rpms drop dramatically. I currently have a 17 pitch prop on my yamaha 90, and I can pull 5400 rpms. when I tried a 19 pitch in the same exact prop, I dropped rpms to 4800 max and speed dropped off dramatically. With the 17 pitch I am able to pull 40 - 41 mph.
That prop looks to be decent, but if you can try different ones, I would consider giving the enertia a try as well...and, it wouldn't hurt to try a 4 blade too...just to see what it's like for handling purposes.
Definitely some things to try and experiment with...can get frustrating if you are not into doing things like that, but if you are, it can be a very fun and rewarding project.
Steve
Edited by VMS 6/9/2009 9:29 AM
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