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Went to the Yamaha website and pulled this info from their performance specs: Max RPM = 6000, ratio 2.15:1.
So...with your boat, you could go to a 21 pitch prop, or even better yet, go to a cupped 20 pitch. The increase in pitch of 2 inches will drop your RPMs about 200 or so...same with the cupped 20. the cupping tends to make the prop grab more, and give you the thrust of a 21. Yamaha has a link to a performance review and they pushed a rig like yours to 43 or so with a 12-5/8 x 21 and the motor was one hole up off the transom. They pulled 5900 rpm.
If your motor is on the transom, raise it to the third bolt hole. Try it there with moderate turning. If you are blowing out at all throttle levels on moderate turns, drop the motor down one hole and try again. If you are pitched correctly and you have a good prop, your boat will feel like it is riding on air (which it should...since there should be very little boat in the water at all). The boat will just loaf along in mid range rpm.
With the correct set-up, you could potentially get yourself another 4 - 5 mph. The biggest change you will see is in your prop selection. 5 props of different brands that have the same pitch and diameter will all run differently and you will be able to tell which runs the best for your rig. I would look at something with high rake (Michigan Rapture, stiletto, Trophy....) You have lots of options with your motor. If you find a dealer that has a selection of demo props for you to try, take a few to the lake and try them out. You will see a difference.
Steve
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