Posts: 3514
Location: Elk River, Minnesota | Hiya,
I'm guessing you might need to look at a 13-1/4 x 15. If memory serves me correctly, that motor has a gear case that is 2:1. That gear ratio will get that boat up and out of the water better than the aforementioned mariner, which I'm guessing has a 1.85:1 ratio but it will not be a speed demon by any means either. I had an alumacraft lunker which was about 100 pounds lighter overall than your tuffy (it weighs in at 750 pounds). I was able to run a 17 pitch prop on my johnson 50hp, but it had a 2.42:1 ratio, which allowed me to turn a higher pitch prop...only because the ratio has a higher first number, where the engine rpm turns a higher amount as compared to the prop.
When the pieces are designed and put together, they all have a limiting factor both for low end torque and top end performance. In the case of these older, heavier rigs, I doubt you will see any increase in speed, but you may be able to get it out of the hole better.
Without a tachometer, it will be hard to determine what pitch you really need, as getting the motor to the upper 1/3 of its rpm range is the ideal situation....even better if you can get it to reach the top rpm in the range. That motor should bring in 5500 rpms at full throttle, so under a light load (you and your gear), that would be what you would want to shoot for. Without a tach, that will be hard to do.
I still think with as heavy as that boat is, a 15 pitch might suit it better....but getting higher than your already posted speed will be a challenge. It may be possible just by getting the rpms in a higher range, but I wouldn't hold my breath.. Definitely worth trying, though...
Steve
Edited by VMS 6/29/2013 10:28 AM
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