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I reckon you're about right with a 17", but you may find an 18 would be better as you're probably not hitting anywhere near peak horsepower at 4850 rpm.
According to Mercury you should be between 5000 and 5750 rpm. Personally I'd head for around 5400-5500 if you're running lightly loaded so you won't be massively overpropped if you load the boat right up. That's based on you using the same prop type but a different pitch.
It's a shame the power/torque graphs aren't online as it'd make prop selection a bit easier.
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