Mercury 80 on 1976 170

[sfmudder]

I have a 1988 Mercury 3 cyl. 80 HP on my 1976 170 Aquasport, had a bent up 13.25 X 19 al. prop on it, I put a 13.5 X 17 Al. on it and it will turn 6000 RPMs trimmed out, I now have a 13 X 19 stainless on it and it planes very well and runs out much better, rpms are 5800. My question would be at what RPMs should this engine run at, and how much decrease in rpms does an inch in pitch give? Thanks in advance for any help.

[Dhadley]

You're OK where you're at but if you wanted to you could send the prop to a shop and have them add a little cup. They can place the added cup to improve bow lift, stern lift or overall lift. That will pull the rpm down just a little and increase the bite.

As for pitch vs rpm change you have to compare the same brand, model and style prop. General rule of thumb (very general) is 200 rpm per inch of pitch. But only when comparing the same props.