What Size Prop should I use?

[KQQLCAT]

I have installed a tachometer on my 20' pontoon boat becuase I thought that my motor was not reving high enough. I have a 2005 Mercury 60HP 4 stroke bigfoot and now I know it's doing 4,000 rpm's at full throttle at about 19-20mph.
I have a 15 pitch prop on there now. What should I go to.
Thanks

 

[guyjg]

If the RPM limit is 5,000 for the engine then you would drop the 3 blade prop pitch by 5 units (5 X 200 RPM = 1,000 RPM) and use a 10 pitch prop. Sine you are pushing a barge the top end speed is not as important at getting it up on plane. Therefore, consider using a 4 blade prop in the 11-12 pitch range.