Prop for 115 2S on SF 20

[Fr. Frank]

The inline-six motors are all Mercury, no Yammie stuff there. They were nicknamed "Tower of Power", and Mercury made them in everything from 70 to 165 hp. They are very good motors. The 115 Mariner should be more than adequate to push a 20'. I agree with going to a smaller pitch prop to keep the rpm's up. PowerTech will custom pitch any of their props for a very small fee.

Here is a very good prop for what you describe as your needs. PowerTech prop

One caveat: the inline 6 motors are naturally balanced (no balancer) and they ALL will self-destruct if run for more than a few minutes at 6600 to 6800 rpm. For some reason, this rpm sets up a harmonic vibration that causes connecting rod failure. Fortunately, if you are not racing, you have no reason to need more than 6200.

In my much younger days I got to oversee the destruction of 2 XS1500 motors from harmonics while racing.
It would be an understatement to say I love the "Tower of Power". I still have 2 inline six motors. The '73 XS1500-EL produced a genuine 165 hp and weighed only 274 lbs. (Mine were on the back of a 14' boat. Yeehaa )