I believe the max RPM for your 2009 Merc 115 4-stroke is quite a bit higher than the R’s you are running, and i suggest you confirm your RPM spec’s. A lot of guys, myself included, aren’t used to spinning our outboards in the 6000 range, which is where many 4-strokes run. I ran a 15 pitch stainless steel 3 blade on a 1775 Pro V with a 115 Merc 4 stroke. It ran 6250 RPM with a medium load, which happened to be my motor’s approximate max RPM, and around 41.5 MPH. I had great dead in the water to on plane performance, which was something I wanted.