Hey Waterwolf,

Try this off Yamaha's site. The 21" seems like a bit much, but they were spinning it up just fine. SS will drop a bit as you will be getting much better bite and less flex in the blades, therefore less slip. They had a real light load as well, with cool temps. Great motor though. You'd probably be right on the money with the 21" @5500, maybe a bit more.

Good fishin',

Tyler Campbell

http://www.yamaha-motor.com/products/otb/perfbull/marinepdfs/crestliner/bulletin_otb_4StrokePerf_MidThrustJetPort_115hp_01164-CRS-Z.pdf