[Bushwacker]

Just to give you some more numbers for comparison, I bought a new 115 for my Seafari in 1975, when motor HP was rated at the powerhead instead of at the prop as it is now. That motor was about 90 hp at the prop. It would run about 33-34 mph @ ~ 5500 rpm running a 13.75" dia x 15" pitch SST prop. Cavitation plate was about 1" above bottom.

Something doesn't seem right here. If I assume same gear reduction in your lower unit as my old 115 had (2:1), the prop slippage calculation with your speed/rpm numbers works out to be about 38%. That's a LOT of prop slippage . . it should be somewhere around 20%!

Are you sure the tach is accurate? The original OMC tach I bought with my new 115 was off by several hundred rpm!

If tach is accurate, prop is really 17" pitch and gear reduction is really 2:1, then you really do have a very inefficient setup. Motor may be mounted too high or trimmed out too high. With motor trimmed so cavitation plate is parallel to bottom of hull, extend a straight edge back from the bottom and measure distance from it up to cavitation plate. If it's more than 1-1.5" you may be getting a lot of ventilation so prop is not getting a good "bite" on the water.