To add to the confusion: my Yamaha 2 cycle, when new, pushed me along at about 6.2 -and did this reliably. The motor, I am now told has low compression from wear, and is about the equivalent of a 6. It does about 5.8. The big difference is that it doesn't take much now to slow down the hull speed (slight head wind, waves, etc.), whereas earlier in its life, the hull speed stayed in the 6.0 range regardless. So the power of the motor affects more than speed on flat water.