According to Meeler, the focus is on pitch. One inch of pitch equals approximately 150 to 200 rpm at wide-open throttle. Outboards have a recommended WOT and the right prop keeps the engine in that sweet spot. The wrong propeller causes the engine to labor or over-rev, shortening its life and decreasing performance. For nearly all Yamaha four-strokes, ideal WOT is 5,700 to 6,000 rpm, Meeler says. So if your boat is currently revving to only 5,400 rpm, subtracting 2 inches of pitch should put it in ideal operating range.