Kev,

The number given by O.S. is the peak horsepower, which happens at that RPM number. It's a calculated number, based upon testing by the factory. Depending upon the engine, though, you'll never run it anywhere near the horsepower peak. Use the propellers suggested in the instructions, and you'll be fine.

Since you didn't mention which engine you're asking about, we can't give you any recommendations for propeller and operating RPM.