Next on the list of factors is pressure. When you increase the surrounding pressure, you can usually dissolve more gases in the liquid. Think about your soda can. It is able to keep the fizz inside, because the contents of the can are under higher pressure.
Assuming a prop, it depends on how you are propped but with a redline of 6000 rpm, most outboards perform best with a cruise in the 4000-4400 rpm range. If you want top fuel economy, get a floscan or such and see what is the most efficient for your engine and prop.
Long Shaft. The shaft is the portion of the outboard motor that connects the top of the engine to the propeller. For a long shaft design, the distance between the top of the motor and the propeller generally is 20 inches. Long shaft outboard motors are favorable for boat transoms that are 18 to 21 inches high.