The Golden Rule of Boat Propellers
The correct prop for your boat will make your engine turn within it's maximum rated RPM range at full throttle.
To apply this rule and make it work, you need to know three things:
- What is the maximum RPM you are able to achieve now? (provided by your tachometer)
- What is the maximum RPM range of your motor? (provided by your owners manual or dealer)
- What size and model propeller you are currently running? (provided by numbers stamped into propeller)
Many boaters complain that this is not relevant, since they never run their engines at full throttle. But the engine manufacturer has designed the engine to achieve it's maximum power, efficiency and longevity when propped to achieve it's maximum RPM. Ignoring this rule risks damaging your engine. If your current prop runs within that range, then it's a candidate for being the correct propeller. (more on that later) If the RPM is too low, you need to decrease the loading caused by the propeller, usually by reducing the pitch. If the RPM is too high, then a pitch increase is in order. The change in RPM caused by a one-inch pitch change varies by application, but it's usually 150-250 RPM for most outboards and stern drives.
Most engines have a maximum RPM range of, let's say 5,000-5,500. A prop that turns 5,000 RPM will offer a better cruise speed and efficiency, while a lower pitch propeller that turns 5,500 RPM will provide a better hole shot. Both are potentially the correct propellers. The correct propeller for you is the one that matches how you use your boat. A heavy water skier will likely want the lower pitch, while a cruiser will go for the higher pitch.