What is the most important thing when choosing a propeller? Pitch. No matter what size engine or what kind of boat, if the propeller isn’t the right pitch, the boat isn’t going to live up to its potential.
Pitch is the theoretical distance, in inches, a propeller moves forward every time it turns (i.e. a 19” prop should move forward 19” with each revolution; in reality, it’s a bit less, because the propeller isn’t 100% efficient).
Here’s the skinny: You want to be able to run the engine at, or near, the manufacturer’s recommended maximum RPM at full throttle, trimmed up for speed, with a typical load in the boat – and if you can, your propeller is the right pitch. Too much pitch (overpropped) — the engine won’t reach its max RPM, is sluggish getting on plane, and has poor throttle response. Not enough pitch (under-propped)—the boat pops on plane and accelerates with confidence, easily exceeding the recommended full throttle RPM. Neither condition is good for engine longevity or fuel economy.
The cure? Since every inch of pitch is worth about 150-200 RPM, decreasing pitch should provide a proportionate increase in RPM – drop pitch an inch, gain a couple of hundred RPM. Conversely, increasing pitch usually results in a RPM decrease – go up an inch, lose a couple of hundred RPM.
After you have the pitch part of the equation figured out, then the propeller selection process can continue. Steve McLelland, Sales and Marketing Manager of Precision Propeller Industries tells us, “Different styles of propellers offer unique performance characteristics – more bow lift, quicker hole shots, and the like. Once we’ve determined the correct pitch range for a given boat, then we can start talking about what your performance goals are and what propeller will help you get there.”