The theoretical pitch of a propeller is the distance it would move forward if it were cutting through a solid. It’s like a screw, a 16” diameter propeller with an 11” pitch would move forward 11” in one rotation, if it were cutting through Jell-O. Of course in the water it never goes this far, because water isn't a solid. In the water it goes only about 50% of that distance.
So why do we use pitch if it isn't the actual distance the propeller travels? Basically it is a reference point that through experience we know that a specific diameter and pitch will load an engine correctly. To size a propeller we need to know the following:
- Boat Make and Model
- Engine Make and Model
- Transmission Reduction Ratio
- Limitation in space for the propeller if any
- Diameter and taper of the propeller shaft
With these 5 things we can provide you with a propeller that will load the engine as it is designed and provide efficient propulsion.
Sizing propellers is part science and part art. After 27 years of doing this I am still learning, and that is what makes the job fun.