For vessels in the displacement-speed to roughly 35-knot range, the larger the propeller's diameter is, the more efficiently it will perform. However, it's worth planting this seed now: Diameter is the single most important factor governing propeller performance. The effects of diameter will be discussed later. For a propeller with an odd number of blades (on which a straight line cannot be drawn in this manner because no two blades directly oppose each other), diameter can be calculated by measuring from the center of the hole in the middle of the propeller to any blade tip (the radius) and doubling this figure. There's no mystery in the first the diameter is the distance from the tip of one blade to the tip of another blade, measured through the center of the propeller. The two terms that are most often associated with propellers are diameter and pitch. Often referred to as a screw or wheel, the propeller has a substantial lexicon that is easily deciphered and demystified. The propeller industry is replete with terms-many of which you may have hoard but might not fully understand-that describe the various components, characteristics, and performance aspects of the most important part of your vessel's running gear: the prop. Although it is by no means comprehensive or all-encompassing, I hope you find it enlightening and useful. What follows is the tip of the proverbial iceberg-just enough information on this fascinating, but often overlooked, component to guide you between the rocks and shoals of propeller design, selection, and service. Propeller 'science' is still described by some as a black art.

Dave Gerr, noted naval architect and author of Propeller Handbook, believes the volume of information written about propellers is a sure sign that we still do not fully understand them, and I concur. Marine propellers are a subject upon which volumes have been written.

Boat propeller pitch chart