These are the basic principals to aid in the understanding, selection and application of boat props.

Diameter: This is usually the first number in a propellers identifying marks. For example, a prop may have 15x17 stamped on the hub of the propeller. In this case the propeller is a 15 inch diameter with a 17" pitch(we will get to pitch later). Diameter is the size of a theoretical circle around the blade tips. It can be measured from tip to tip on 2 & 4 blade props or from the center of the hub to a blade tip and multiplied x2. A larger diameter prop with a lower pitch is preferred in applications where the vessel may have a lower power to weight ratio such as a workboat, heavy cruiser or houseboat. On the other side of the spectrum, most high speed craft with a high power to weight ratio will use a smaller diameter propeller with a greater pitch.

Pitch: Pitch is the distance that a propeller is designed to travel through the water in one rotation or 360°. If you have a propeller with a 17" pitch, well, it should move 17" forward through the water with one rotation. The greater the pitch the farther the propeller will travel, but more energy input is required.