Advanced hydrodynamic theory aside, the essence of propellers can be broken down into simple terms. Prop diameter is the measurement in inches across the circle traced by the rotating blade tips. Pitch is the distance in inches a propeller moves forward in one complete rotation. Props are designated by both. For instance, one stamped "14 1/2 5 17" would have a diameter of 14 1/2 inches with 17 inches of pitch. In laboratory conditions, this boat would move forward 17 inches with one revolution of the prop. "Slip" is the term used to describe the difference between abstract and actual travel, while ventilation refers to air from the water's surface or engine exhaust gases sucked into the blades. For practical purposes, however, pitch is the key consideration.