The pitch of a propeller is defined as “the distance a propeller would move in one revolution if it were moving through a soft solid, like a screw through wood.” For example, a 21-pitch propeller would move forward 21 inches in one revolution.
Propeller pitch explained: Think of a propeller as you would a car’s axle ratio. The lower the ratio, the more pulling power from a standstill. The same is true with a prop. The lower the prop pitch, the better your hole-shot. However, this comes at a price: top speed. The lower pitch makes the engine reach maximum rpm at slower speeds.
Alternatively, a higher pitch will deliver greater top speeds, but slower acceleration. Be aware that lower-horsepower engines can bog down if fitted with an outboard propeller with too high a pitch and diameter, and that can wear heavily on internal engine parts.
If you’re changing pitch on a recreational boat, remember that each inch of pitch is worth about 200 rpm. Lowering the pitch will increase rpm and vice versa. For example, going from a 23-pitch to a 21-pitch propeller will increase engine rpm by about 400 revolutions.
The trick is to chose a propeller that delivers acceptable acceleration and top speed. A qualified dealer or marina can help with selection, but you need to be prepared with some vital information. You will need to know the type of engine and its horsepower rating, the boat’s weight and hull style, and the type of propeller you are running now: number of blades, diameter and pitch. If you don’t know, diameter and pitch usually are stamped somewhere on the prop. Also know the engine’s rpm at wide-open throttle when your boat is running with a typical load and trimmed for best speed.