Propeller pitch is one of the most important, yet one of the most misunderstood factors of a boat’s performance. The right prop pitch makes a world of difference on how a boat maneuvers in the water.
In layman’s terms, think of a propeller’s movement like a screw and a piece of wood: as the screw is turned, it moves forward into the wood.
A propeller works in the same way through water, and because of this similarity, boat propellers are often referred to as screw propellers. As a propeller rotates, its blades push against the water, and the resistance of the water against the blades pushes the prop – and with it the boat – forward through the water. The distance a propeller moves forward for each revolution is the pitch.
Prop Pitch Measured
Pitch is measured in inches. For example, if a prop travels forward 21 inches when rotated once, the pitch of that prop is 21 inches. To help identify a propeller, the pitch is stamped or engraved on the prop along with its diameter noted first and the pitch last. For example, a propeller stamped 14 ¾ x 18 has a 14 ¾ inch diameter and an 18 inch pitch.
Pitch and Slip
The pitch is the theoretical distance a propeller moves forward for each revolution. However, the difference between a prop’s theoretical distance traveled and actual distance traveled is known as slip, or prop slip.
Propeller pitch how it works
The actual distance a propeller moves forward for each revolution is somewhat less, because it’s travelling through a liquid, not a solid, and also encounters limiting factors such as drag and cavitation. For example, a prop with a 21-inch pitch (theoretical distance) may only travel 18 inches through water (the actual distance), giving a prop slip of 14 percent.