What is Propeller Pitch?

Prop pitch is one of the most important, yet misunderstood factors of a boat’s performance. The right prop pitch makes a world of difference on how a boat moves, and if you own one, you need to know what it is.

The textbook definition of prop pitch is: The theoretical distance a propeller will travel forward through a solid when turned one full revolution. In this article, we touch on prop pitch and how it affects your boat’s performance.

Prop Pitch Simplified

Think of a propeller’s movement like a screw turning inside a piece of wood. A propeller works in the same way through water, and because of this similarity, boat props 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 the boat forward through the water. The distance a propeller moves forward for each revolution is the pitch.

Measuring Prop Pitch

Pitch is measured in inches. The pitch is usually stamped or engraved on the prop, along with its diameter noted first and the pitch last.

• A propeller stamped 14 ¾  x 18 has a 14 ¾  inch diameter and an 18 inch pitch.

• When a prop travels forward 21 inches in one rotation, the pitch is 21 inches.

Prop 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. The actual distance a propeller moves forward for each revolution is somewhat less, and it 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.

Prop Pitch and Performance

A propeller's pitch has a major effect on a boat's performance. So if you can fit the biggest outboard with the highest horsepower to your boat but the prop's pitch is off, you won't get use of all that power.

• Smaller prop pitch offers the best hole shot, acceleration, and pulling power at low speed, but can’t reach a high top speed.

• Larger prop pitch offers the best top speed, but lacks hole shot and performance at low speeds. 

If the pitch is too low, the motor will easily reach and exceed its safe RPM limit, leading to wear and damage. However, if the pitch is too high, the motor will get overworked trying to reach its operating RPM range and the same could happen.

The Correct Prop Pitch

Because there’s always a tradeoff between acceleration and top speed, finding the correct propeller pitch depends on how the boat is used. For example, a typical bass fishing boat may have a prop pitch of 25 inches, while a bowrider used for watersports may be better off with a 17- to 19-inch prop pitch. Always check if your boat’s propeller has the correct pitch, and to choose the right propeller if it doesn’t. 

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