What is Propeller Pitch?
Propeller pitch is one of the most important, yet one of the most misunderstood factors of a boat’s performance.
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’ll shed some light on what prop pitch is and give you some basic knowledge on how it affects your boat’s performance.
Prop Pitch Simplified
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, with the 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.
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%.
The Effect of Pitch on Performance
A propeller’s pitch has a bigger effect on a boat’s performance than virtually any other variable.
You can fit the biggest outboard with the highest horsepower to your boat, but if the prop’s pitch is wrong, you won’t get to use of all that power. A smaller pitch offers the best hole shot, acceleration, and pulling power at low speed, but can’t reach a high top speed. Conversely, a larger pitch offers the best top speed, but lacks hole shot and performance at low speeds.
If the prop’s pitch is too low, the motor will easily reach and exceed its safe RPM limit, leading to wear and the potential for damage. However, if the pitch is too high, the motor will be overworked trying to reach its operating RPM range, which also leads to wear and damage.
The Correct Prop Pitch
Because there’s always a tradeoff between acceleration and top speed, finding the correct propeller pitch will depend on how the boat is to be used.
A typical bass boat used for fishing may have a prop pitch of 25 inches, while a bowrider used for waterskiing and wakeboarding may be better off with a 17- to 19-inch prop pitch.
With a basic understanding of what prop pitch is and how it works for or against performance, it would be wise to check if your boat’s propeller has the correct pitch, and to choose the right propeller if it doesn’t.