Enter your engine RPM, boat speed, gear ratio, and propeller slip to calculate the ideal prop pitch for your boat. You can also reverse the calculation — enter a known pitch to find your theoretical boat speed. Results include prop slip percentage and desired pitch so you can fine-tune your setup for peak performance.
Desired Prop Pitch
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Theoretical Pitch (0% Slip)
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Actual Prop Slip
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Theoretical Boat Speed (Current Pitch)
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Propeller RPM
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Propeller pitch is the theoretical distance a propeller would travel forward in one full rotation if there were no slip — similar to how a screw moves through wood. For example, a 21-inch pitch prop would move 21 inches forward per revolution in a perfect (zero-slip) scenario. It is one of two main specs used to describe a prop, along with diameter.
Prop slip is the difference between the theoretical distance a propeller should travel and the actual distance it travels through water. It occurs because propeller blades must have an angle of attack to generate thrust. Slip is not a sign of inefficiency — it is a necessary byproduct of generating forward thrust. Most planing boats operate in the 5–20% slip range.
The formula is: Prop Pitch = (Speed × Gear Ratio × C) / (Engine RPM × (1 − Slip)), where C is a unit conversion constant (1056 for MPH, 1215.3 for knots, 1695.6 for KPH). You need your GPS-verified boat speed, gear ratio, engine RPM at full throttle, and estimated slip percentage to get the result.
To find theoretical speed from a known pitch: Speed = (Pitch × Engine RPM) / (Gear Ratio × C), where C is the same unit conversion constant. This tells you the maximum theoretical speed your current propeller should achieve at a given RPM, assuming zero slip.
A higher pitch prop can achieve greater top speed if your engine has sufficient power to turn it at rated RPM. However, a pitch that is too high will lug the engine below its optimal RPM range, reducing efficiency and potentially causing damage. A lower pitch accelerates faster but limits top speed. The goal is to reach max RPM at full throttle.
Most planing hull boats fall in the 10–20% slip range under normal conditions. High-performance racing setups can achieve 5–10% slip. If your slip is negative, your propeller is over-pitched relative to your speed and RPM. Displacement hulls and heavily loaded boats may have higher slip values.
A 21-inch pitch prop theoretically moves the boat 2 inches farther per revolution than a 19-inch prop. In practice, a higher pitch prop can add 200–400 RPM of load. If your engine RPM is at or above the recommended range with a 19-inch prop, switching to a 21-inch will increase top speed. If it is already below range, stick with 19 or go lower.
The gear ratio is the ratio of engine crankshaft rotations to propeller shaft rotations. Common outboard ratios range from 1.75:1 to 2.33:1. You can find your gear ratio in your engine or drive unit owner's manual, or on the manufacturer's website. Enter it as a decimal — for example, 1.85 for a 1.85:1 ratio.