Prop Slip Calculator
Calculate the theoretical slip of a boat propeller based on boat speed, engine RPM, gear ratio, and propeller pitch.
Published by:
isCalculator Editorial Team
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How to Use the Prop Slip Calculator
This calculator determines the propeller slip percentage—the difference between theoretical and actual boat speed. Follow these steps:
- Enter Boat Speed — Input your actual speed (in mph or km/h, as selected below).
- Enter Engine RPM — The engine’s revolutions per minute (e.g., 5000).
- Enter Gear Ratio — The ratio of engine RPM to propeller RPM (e.g., 1.5).
- Enter Propeller Pitch — The pitch of your propeller in inches (e.g., 21).
- Select Unit — Check “Use mph” for miles per hour, or uncheck for km/h.
- Click “Calculate Slip” — The result shows slip as a percentage (0–100%). A lower percentage indicates better efficiency.
Note: All inputs must be positive numbers. The theoretical speed is derived from prop RPM and pitch, then compared to your actual speed to compute slip.
Previous Results
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Frequently Asked Questions
Prop slip is the difference between the theoretical speed of a propeller and the actual boat speed, expressed as a percentage. It accounts for efficiency losses.
Typical prop slip for a well-matched boat and propeller is between 10% and 20%. Higher slip indicates inefficiency.
Reducing prop slip can be achieved by adjusting propeller pitch, ensuring proper engine trim, reducing weight, or choosing a more efficient propeller design.