Prop Slip Calculator
This calculator helps you determine the efficiency of your boat's propeller. Propeller slip is the difference between the theoretical distance the prop should travel in one revolution and the actual distance it travels. Understanding this value is crucial for optimizing your boat's performance.
What is Propeller Slip?
Propeller slip is a measure of a propeller's efficiency. In a perfect world, a propeller with a 19-inch pitch would move the boat forward 19 inches with every full rotation. However, because water is a fluid and not a solid, some "slippage" occurs. Prop slip is the percentage difference between this theoretical distance and the actual distance your boat travels. A certain amount of slip is necessary to create thrust, but excessive slip indicates a problem or an inefficient setup.
Why is Calculating Prop Slip Important?
Calculating prop slip is a key diagnostic tool for any boater serious about performance and fuel economy. It helps you:
- Assess Performance: Determine if your current propeller is the right match for your boat and engine combination.
- Diagnose Problems: Unusually high slip can indicate issues like a spun prop hub, ventilation (air being drawn into the prop), cavitation (water vaporizing on the blade surface), or a damaged propeller.
- Optimize Setup: A low slip percentage generally means your setup is efficient. If slip is too high, you might need a different style of prop, a different pitch, or an adjustment to your engine's mounting height.
How to Gather Your Numbers
- Engine RPM: With your boat safely on the water, run it at Wide Open Throttle (WOT) and note the maximum RPM your engine achieves.
- Gear Ratio: This can be found in your engine or outdrive's owner's manual. It's often expressed as a ratio like 2.00:1; for the calculator, you would enter 2.00.
- Propeller Pitch: This number is stamped on the propeller itself, usually on the barrel or one of the blades. It's the second number in a sequence like "14.5 x 19". In this case, the pitch is 19.
- Actual Speed: While running at WOT, use a reliable GPS device (not the boat's speedometer) to record your top speed in Miles Per Hour (MPH).
Understanding Your Results
The ideal prop slip percentage varies by boat type:
- High-Performance Bass Boats: 5% to 10% is considered very good.
- General Purpose Planing Hulls (Outboards/Sterndrives): 10% to 15% is a healthy range.
- Heavier Offshore Boats: 15% to 20% can be normal due to the greater water resistance.
- Workboats & Displacement Hulls: Slip can be 20% or higher and still be considered acceptable for the application.
If your slip is over 20% (for a typical planing hull), it's a sign that you should investigate your setup for potential improvements.