Prop Slip Calculator
Prop Slip Calculator
Calculate propeller slip and desired propeller pitch with this easy-to-use marine calculator. Optimize your boat's performance by understanding propeller slip ranges.
Current Propeller Data
Your measured top speed verified by GPS
Transmission gear ratio (typically 1.5:1 to 2.5:1)
Maximum engine RPM achieved at top speed
Pitch of your current propeller in inches
Desired Propeller Pitch Calculator
Calculate the needed pitch for new power or gear ratio settings
New target maximum RPM
Your target top speed
Will use the calculated slip % from above
Slip Formula
slip % = (1 - (actual speed × 1056) / (RPM × pitch / gear ratio)) × 100
Results
12.50% Slip
Normal Slip Range
Theoretical Speed
37.14 mph
Without slip
Actual GPS Speed
32.50 mph
Measured speed
Performance Analysis
Propeller RPM:
2,750 RPM
Slip Rating:
Normal Range
Desired Pitch:
21.5 inches
New Prop Recommendation:
Increase pitch 2.5"
Information & FAQs
What is Prop Slip?
Slip is the difference between actual and theoretical travel due to the necessary angle of attack of the propeller blades. It is not an efficiency measure, but rather a natural characteristic of propeller design. The propeller needs to be at an angle to "bite" into the water and generate thrust.
Reading Your Results
What is a normal slip range?
Typical slip range is 5-20%. Readings outside this range may indicate incorrect data entry or potential for propeller setup improvement. Negative slip or slip above 20% usually suggests an opportunity to optimize your propeller configuration.
What causes high propeller slip?
High slip can be caused by a propeller with too much pitch for your engine power, damaged or bent blades, excessive boat weight, marine growth on the hull, or incorrect engine height. Environmental factors like strong currents or winds can also increase slip.
How do I determine the desired propeller pitch?
When adjusting power or gear ratio, input your new target RPM range, desired top speed, and previous slip percentage to calculate the needed propeller pitch. This helps you estimate what propeller pitch you'll need to achieve your performance goals with new engine or drive settings.
What is propeller pitch?
Propeller pitch is the theoretical distance a propeller would move forward in one complete revolution. It's typically measured in inches. A 19" pitch propeller would theoretically move forward 19 inches in one revolution if there were no slip.
Can propeller slip be negative?
While technically possible in the calculations, negative slip usually indicates an error in your measurements or data entry. It would mean your boat is traveling faster than theoretically possible. Check your GPS speed, RPM, gear ratio, and propeller pitch measurements if you get a negative slip result.
Does higher pitch mean more speed?
Not necessarily. While higher pitch can potentially provide more speed, it also requires more power. If your engine can't maintain its optimal RPM range with a high-pitch propeller, performance will actually decrease. The ideal pitch balances speed potential with your engine's power characteristics.
Is slip the same as efficiency?
No, slip is not an efficiency measure. It's a natural characteristic of propeller design since the propeller blades must be at an angle to generate thrust. A properly matched propeller will still have slip in the normal range (5-20%).
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