Prop Slip Calculator

How to use this prop slip calculator

What this calculator does

This calculator estimates two connected quantities for a propeller-driven boat: the pitch-based Theoretical boat speed and the resulting Propeller slip. It uses engine speed, drive reduction, propeller pitch, and your measured boat speed. The result is a diagnostic comparison, not a direct measurement of propeller efficiency, thrust, cavitation, or engine health. Propellers need some slip to produce useful thrust, so a nonzero result is not automatically a fault. Mercury Racing's explanation of how propeller slip relates to pitch and thrust is useful background when interpreting the percentage.

When to use it

Use the calculator when you want to compare a speed run with the speed implied by propeller pitch and shaft speed, check whether a setup change moved calculated slip up or down, investigate an unexpectedly slow boat after confirming engine RPM, or compare repeatable tests made with different propellers. It is most useful when RPM and speed are recorded under consistent load, trim, water, and wind conditions.

How to calculate

1. The calculator opens with a ready-to-use demonstration: 6000 rpm, a 2:1 gear ratio, a 24-inch propeller pitch, and 60 mph actual speed. The results are already calculated, and Download Excel is immediately available for the same example.
2. Replace Engine/crankcase rpm with the engine speed from the test point. Enter Gear ratio as engine revolutions per propeller revolution, such as 2 for a 2:1 reduction.
3. Enter Propeller pitch and choose Pitch unit. If you switch between inches and centimeters, the current pitch value is converted so the physical pitch stays the same.
4. Enter Actual boat speed and choose Speed unit. The calculator updates live; switching mph, km/h, or kn converts the current speed and keeps the slip calculation consistent with the selected unit system.
5. Read Theoretical boat speed first, then compare it with Propeller slip. Use Download Excel when you want the current inputs and results in a real workbook. Reset clears the demonstration and all entered data; after Reset, Excel export is disabled until a complete valid input set is entered again.

Input guide

Engine/crankcase rpm is a required positive number in revolutions per minute. A realistic example is 6000 rpm. Higher RPM raises theoretical speed when pitch and gear ratio are unchanged. Enter the engine's actual test RPM, not a catalog maximum unless the boat was really operating there; zero, negative, scientific notation, or ambiguous decimal-comma input is rejected.

Gear ratio is a required positive decimal with no unit. A value of 2 means the engine turns twice for one propeller revolution. Increasing the gear ratio lowers propeller shaft speed and therefore lowers theoretical boat speed for the same engine RPM and pitch. A common mistake is reversing the ratio and entering 0.5 for a stated 2:1 reduction.

Propeller pitch is a required positive length. A realistic example is 24 with Pitch unit set to inches. Higher pitch increases theoretical distance advanced per propeller revolution and therefore increases theoretical speed in this idealized model. Do not confuse pitch with propeller diameter; manufacturer markings often include both dimensions.

Pitch unit is required and accepts inches (in) or centimeters (cm). It changes how the pitch input is interpreted and converts the current value when switched. For example, 24 in becomes 60.96 cm. The common mistake is changing the unit label mentally without converting the number; the calculator handles that conversion for you.

Actual boat speed is a required nonnegative measured speed. A realistic example is 60 mph. Raising this value while the theoretical speed stays fixed reduces calculated slip and can make slip negative if measured speed exceeds the model's theoretical speed. Use a stabilized GPS or speed reading when practical, and avoid mixing a speed from one run with RPM from another.

Speed unit is required and accepts mph, km/h, or kn. It controls the displayed and entered speed unit. Switching the unit converts the current actual speed and the theoretical result together, so the slip percentage remains the same apart from display rounding.

Output guide

Theoretical boat speed is an estimate of speed with zero slip under the pitch-and-RPM relationship. It is shown in the selected speed unit and is driven by engine RPM, gear ratio, propeller pitch, pitch unit, and speed unit. A larger value does not guarantee the boat can achieve that speed; it is the comparison baseline used by the slip formula.

Propeller slip is the percentage difference between theoretical and actual speed. It is driven by all four measured/setup inputs. A value near zero means actual speed is close to the pitch-based theoretical value. A positive value means actual speed is lower. A negative value means the measured speed exceeds the model baseline and usually deserves a check of units, gear ratio, pitch, current/wind, and instrument readings rather than being treated as literal reverse slip.

Worked example

With the startup values, theoretical speed is 24 × 6000 ÷ (2 × 1056) = 68.18 mph. Propeller slip is then (68.18 – 60) ÷ 68.18 × 100 = 12.00%. Those are the values shown on first open and written into the startup Excel workbook. If actual speed fell to 55 mph while the other inputs stayed unchanged, calculated slip would rise because the gap from the same theoretical speed became larger.

Learn more

For the physical meaning of pitch, Suzuki Marine describes propeller diameter and pitch, including the idea that pitch is the theoretical forward travel per revolution. Yamaha's propeller FAQ is also useful for reading propeller markings and understanding why propeller choice is application-specific.