Boat Propeller Pitch and Slip Calculator

Calculate propeller slip, no-slip or actual boat speed, required nominal pitch, engine RPM, or drive ratio. The math runs locally in this browser tab.

A calculated pitch is a comparison value, not a propeller recommendation Propeller selection also depends on engine operating range, diameter, blade design, cup, rake, hull, load, mounting height, ventilation, cavitation, and handling. Verify specifications and testing with the engine, drive, boat, and propeller manufacturers or a qualified marine professional.

Propeller and performance data

Calculation
Known values

Use the marked nominal pitch, not diameter.

Use measured engine RPM at the same speed.

Engine revolutions per one propeller revolution.

Use steady speed through water when available.

Needed when solving a value other than slip.

Inputs are processed only in your browser and are not added to analytics events.

Calculation result

Enter the known values and calculate.

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Propeller slip formula

Pitch is the theoretical forward travel of the propeller in one revolution. A drive ratio of 1.75:1 means 1.75 engine revolutions for each propeller revolution.

Propeller RPM = engine RPM ÷ gear ratio

No-slip speed (mph) = pitch (in) × propeller RPM ÷ 1056

Slip (%) = (no-slip speed − actual speed) ÷ no-slip speed × 100

The constant 1056 comes from 63,360 inches per mile divided by 60 minutes per hour. The calculator converts knots and km/h to mph internally before applying the same relationship.

What the result means

Slip compares theoretical pitch travel with observed travel. It is not, by itself, a measure of propeller efficiency. Water is not a solid, and a working propeller needs blade angle of attack to create thrust.

Use measurements recorded together at a stable operating point. GPS speed over ground includes current; for a closer performance comparison, test in opposite directions and average the runs, or use reliable speed-through-water data.

Recheck unusual results. Negative slip or a very high percentage can indicate mismatched speed and RPM observations, an incorrect drive ratio, tachometer or speed error, ventilation, damage, or a pitch value that does not represent effective behavior.

How to use this calculator

  1. Choose the unknown. For a normal slip check, select Propeller slip.
  2. Enter pitch in inches, engine RPM, and the published gear reduction. Enter gear ratio as engine revolutions per propeller revolution.
  3. Enter speed observed at the same RPM. Select mph, knots, or km/h.
  4. Calculate, then review theoretical speed, propeller RPM, and the slip comparison table.
  5. Repeat under comparable load and conditions. Verify every value before interpreting a change as a propeller problem.

Example propeller slip calculation

With 19 inches of pitch, 6,000 engine RPM, a 1.75:1 drive ratio, and 55.5 mph actual speed, propeller RPM is approximately 3,428.6. The theoretical no-slip speed is 61.69 mph, so calculated slip is about 10.03%.

(61.69 − 55.50) ÷ 61.69 × 100 = 10.03% slip

Frequently asked questions

What is the difference between propeller pitch and diameter?

Diameter is the width of the circle swept by the blade tips. Pitch is the theoretical forward distance traveled in one revolution through a solid. This calculator uses pitch, not diameter.

What does a 2.00:1 gear ratio mean?

The engine rotates twice for one propeller-shaft revolution. Divide engine RPM by 2.00 to find propeller RPM.

Should I use GPS speed to calculate slip?

GPS provides speed over ground, so current affects it. For a closer estimate, make stable runs in opposite directions under similar conditions and average them, or use reliable speed-through-water data.

Is lower calculated slip always better?

No. Slip is necessary to produce thrust and is not the same as total propeller efficiency. A useful value depends on the hull, propeller, operating point, and quality of the measurements.

Why can a cupped propeller change the result?

Cup and blade geometry can make a propeller behave as though its effective pitch differs from the nominal stamped pitch. The calculator only knows the numeric pitch entered.

Can I select a new propeller from required pitch alone?

No. The calculated pitch preserves the entered speed, RPM, ratio, and slip assumption only. It does not evaluate engine loading, diameter, blade area, cup, rake, material, clearance, cavitation, ventilation, handling, or manufacturer limits.

Limits and safety disclaimer

  • The model is a steady-state kinematic estimate. It does not predict acceleration, thrust, horsepower, fuel use, cavitation, ventilation, handling, or structural loads.
  • Nominal pitch is treated as constant. Real blade geometry, flex, cup, rake, manufacturing tolerances, and water flow can change effective behavior.
  • Speed, RPM, pitch, ratio, load, trim, mounting height, current, wind, and water conditions must describe the same test point for a meaningful comparison.
  • Never exceed the engine manufacturer's full-throttle RPM range or any boat, drive, or propeller rating to chase a calculated value.

Safety disclaimer: This educational calculator does not determine whether a propeller or operating setup is safe, compatible, or suitable. Follow manufacturer documentation and use qualified marine service guidance before changing or testing propulsion equipment.

Methodology and sources

Last reviewed: August 1, 2026. Definitions, inputs, interpretation, and the pitch/ratio relationship were checked against manufacturer material:

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