What is propeller pitch?

Pitch is the theoretical distance a propeller would advance through the water in one complete rotation if there were no slip — similar to the lead of a screw thread. A 21-inch pitch prop would advance 21 inches per revolution in a solid medium. In practice, water gives way and actual advance is always less than theoretical pitch.

What is propeller slip and what is a normal value?

Slip is the difference between theoretical and actual speed, expressed as a percentage. Planing boats typically run 10–20% slip at WOT; displacement hulls and heavily loaded boats commonly see 20–40%. Higher slip means more energy is lost to water movement rather than boat movement. Very low slip (< 5%) may indicate under-propping.

What gear ratio should I enter?

The gear ratio is the reduction between the engine crankshaft and the propeller shaft. Check your engine or drive owner's manual — common values are 1.5:1, 1.87:1, 2.0:1, and 2.47:1. Enter the ratio as a decimal, e.g. 1.87. For direct-drive boats the ratio is 1.0.

Why does the speed formula use 1,056?

The constant 1,056 converts (inches × RPM) into knots. It combines the inches-to-feet and feet-to-nautical-miles conversions with 60 minutes per hour: 1 knot = 6,076.1 feet per hour = 72,913 inches per hour. Dividing RPM × pitch (inches/rev) by 1,056 gives knots directly.

How do I choose between a 3-blade and 4-blade prop?

Three-blade props generally offer higher top speed and efficiency at WOT. Four-blade props provide better hole shot, smoother running, and more holding power when loaded. The pitch calculation method is the same for both — blade count mainly affects thrust character and vibration, not the speed formula.

My actual speed is higher than theoretical speed — is that possible?

In theory, actual speed cannot exceed theoretical speed because slip should always be positive. If your GPS speed exceeds the theoretical figure, the gear ratio entered may be incorrect, or the pitch stamped on the propeller may not match the actual manufactured pitch. Re-check your gear ratio first.

Propeller Pitch Calculator

Name: Propeller Pitch Calculator
Availability: InStock
Author: Nham Vu

Enter propeller pitch, RPM, and actual boat speed to find theoretical speed, slip percentage, and optimal pitch for your target performance.

Propeller Parameters

Actual Speed Unit

Propeller Pitch (inches)

Stamped on the propeller hub, e.g. 14.5 × 19 means 19-inch pitch.

Engine RPM at WOT

Gear Ratio (e.g. 1.87 for 1.87:1)

Actual GPS Speed (knots, optional)

Leave blank to skip slip calculation.

Pitch Optimizer

Target Speed (kts)

Target RPM

Expected Slip (%, typically 10–25)

15%

Enter propeller parameters and click Calculate.

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Summary

Enter propeller pitch, RPM, and actual boat speed to find theoretical speed, slip percentage, and optimal pitch for your target performance.

How it works

Enter your propeller pitch in inches and the number of blades (for reference display).
Enter engine RPM at wide-open throttle (WOT) and the gear ratio of your drive.
The calculator computes theoretical speed using the formula: (Pitch × RPM) ÷ (Gear Ratio × 1,056) = speed in knots.
Enter actual measured boat speed (knots or km/h) to compute slip percentage.
Use the pitch optimizer to find what pitch achieves a target speed at a given RPM.

Use cases

Verify that a new propeller will allow the engine to reach WOT RPM range.
Diagnose over-propping — engine cannot reach rated RPM — or under-propping — engine revs too high.
Calculate slip on your current prop to gauge efficiency losses.
Find the correct pitch for a replacement propeller after changing engines.
Compare theoretical performance between two propeller pitch options.
Confirm that a used boat's stated top speed matches its propeller specifications.
Size a propeller for a repower where target speed and rated RPM are known.

Frequently Asked Questions

Last updated: 2026-07-22 · Reviewed by Nham Vu