Prop Pitch Calculator

How to use this propeller pitch calculator

What this calculator does

This calculator links five quantities used in a common propeller-speed relationship: Boat speed, Gear ratio, Engine/crankcase speed, Propeller slip, and Propeller pitch. Choose any one with Solve for, then provide the other four. The calculator solves the selected quantity and also shows propeller shaft RPM, the corresponding no-slip theoretical speed, and the effective forward advance per propeller revolution. It is a kinematic estimate, not a propeller-selection guarantee. Actual performance can vary with hull resistance, load, trim, altitude, water conditions, propeller design, ventilation, and the engine's recommended operating range.

When to use it

Use it when checking whether an observed top speed is plausible for a known propeller, estimating a pitch needed for a target speed, estimating slip from a GPS speed test, or checking the gearing/RPM assumptions behind a performance comparison. It is especially useful for screening calculations before an on-water propeller test. For practical context, Mercury Marine's explanation of propeller diameter and pitch describes pitch as theoretical travel per revolution and explains why pitch changes affect engine RPM and performance.

How to calculate

1. The calculator opens with a ready-to-use demonstration: 50 mph boat speed, a 2.00:1 gear ratio, 5,500 rpm, and 20% slip, giving a 24.00 in pitch. Its Excel workbook is validated and available immediately.
2. Choose the quantity you want under Solve for. That field becomes read-only; the other four become the required inputs.
3. Enter the known values. Boat speed unit can be mph, km/h, or knots; changing the unit converts an entered boat speed rather than merely relabeling it.
4. Read the main result, then use Propeller RPM, Theoretical speed, Effective advance, and the calculation table to verify the intermediate relationships.
5. Select Download Excel to export the current valid state as a real .xlsx workbook. Reset clears the demonstration and all data fields; after reset, Excel export stays disabled until a complete valid set of inputs is entered again.

Input guide

Solve for is required and selects the unknown: Propeller pitch, Boat speed, Propeller slip, Engine/crankcase speed, or Gear ratio. For example, choose Propeller pitch when speed, gearing, RPM, and slip are known. A common mistake is selecting one quantity as the target and then expecting its typed value to remain fixed; the target is deliberately recalculated.

Boat speed is a required positive decimal unless it is the target. Use the adjacent Boat speed unit control to choose mph, km/h, or knots. The accepted numeric convention uses a decimal point and optional thousands separators, such as 50, 50.5, or 1,250.5; scientific notation and decimal commas are rejected to avoid ambiguity. Higher boat speed requires more pitch, more RPM, less gearing reduction, or less slip when the other values are fixed.

Gear ratio is required and must be greater than zero unless it is the target. Enter engine revolutions per propeller revolution, such as 2 for a 2.00:1 reduction. Do not enter the inverse ratio. A larger numerical ratio means the propeller turns more slowly for the same engine RPM, so more pitch is required to sustain the same speed.

Engine/crankcase speed is required in rpm and must be greater than zero unless it is the target. A realistic example is 5,500 rpm. Higher RPM increases theoretical travel rate, so less pitch is needed for the same speed if gearing and slip are unchanged. Use the actual test RPM rather than a tachometer redline that the engine did not reach.

Propeller slip is required as a percentage from 0 up to, but not including, 100 unless it is the target. Enter 20 or 20% for twenty percent. Slip represents the portion of geometric pitch that does not become forward boat travel. Higher slip lowers actual speed for a given pitch and RPM. Do not enter 0.20 when you mean 20%; in this field 0.20 means 0.20%.

Propeller pitch is required in inches and must be greater than zero unless it is the target. A realistic example is 24 in. Pitch is theoretical advance per propeller revolution in a solid medium. A higher pitch raises the theoretical travel per revolution, but actual engine RPM and acceleration can change after fitting a different propeller, so the calculator should not be used as the sole basis for purchasing or setup decisions.

Output guide

Primary result is whichever quantity is selected under Solve for. It is an exact result of the stated kinematic relationship for the supplied assumptions, not a guarantee of measured performance. The header Target pill echoes the selected unknown, Prop RPM mirrors the Propeller RPM result, and No-slip speed mirrors Theoretical speed. Propeller RPM is engine RPM divided by gear ratio. Theoretical speed is the no-slip boat speed for the current pitch, RPM, and gearing; actual boat speed will be lower whenever slip is positive. Effective advance is pitch × (1 – slip), expressed in inches per propeller revolution. The Calculation details table repeats the five core values plus these intermediates so the relationship can be audited row by row.

Worked example

In the opening example, speed is 50 mph, gear ratio is 2.00:1, engine speed is 5,500 rpm, and slip is 20%. For mph, the conversion constant is 1,056. The pitch calculation is 50 × 2 × 1,056 ÷ [5,500 × (1 – 0.20)] = 24.00 in. Propeller RPM is 5,500 ÷ 2 = 2,750 rpm. Effective advance is 24 × 0.80 = 19.20 in/rev. Without slip, the same pitch, RPM, and gearing would imply 62.50 mph; applying 20% slip reduces that to 50.00 mph. Those values match the first-open controls, results, detail table, and downloadable workbook.