Estimate prop diameter and pitch from speed, rpm, power. Review formulas, examples, exports, and charts. Built for engineering checks with practical adjustable design inputs.
This calculator estimates prop diameter and pitch from power, shaft speed, target speed, slip, blade count, and application loading. It is useful for early-stage engineering checks when you need a practical starting point before comparing manufacturer charts or test data.
The page returns recommended diameter and pitch, shaft RPM, pitch-to-diameter ratio, advance ratio, tip speed, and an adjustable slip chart. You can also export the output to CSV or PDF and compare your result against the sample reference table below.
| Case | Power | Engine RPM | Gear Ratio | Target Speed | Slip | Blades | Diameter | Pitch |
|---|---|---|---|---|---|---|---|---|
| Light fast hull | 90 hp | 3600 | 2 | 24 knots | 14% | 3 | 18.54 in | 18.84 in |
| Balanced cruiser | 150 hp | 3200 | 2 | 28 knots | 12% | 3 | 22.72 in | 24.17 in |
| Heavy load setup | 220 hp | 3000 | 2.3 | 22 knots | 18% | 4 | 28.23 in | 25.00 in |
The calculator estimates shaft speed first:
Shaft RPM = Engine RPM ÷ Gear Ratio
Pitch is then estimated from the target advance speed and slip:
Pitch = Speed ÷ (n × (1 − Slip))
Here, n is shaft revolutions per second, and slip is entered as a decimal fraction. This gives geometric pitch per revolution.
Diameter is estimated using a propeller power scaling relation:
D = (P ÷ (ρ × n3 × Cp))1/5
In this relation, P is shaft power in watts, ρ is fluid density, and Cp is a power coefficient. Blade count selects a practical default coefficient, while the application factor shifts the diameter for light or heavy loading.
The chart shows how the required pitch changes when slip changes while speed and shaft RPM stay fixed.
Use the result as a starting point. Final prop selection should always be checked against blade clearance, cavitation limits, engine loading, and manufacturer prop family recommendations.
It estimates recommended prop diameter and pitch from power, shaft speed, target speed, slip, density, blade count, and loading assumptions. It is best for preliminary engineering sizing.
No. Real performance depends on blade area, cup, rake, hub design, clearance, cavitation behavior, and the prop series chosen by the manufacturer.
Gear ratio converts engine RPM to shaft RPM. Propeller diameter and pitch must be sized from shaft speed, not engine speed alone.
Use measured or expected slip from similar setups. Fast efficient systems often use lower slip, while heavy or loaded systems usually need higher slip values.
Blade count changes the practical loading coefficient. More blades can absorb power differently, which shifts the estimated diameter used for a comparable operating point.
Use a custom coefficient when you already have design data, test data, or a known propeller family. Otherwise, the built-in defaults are better for quick estimates.
It shows how required pitch changes when slip changes while your selected speed and shaft RPM remain fixed. This helps you test sensitivity quickly.
Yes, it suits preliminary engineering checks where power, speed, slip, and shaft RPM drive sizing. Always validate final hardware with detailed design data.
Important Note: All the Calculators listed in this site are for educational purpose only and we do not guarentee the accuracy of results. Please do consult with other sources as well.