propeller calculator

What this propeller calculator does

This propeller calculator is designed for marine setups where you know engine RPM, lower-unit gear ratio, and prop pitch. It helps you quickly estimate how fast a boat should go under ideal conditions and how that changes once propeller slip is included.

In practical terms, it answers questions like: “Is this 21-pitch prop close for my target speed?” and “If my GPS speed is lower than expected, what slip am I actually running?”

Core formulas used

1) Propeller shaft RPM

The propeller turns slower than the engine because of gearing: Prop RPM = Engine RPM / Gear Ratio. Example: 5500 RPM engine with a 2.00:1 gear ratio gives 2750 prop RPM.

2) Theoretical speed (zero slip)

Pitch is the distance the prop would move forward per revolution in a perfect medium. In mph, the common formula is: Speed = (Pitch in inches × Prop RPM × 60) / 63,360.

3) Actual speed with slip

Real water isn’t a solid thread, so there is always some loss: Actual Speed = Theoretical Speed × (1 − Slip/100). Lower slip usually means better prop efficiency for that setup and load.

How to use this calculator

• Enter your engine RPM at wide-open throttle or your test condition.
• Enter your gear ratio exactly as listed by the manufacturer.
• Enter prop pitch in inches.
• Add an expected slip percentage (10–15% is common for many planing hulls).
• Optionally enter observed GPS speed to calculate an estimated real-world slip.

Understanding the inputs

Engine RPM

Use real measured RPM, not a guessed value. Tachometer error can significantly skew speed estimates.

Gear ratio

A 2.00:1 ratio means the crankshaft rotates twice for each single prop revolution. Small ratio changes can create surprisingly large speed differences.

Pitch

Higher pitch generally increases potential top speed but can reduce acceleration and raise engine load. Pitch should be selected to keep the engine in its recommended WOT RPM range.

Slip

Slip is not “bad” by itself; it is a normal hydrodynamic reality. The right target depends on hull type, setup height, trim, load, and water conditions.

Typical slip ranges (rough guide)

• High-performance setups: ~5% to 10%
• General planing boats: ~10% to 18%
• Heavy loads / less optimized setups: ~18% to 30%

Example scenario

Suppose you run 5500 RPM, 2.00:1 gears, and a 21-pitch prop. Theoretical speed is just under 55 mph. If your slip is 12%, your adjusted speed is around 48 mph. If your GPS confirms about 48 mph, your setup is likely behaving as expected.

Optimization tips

• Verify tach and GPS accuracy before changing hardware.
• Test one change at a time (pitch, engine height, trim technique, load distribution).
• Watch engine operating range; avoid over-propping and lugging the motor.
• Record weather, water state, and fuel/load for comparable test runs.

Limitations

This calculator is intentionally simple and fast. It does not model blade design, cup, rake, hull drag curves, cavitation behavior, ventilation, altitude effects, or current and wind. Use it as a planning and comparison tool, then validate with on-water testing.