# Prop Slip Calculator

Calculate propeller slip percentage from RPM, pitch, and boat speed. Find theoretical vs actual speed and optimize your prop setup.

## What this calculates

Propeller slip is the difference between the theoretical distance a prop should move through water per revolution (based on its pitch) and how far it actually moves. Some slip is normal and necessary for thrust. Typical slip ranges from 10% to 20%, but values outside this range can signal a prop mismatch, hull fouling, or engine issues.

## Inputs

- **Engine RPM** (RPM) — min 0 — Propeller shaft RPM (after gear reduction if applicable).
- **Propeller Pitch** (in) — min 0 — Theoretical distance the prop moves in one revolution.
- **Actual Boat Speed** (mph) — min 0
- **Gear Ratio** (:1) — min 0.1 — Lower unit gear ratio. Use 1.0 for direct prop shaft RPM.

## Outputs

- **Prop Slip** (%) — Percentage of theoretical speed lost to slip
- **Theoretical Speed** (mph) — Speed if the prop had zero slip
- **Speed Lost to Slip** (mph) — Difference between theoretical and actual speed
- **Prop Shaft RPM** (RPM) — RPM at the propeller after gear reduction

## Details

The theoretical speed of a boat is calculated from RPM and propeller pitch. The formula is:

Theoretical Speed (mph) = (Prop RPM x Pitch in inches) / 1056

The constant 1056 converts inches per minute to miles per hour (12 in/ft x 5280 ft/mi / 60 min/hr).

Slip percentage is then:

Slip % = ((Theoretical Speed - Actual Speed) / Theoretical Speed) x 100

  - 10-15% slip is typical for well-matched props on planing hulls

  - 15-20% slip is common on heavier displacement boats

  - Over 25% slip suggests the prop is too small, the hull is fouled, or the engine is underpowered

  - Under 5% slip may indicate the prop is over-pitched, potentially overloading the engine

If you have a gear reduction lower unit, remember to divide the engine RPM by the gear ratio to get the actual prop shaft RPM before calculating. Many outboards and stern drives have gear ratios between 1.75:1 and 2.5:1.

## Frequently Asked Questions

**Q: What is a normal prop slip percentage?**

A: Most boats run between 10% and 20% propeller slip. Planing hulls with well-matched props typically see 10-15% slip, while heavier displacement hulls run closer to 15-20%. Slip below 5% or above 25% usually signals a problem with prop selection or hull condition.

**Q: How do I reduce propeller slip?**

A: You can reduce excessive slip by choosing the right prop diameter and pitch for your boat, keeping the hull bottom clean, checking that the engine reaches its recommended wide-open-throttle RPM range, and making sure the prop is not damaged or cavitating. Switching to a four-blade or cupped prop can also help in some cases.

**Q: Do I use engine RPM or prop shaft RPM?**

A: You need prop shaft RPM for the calculation. If your tachometer shows engine RPM, divide by the lower unit gear ratio to get prop shaft RPM. For example, 5000 engine RPM with a 2.0:1 gear ratio gives 2500 prop shaft RPM. Enter the gear ratio in this calculator and it handles the conversion for you.

**Q: Why is some propeller slip necessary?**

A: A propeller generates thrust by pushing water backward. Zero slip would mean the prop is moving through the water without pushing any, like a screw in a solid block. The slip represents the water being accelerated rearward, which is what propels the boat forward. Without slip, there would be no thrust.

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Source: https://vastcalc.com/calculators/physics/prop-slip
Category: Physics
Last updated: 2026-04-08