Prop Slip Calculator
Calculate propeller slip percentage to assess propeller efficiency and performance
Calculate Propeller Slip
Theoretical distance per revolution
Engine speed at full throttle
Engine to propeller shaft ratio
Measured boat speed
Calculation Results
Formula used: Slip = ((Theoretical Speed - Actual Speed) / Theoretical Speed) × 100
Theoretical Speed: (Pitch × RPM) / (Gear Ratio × 1056)
Speed difference: 8.2 MPH
Performance Analysis
Example Calculation
Boat Performance Assessment
Engine: 6000 rpm rated engine
Gear Ratio: 2:1
Propeller Pitch: 24 inches
Actual Top Speed: 60 mph
Step-by-step Calculation
1. Theoretical Speed = (24 × 6000) / (2 × 1056) = 68.18 mph
2. Slip = ((68.18 - 60) / 68.18) × 100
3. Slip = (8.18 / 68.18) × 100
4. Slip = 12%
This is within the normal range of 10-20% slip
Slip Range Guide
0-10% Slip
Excellent efficiency
Optimal propeller performance
10-20% Slip
Normal range
Typical for most boats
20-30% Slip
Higher than normal
Check propeller condition
30%+ Slip
Poor performance
Propeller issues likely
Factors Affecting Slip
Weather Conditions
Wind, waves, and current affect performance
Boat Load
Heavy loads increase slip percentage
Propeller Condition
Damage or fouling increases slip
Hull Condition
Clean hull reduces resistance
Engine RPM
Slip varies with engine speed
Understanding Propeller Slip
What is Propeller Slip?
Propeller slip is the difference between the theoretical distance a propeller should advance per revolution (its pitch) and the actual distance it advances through the water. This occurs because water is a fluid medium, unlike the solid interaction of a screw and nut.
Why Does Slip Occur?
- •Water displacement and turbulence around the propeller
- •Hull resistance and drag forces
- •Propeller blade angle and efficiency
- •Loading conditions and environmental factors
Slip Calculation Formula
Slip = ((Theoretical Speed - Actual Speed) / Theoretical Speed) × 100
- Theoretical Speed: Speed based on propeller pitch and RPM
- Actual Speed: Measured boat speed from speedometer
- Slip: Percentage difference (typically 10-20%)
Note: Negative slip can occur with favorable wind/current conditions, where the boat moves faster than theoretical speed.
Optimizing Propeller Performance
Understanding propeller slip helps optimize boat performance and diagnose issues. Consistent slip measurements can indicate propeller health and efficiency.
Low Slip (0-15%)
Indicates efficient propeller operation and good matching to boat/engine
Normal Slip (15-25%)
Acceptable range for most recreational boats under normal conditions
High Slip (25%+)
May indicate propeller damage, incorrect sizing, or heavy loading