Boat Prop Slip Calculator

Propeller Slip Formula:

\[ PS = 100 \times (1 - \frac{V}{\frac{RPM \times PP}{GR \times 1056}}) \]

Speed (V):

mph

RPM:

rev/min

Pitch (PP):

inches

Gear Ratio (GR):

dimensionless

Propeller Slip (PS):

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1. What is Propeller Slip?

Propeller slip is the difference between the theoretical distance a propeller should move through the water and the actual distance it travels. It's expressed as a percentage and indicates the efficiency of propeller performance.

2. How Does the Calculator Work?

The calculator uses the propeller slip formula:

\[ PS = 100 \times (1 - \frac{V}{\frac{RPM \times PP}{GR \times 1056}}) \]

Where:

\( PS \) — Propeller slip percentage (%)
\( V \) — Boat speed (mph)
\( RPM \) — Engine revolutions per minute (rev/min)
\( PP \) — Propeller pitch (inches)
\( GR \) — Gear ratio (dimensionless)
1056 — Conversion constant

Explanation: The formula calculates the theoretical speed based on propeller pitch and engine RPM, then compares it to the actual speed to determine slip percentage.

3. Importance of Propeller Slip Calculation

Details: Propeller slip is crucial for optimizing boat performance. Lower slip percentages indicate better efficiency, while high slip may indicate propeller issues, incorrect sizing, or hull problems.

4. Using the Calculator

Tips: Enter boat speed in mph, engine RPM, propeller pitch in inches, and gear ratio. All values must be positive numbers. Typical propeller slip ranges from 5% to 25% for most boats.

5. Frequently Asked Questions (FAQ)

Q1: What is a good propeller slip percentage?
A: For most boats, 10-15% slip is considered good. High-performance boats may achieve 5-10%, while heavily loaded or inefficient hulls may have 20-25% slip.

Q2: Why does propeller slip occur?
A: Slip occurs due to water displacement, cavitation, hull drag, and the fact that propellers don't move through a solid medium but through water that can slip past the blades.

Q3: How can I reduce propeller slip?
A: Proper propeller sizing, maintaining clean hull surfaces, correct trim angle, and ensuring the propeller is undamaged can help reduce slip.

Q4: What does negative slip mean?
A: Negative slip is physically impossible and indicates measurement error, typically in speed, RPM, or propeller pitch data.

Q5: Does slip vary with boat speed?
A: Yes, slip typically decreases as boat speed increases up to a point, then may increase again at very high speeds due to cavitation.