1. What is the Propeller Speed Equation?

The propeller speed equation estimates a boat's theoretical top speed based on engine RPM, propeller pitch, gear ratio, and propeller slip. It helps boaters select the right propeller for optimal performance.

2. How Does the Calculator Work?

The calculator uses the propeller speed equation:

Where:

• \( RPM \) — Engine revolutions per minute
• \( Pitch \) — Propeller pitch in inches
• \( Gear Ratio \) — Drive gear reduction ratio
• \( Slip \) — Propeller slip as a fraction (typically 0.1-0.2)

Explanation: The equation calculates theoretical speed by accounting for how much the propeller advances per revolution, adjusted for gear reduction and slip.

3. Importance of Propeller Speed Calculation

Details: Proper propeller selection ensures your engine operates in its optimal RPM range, maximizes fuel efficiency, and prevents over-revving or lugging the engine.

4. Using the Calculator

Tips: Enter engine RPM at wide-open throttle, propeller pitch in inches, gear ratio (e.g., 2:1 = 2.0), and typical slip (10% = 0.1). All values must be positive numbers.

5. Frequently Asked Questions (FAQ)

Q1: What is propeller slip?
A: Slip is the difference between theoretical and actual speed due to water not being a solid medium. Typical slip is 10-20% for most boats.

Q2: How do I find my gear ratio?
A: Check your outboard/sterndrive specifications or count drive shaft rotations vs propeller rotations when manually turning the engine.

Q3: Why is actual speed often less than calculated?
A: Hull drag, weight, water conditions, and engine performance all affect actual speed beyond just propeller calculations.

Q4: What's a good pitch for my boat?
A: Ideal pitch depends on boat type/weight and engine power. The goal is to reach manufacturer's recommended WOT RPM range.

Q5: How does changing prop diameter affect speed?
A: Diameter isn't in this simplified equation but affects performance. Larger diameters generally provide more thrust but may reduce RPM.