1. What is a Prop Size Calculator by Boat?

Definition: This calculator estimates the optimal propeller pitch based on boat speed, engine RPM, and propeller slip factor.

Purpose: It helps boat owners and marine professionals select the right propeller for optimal performance and fuel efficiency.

2. How Does the Calculator Work?

The calculator uses the formula:

Where:

• \( Pitch \) — Recommended propeller pitch (inches)
• \( S \) — Boat speed (miles per hour)
• \( RPM \) — Engine revolutions per minute
• \( Slip \) — Propeller slip factor (decimal, typically 0.05-0.25)

Explanation: The formula calculates theoretical pitch needed based on actual performance, accounting for slip (the difference between theoretical and actual movement through water).

3. Importance of Proper Prop Selection

Details: Correct propeller sizing ensures optimal engine performance, fuel efficiency, and prevents over-revving or lugging the engine.

4. Using the Calculator

Tips: Enter your boat's actual speed at wide-open throttle (WOT), the engine RPM at that speed, and estimated slip factor (default 0.12).

5. Frequently Asked Questions (FAQ)

Q1: What is propeller slip?
A: Slip is the difference between theoretical and actual movement through water, caused by factors like hull drag and water conditions.

Q2: What's a typical slip factor?
A: Most boats have 10-15% slip (0.10-0.15). High-performance boats may be 5-8%, while heavy displacement hulls may be 20-25%.

Q3: How do I measure my actual slip?
A: Use the formula: \( Slip = 1 - \frac{Actual Speed}{Theoretical Speed} \). Theoretical speed = (Pitch × RPM) / 1056.

Q4: Should I round the calculated pitch?
A: Yes, propellers typically come in even-numbered pitches (e.g., 17, 19, 21 inches). Round to nearest available size.

Q5: What if my WOT RPM is below manufacturer's recommendation?
A: You may need a lower pitch propeller to allow higher RPMs and proper engine loading.