Boat Propeller Size Calculator

Understanding Propeller Specifications A boat propeller is described by two numbers: diameter × pitch, always in inches. For example, a “14 × 19” prop has a 14-inch diameter and 19-inch pitch. The diameter is the circle swept by the propeller blades as they rotate. The pitch is the theoretical distance the prop would advance forward in one revolution if turning in a solid medium with no slip — like a screw moving through wood.

What Pitch Controls Pitch is the primary tuning variable for performance. Higher pitch means the prop grabs more water per revolution, giving higher top speed — but it puts a heavier load on the engine, making it harder to reach maximum RPM. Lower pitch reduces top speed but allows the engine to spin freely to its WOT (Wide Open Throttle) RPM range, improving acceleration (hole shot) and efficiency at cruise. The goal is always to select a pitch that allows the engine to reach its manufacturer-specified WOT RPM range when the boat is fully loaded.

The RPM Rule of Thumb If your engine is below its target WOT RPM range with your current prop, reduce pitch by 1 inch per approximately 150–200 RPM shortfall. If above the RPM range, increase pitch by the same logic. Each 1 inch of pitch change typically moves RPM by 150–200 RPM.

Propeller Slip No propeller is 100% efficient. Slip is the difference between theoretical advance (pitch × RPM) and actual boat speed. A well-matched prop on a planing hull typically slips 10–15% at WOT. Heavy boats and displacement hulls may slip 15–25%. High slip means the propeller is inefficient and losing energy to cavitation and turbulence. Low slip means excellent propeller-to-hull matching.

3-Blade vs. 4-Blade Props Three-blade propellers are the most common: they optimize for top speed and efficiency at WOT. Four-blade props provide better low-RPM torque, improved fuel economy at cruise speeds, better performance in rough water, and reduced vibration — but they typically cost more and give slightly lower top speed. Five-blade props are used in specialized performance and commercial applications.

Stainless vs. Aluminum Aluminum props are less expensive (often $80–$200), easily repaired, and perfectly adequate for most recreational use. Stainless steel props are 3–4× more expensive but are more rigid (flex less, which improves efficiency), more durable, and more resistant to damage from rocks and debris. Stainless props are preferred for performance boats and commercial use.