Boat Propeller Size Calculator

Understanding Propeller Specifications A boat propeller is described by two numbers: diameter × pitch, always in inches. A “14 × 19” prop has a 14-inch diameter and 19-inch pitch. The diameter is the circle swept by the blades. The pitch is how far the prop would advance in one revolution if it were turning through a solid, the way a screw moves through wood.

This calculator works on pitch. Diameter is set by your gearcase and is not something you pick freely, so on almost every outboard and sterndrive the prop you buy comes in one diameter for a given family and you tune with pitch.

What Pitch Controls Pitch is the main tuning variable. More pitch grabs more water per revolution and raises top speed, but it loads the engine harder and makes maximum RPM harder to reach. Less pitch costs top speed and lets the engine spin up to its WOT (Wide Open Throttle) range, which improves hole shot and keeps cruise efficient. The target is always the same: pick the pitch that lets the engine reach its manufacturer-specified WOT range with the boat loaded the way you actually use it.

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, and they favor top speed and efficiency at WOT. Four-blade props give better low-RPM torque, better economy at cruise, better bite in rough water, and less vibration. They cost more and usually give up a little top speed. Five-blade props show up in specialized performance and commercial work.

One thing to expect before you order: dropping pitch to fix a lagging engine almost always costs top speed. That is the trade, not a mistake in the numbers. You are buying back RPM, hole shot, and engine life with a couple of miles per hour.

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.