What Is Propeller Pitch?
Propeller pitch is the theoretical distance a propeller would advance in one complete revolution if it moved through a solid medium with no slippage, much like a screw advancing through wood. It is measured in inches (or millimeters) and is one of two primary specifications for any propeller, the other being diameter. A propeller labeled "14 x 19" has a 14-inch diameter and a 19-inch pitch.
The pitch determines the load on the engine and the theoretical top speed. A higher-pitch propeller moves more water (or air) per revolution and is like a higher gear in a transmission -- it provides more top-end speed but requires more power and results in slower acceleration. A lower-pitch propeller is like a lower gear, providing better acceleration and load-carrying ability at the cost of top speed. Selecting the right pitch is critical for engine performance, fuel efficiency, and longevity.
Pitch Formulas
The constant 1056 converts from inches per minute to miles per hour (12 inches/foot x 5280 feet/mile / 60 min/hour = 1056). The blade angle varies along the blade radius; it is measured at the 75% radius position (0.75R) by convention. Slip is the difference between theoretical and actual speed, expressed as a percentage. Typical marine prop slip ranges from 10% to 30%.
Common Marine Prop Sizes
| Engine HP | Diameter (in) | Pitch (in) | Application |
|---|---|---|---|
| 25-50 | 10-11 | 13-15 | Small outboard / pontoon |
| 75-115 | 13-14 | 17-21 | Bass boat / runabout |
| 150-250 | 14-15.5 | 19-25 | Ski boat / wakeboard |
| 250-400 | 15-16 | 21-28 | Offshore / performance |
Prop Selection Guide
- Increase pitch by 2 inches: Expect about 200-400 RPM drop and higher top speed (if engine can still reach operating range).
- Decrease pitch by 2 inches: Expect about 200-400 RPM increase and better acceleration / hole shot.
- Operating range: Always ensure your engine reaches its recommended wide-open-throttle RPM range with the selected prop. Running below range strains the engine.
- Material matters: Stainless steel props have less flex than aluminum, resulting in less slip and better performance but at higher cost.
Frequently Asked Questions
What is the ideal slip percentage?
For most recreational boats, slip between 10% and 20% is normal and acceptable. High-performance boats with stainless steel props may achieve as low as 5-10% slip. Heavily loaded boats or those with fouled hulls may see slip exceeding 30%. If your slip is unusually high, check for a damaged prop, hull growth, or an undersized propeller.
Does pitch affect fuel economy?
Yes. Too much pitch causes the engine to lug (run below optimal RPM), consuming more fuel per mile. Too little pitch causes the engine to over-rev, wasting energy. The sweet spot is a pitch that lets the engine run at its most efficient RPM for your typical cruising speed, which is usually around 60-70% of wide-open throttle RPM.
Can I calculate pitch from blade angle?
Yes. Pitch = pi x D x (r/R) x tan(blade angle), where D is diameter, r is the measurement radius, and R is the full radius. Since blade angle varies along the radius, always measure at the same reference position (typically 0.75R) for consistent comparison between propellers.