Boat Propeller Sizing Guide
Why Propeller Size Matters
A boat propeller converts engine power into moving water. The chosen diameter and pitch decide how strongly that water is pushed. Diameter describes the circle swept by the blades. Pitch describes the theoretical forward travel during one full turn. A good match lets the engine reach its intended wide open throttle range.
Pitch, Slip, and Speed
Propeller sizing starts with shaft speed. The engine speed is divided by the gear ratio. The target boat speed then sets the needed pitch after slip is included. Slip is normal because water is not solid. Heavy boats, rough bottoms, and high loads usually increase slip. Fast planing hulls often use less slip when trimmed well.
Diameter and Blade Load
Diameter is estimated from pitch, hull type, blade count, and power demand. A displacement hull normally needs more blade area. A light planing hull can use a smaller diameter with higher pitch. More blades can carry load smoothly, but they may add drag. The calculator blends these effects into a practical starting size.
Testing Before Selection
The result should not be treated as a final purchase order. Propellers are affected by hull shape, trim angle, mounting height, ventilation, cavitation, and manufacturer blade design. Two propellers with the same stamped size may behave differently. Cup, rake, blade area, and material can change speed and engine load.
Reading the Result
Use the pitch range to compare nearby sizes. If engine RPM is too high, increase pitch or diameter carefully. If RPM is too low, reduce pitch or diameter. A one inch pitch change often changes engine speed noticeably. Test with normal fuel, crew, and gear aboard.
Current Propeller Diagnosis
The speed estimate helps diagnose current performance. Enter an existing pitch to see its theoretical speed. Compare it with measured speed to estimate slip. High slip can point to ventilation, wrong height, poor trim, damage, or excessive load. Low slip may indicate optimistic speed data or tachometer error.
Final Notes
This tool gives a structured physics based estimate. It supports planning before water testing. It also helps explain why a propeller feels slow, overloaded, or under pitched. Keep records of weather, water condition, engine height, trim setting, fuel load, and GPS speed for comparisons during repeat tests. Always check engine maker RPM limits and boat maker guidance. Use verified testing before buying any final propeller size.