Propeller Sizing Calculator

Propeller diameter and pitch answer two different physical questions, and this calculator's formulas keep them cleanly separated. Diameter is sized from the classic simplified Crouch method, which relates blade diameter to engine horsepower and propeller RPM (engine RPM divided by your gear reduction ratio) -- more power at a given RPM calls for a bigger disc to absorb it, while spinning faster at the same power lets a smaller diameter do the job. Pitch, on the other hand, is sized from your target boat speed and propeller RPM with an assumed 15% slip factor -- it answers "how far should this prop notionally travel per revolution to hit that speed," and engine horsepower is never a factor anywhere in that math. That separation means the two outputs respond to different inputs: engine horsepower moves diameter but never pitch, and boat speed moves pitch but never diameter.

Propeller RPM (shaft RPM divided by reduction ratio) affects both, but in opposite directions from the two RPM-side inputs -- a higher engine RPM at a fixed reduction ratio raises propeller RPM and shrinks both the recommended diameter and pitch, while a higher reduction ratio lowers propeller RPM for the same engine RPM and grows both. What this calculator doesn't account for: cavitation risk beyond the reported tip speed figure, blade area and blade count (which the simplified formula doesn't model at all), hull-specific loading or wake effects, or the difference between a displacement hull and a planing hull's very different propeller demands. Treat the output as a starting-point selection to discuss with a propeller shop, not a final specification.