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Propeller Pitch & Lower Unit Gear Ratios

Updated: Nov 24, 2023

The lower unit gear ratio of an outboard engine is a reduction gear that controls how power is applied to the water. The gear ratio is calculated by dividing the output speed by the input speed. For example, a gear ratio of 1.50:1 means that the engine must turn 1.5 times to turn the propeller once.

The gear ratio of outboard and sterndrive motors typically ranges between 1.47 and 1.92. For example, a Mercury Bravo One SM drive has gear ratios of 1.35:1 and 1.50:1. The Mercury 50HP has a gear ratio of 2.31:1. The Seven Marine 627 outboards have a range of gear ratios from 2.33:1 to 1.47:1. Race outboard motors typically have a 1:1 gear ratio and use lower-pitch props.

When changing propeller pitch and/or lower units with different gear ratios, it's important to make sure your combination is going to work together.

This chart helps to illustrate the impact of changes; for instance, going from 1.60 to 1.75 gearbox might require a propeller 2-3 pitch sizes bigger (maybe 30 to 32 or 33) to run similar RPMs. Conversely, going from a 1.75 to 1.60 might require a propeller 2-3 pitch sizes smaller (maybe 30 to 27 or 28) to run similar RPMs.

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