Ratio

Ratio

The speed a propeller can propel you depends, at a basic level, on two factors: its pitch and how fast it is turning. The rpm you read on your tachometer tells you how fast the engine itself is spinning, but it does not tell you how fast the propeller is turning. To determine the propeller speed, you must know what the gear ratio of your boat’s transmission is. Propeller rpm is calculated as follows:

Prop rpm = Engine rpm / Gear Ratio

Gear ratios are commonly reported as something-to-1 (something : 1). In most marine applications, the “something” is usually between 1.5 and 2.5 (1.5:1 to 2.5:1). If our boat’s gear ratio is 1.86:1 (a common value on large Mercury Outboards), and the rpm reported by the tachometer is 4800, our propeller rpm would be 2580rpm:

2580rpm = 4800 / 1.86