## Calculating Frequency of Propeller Blade Vibration

Optimizing the performance of Boston Whaler boats
jimh
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### Calculating Frequency of Propeller Blade Vibration

Propeller vibration in outboard engines is often the result of the propeller blades passing behind the gear case and skeg. The flow of water into the propeller blade is altered by the upstream obstruction, and the blade operates in the shadow of this obstruction. This causes a change in the blade loading onto the propeller shaft and on the engine, resulting in a small change in the instantaneous rotation speed, and this is felt as vibration.

The vibration created by the blade shadowing is related to the propeller shaft rotation speed and the number of blades on the propeller. To put some numbers to the frequency of the propeller blades being shadowed, here are two examples.

First, we assume the vibration occurs at an engine speed of 1,000-RPM. Next we apply the reduction gear ratio to find the propeller shaft RPM. For a 90-HP engine the gear ratio would typically be 2:1. This means the propeller shaft rotation speed will be 500-RPM.

If the propeller has three blades, then there are six occurrences of a shadowing for each shaft revolution, or 6-shadows-per-revolution x 500-RPM = 3,000-shadow cycles-per-minute.

The frequency of sounds or vibrations are typically expressed in cycles-per-second, so we have to convert cycles-per-minute to cycles-per-second; we divide 3,000 by 60 to get 50-cycles-per-second or 50-Hz.