What is the purpose of a Rupert's scale?

The correct purpose of a Rupert's scale is to measure the roughness of propellers. This measurement is crucial because the performance and efficiency of a propeller can be significantly affected by its surface condition. A rough surface can lead to increased drag and reduced thrust efficiency, which can negatively impact a vessel's performance. By using the Rupert's scale, operators can evaluate the roughness and wear on propeller blades, enabling them to make informed decisions about maintenance, repairs, or replacements to ensure optimal performance and fuel efficiency for their vessels.