[Leandros]

Wikipedia: "Propeller RPM: The speed of a ship is roughly proportional to the rotation rate of the propeller. This approach is most useful when the propeller has constant pitch. It is less useful on ships with propellers that have variable pitch propellers or no propellers (like sailing ships). A variation of this approach was used by submarines during World War II to determine the speed of targets. The sonar operator would listen to the sound of a target's propeller and would determine its rotation rate. Knowing the propeller rotation rate and visually identifying the type of ship, the target's speed could be estimated".

This would be a nice mission for one of the excellent modders here: A solution for deciding propeller RPM's taken from the sonar and, consequently, estimating the ship's speed compared with the ship type as observed in the periscope....

Fred