Authentic speed measurement in SH4

[musikerz]

Hello, everyone here, I am a new cadet skipper who has just been introduced to the great Silent Hunter 4 and its mods, and still learning to survive in the 100 percent realism and how to use TDC manually.

So my question is how do u guys measure vessels' speed by listening to its propeller rotation? Because i heard that those real skippers did use this method in the history, isn't it?