Having read the war diary of U-480 over in the U-boatarchive I realised that at least some late war U-boats actually estimated regularly the speeds of the merchants and the escorts through listening to their propellers' sounds. Counting the screws and deducting the RPM's per minute from that
That's why I made this SH3MissionEditor Mission where I let German type 34 destroyers run by me at different speeds. I counted the RPM's and took screenshots for a crude guesstimation of speed based on how the smoke coming from the funnels behave
I made a chart like this:
This was something the Germans could have done very easily during the war. Of course it's possible that the type 34 destroyer's screw sounds are totally different to a Town class or River Class
Anyways, I'm planning on maybe adding the chart to Charts Addon one day. Here's the tga for anyone who wants to try it
http://www.2shared.com/file/nMI-xdd1/sliderule.html
The easiest way to try it, is just to drop it into the layout folder in charts Addon. Then it replaces the chart that tells how to multiply with an attack disc.