Ah yes the famous Hanhart watch
I readed some time ago that the scale was meant for timing the torpedo impacts, hence it would act as the current fake red needle displayed in the chrono when a torpedo is shot, showing -according to the set speed- the estimated time of impact.
Here is an interesting link about the use of tachymeters:
http://en.wikipedia.org/wiki/Tacheometer
Given the divisions shown in the external scale (1-20), my bet is that it is probably to allow the commander calculating the speed of his own U-boat, an enemy ships or similar which travels between two marked positions.
The spiral shaped interior scale is clearly meant for longer timings, so probably for the aforementioned torpedo runs.