German U-Boats were instructed to use electric motors during (night) surface attack IF: The conditions for sound-locating are good(*) and that enemy sound-location is anticipated. Sea is calm. Target speed is slow and the U-Boat in relation to the target will allow attack.
*
- acoustical conductivity of the water (temperature, salt content, motion of the sea)
- interference level of the sound-locating vessel
-volume of the sound to be located
Was it only a suggestion from behalf of the germans that they could be heard when running on surface or was it a real threat?
