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? :88)

On a calm day the engine noises of surface vessels can be carried over long distances. If one is on a stationery or noiseless platform, one hear engines of vessel before you can see it.

Therefore I think the advice of using electric engines is not that bad. But I have never been on a ocean. I do not know the elements there.

In this case of being sound-located because of the diesel noise wasn't concerned to be a threat heard via air but underwater (hydrophones).