i dont know it with depthcharches, but i had once also a weird thing:
i was just surfaced and chasing a convoy suddenly a destroyer turns up and i do a crashdive. when it was quiet for a while i get back to periscope depth and raise my periscope. i didnt realise he was very close( i didnt had the hydrophone manned) and he just scratches my periscope, the ships blows and sinks!
