I have got myself into a situation where over 30 destroyers are hovering over me, but on my map, their asdics are not appearing, but they are pinging me, and there is none of the yellow or red circle on me at all, I have been in one place for over 3 days in game time, but they have not run out of ammo.

The red semi circle is not even present.
PS. I was 143 meters deep, with Silent running on.
Is there any way I can edit their depth limit myself?
And their "Ash cans" are not able to reach me? so why do they hover around?
Wouldn't they run out of fuel? Ammo?
I have also noticed them picking me up and finding me at ranges of over 30 kilometers without radar (1939)
After scanning the internet for multiple times a day, I am yet to find out how deep the Asdic can hear.
Because If a Submarines max depth is 200, and the Asdic can hear 200, the submarine is useless and needs to be replaced with a more suitable upgrade, such a new model, but since their is only the VII, it is hard to get a work around this.