Depth Charges?

[Col7777]

Funny this came up, I have recently downloaded the Mediterranean Campaign, and during one of the missions I got detected so I went deep, rigged for Silent Running and headed away from the convoy.

The DD's came hunting, I was creeping away at 4 knots at 160ft. As one of them came straight for me I deployed decoys, that seemed to work, it went over me then dropped DC's a fair distance away.
I just kept moving away, the others also came but concentrated on an area a good way behind me.
I was on the free cam watching them DC an area I wasn't in, there were in the end 7 DD's plus at one stage an aircraft also bombed an area a good way from me.
The DD's eventually gave up and left the area, I surfaced when it was safe I never scored a hit but in a way I felt I did for evading all those hunters and got the crew home safe.

This leads me to a question, this was the first time I deployed decoys, so how far do they go to lead a DD to ignore you, how do they work?
I've seen them in modern sub movies but this is WW2 and in the sim they worked in this case anyway.

Edited to add:
I was in the Porpoise sub and the Japanese DD's seemed to be a few various types.

Col.