135. Creeping attacks.
This was a highly effective form of DC attack where 1 escort stood off circa 800m from a u-boat, keeping it pinged with asdic, without actually attacking. A second escort would then be directed to the position of the u-boat by the first, not using asdic, moving at dead slow, and dropping a pattern. The u-boat had no warning other than "splashes", as it only heard the pinging escort.
https://en.wikipedia.org/wiki/Creeping_attack
It would be interesting if our AI destroyers (and later human operated escorts) were able to work as pairs in this regard, especially if the pinging u-boat of the pair followed up with it's attack once the first boat's charges had detonated...