[irish1958]

After spending several hours investigating this topic, I can't determine how long the disturbance from a DC persists. What I did learn is that the destroyer always looses contact after an attack and it takes a while to reestablish, if it can be. As Stiebler pointed out, an hour or more was not uncommon. Another source stated that there is a one in fifteen chance of establishing contact if the correct procedures were followed but that they rarely were.
Another source stated that it is impossible for a WWII destroyer to re-establish contact with any sub traveling faster than 15 knots as it's sonar was not effective above 15 knots due to water noise from the sonar array of the destroyer. The type XXI, had it been developed, would have be devastating.
So apparently a single destroyer was not very effective against the WWII subs. If a single destroyer was attacking a sub, the tactic was to keep the sub submerged under periscope depth while allowing the convoy to escape. The depth charges were dropped set for shallow explosions to keep the sub down and unable to attack while the convoy moved on. The destroyer would then trail the convoy and to prevent the sub from surfacing and attacking.
Up until late 1942 or early 1943 (April) the US tactics were not to sink the subs but to prevent attacks on the convoy and success was determined not by how many subs were sunk, but how many ships reached port.
This changes in 1943 (for the US).
In the Pacific, the Japanese ran out of destroyers in 1944 so the US subs were unopposed in the last year of the war.
This mod is terrific and really adds to the game.