unfortunately i dont think that the ASW aspect of the sim is recreated to such a level so one could take advantage of such tactics.
i think the ASDIC works to a certain depth and then it just suddenly stops working.
ASDIC and sonar are complex beasts, and i think the trick to making a great subsim is to figure out a way to model or at least account for all of these little complexities.
I have often made attempts to hide under the convoy - but not for the reasons you have stated... but simply because the collision avoidance programming of the AI causes them to frequently break off attacks in order to avoid ramming other ships, even if the ship is in the process of sinking.
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