What's the best way to approach convoys post '43

[Tonnage_Ace]

Anyone have any ideas, so far I've been putting myself in front of the convoy's path at periscope depth and waiting till a ship gets close enough so I can get the heading and speed down, but DD's seem to pick me up with ASDIC even when I'm at a dead stop. 92% realism and I'm sporting a brand new IXD2. Maybe I shouldn't be right ahead of the convoy but rather on it's flank when I take my readings but when your in the convoy's path it you can take the heading/speed set up the TDC and fire a salvo then let the convoy pass and hit it again later, this is what I've been doing but after '43 it seems the enemies ASDIC kicks in even if your at a dead stop and under water. Love to hear from 'pros' on this issue.