I'm currently at Oct 1939 and my orders now have me navigating the shallow waters of the English Channel. During my last patrol I sunk 11 ships before I even reached my patrol grid. A couple of them under the shadow and wake of a DD.
I learned from a reading a number of posts here that it seems that an ASDIC's abilty to pick you up heavily depends on an engine's RPM. So when I come in contact with a destroyer and he knows I'm around I switch my speed control from the Ahead Slow/Ahead Flank format to selecting specifc speeds in knots. I rig for silent running and go to 1-2 knots. I also tried to maintain a depth that was anywhere from 5-10 meters above the bottom (the water was about a total of 35 or 40 meters deep). This, coupled with turning into him when his search pattern brings him on a closing heading and continuing past him using his own screws to mask me.
The one thing you have to be careful about is if he does detect you and goes on a Depth Charge run is going to Flank speed to aviod the Charges. Timing is everything. If you're not careful he'll hear you after the last charge has gone off and come around for another run... which can happen more often than not.
I was able to slip away from a couple of destroyers doing this when I had little to no diving room (which made me nervous as all hell). Two of them I even put myself in a perfect firing position when I came up to periscope depth to check the surface. I don't really think that putting two of Winston's annoying destroyers on the bottom is a waste of an eel, especially considering the fact that all the merchant ships I encounter are still unarmed. Making deck gun operations perfect... weather permitting.