Back when I was playing SHIII I used this method with great success.
http://www.subsim.com/radioroom/showthread.php?t=110619
Since I encounter lots of poor visbility conditions in the North altantic this method worked brilliantly. In fact I started to use it for every contact since I found myself submerge most of the time due to pesky aricraft dropping bombs on me.
Mittelwaechter had a matching video unfortnately the rapidshare link he used no longer works. Wished he would post it up on youtube.