I second both Charlie and alphaBeta. I can't envision how late war would be survivable without the sonar mod - assuming that late war is survivable in any case!
As a bit of a postscript, since my earlier problems I've encountered two more large convoys. In both cases I was able to sneak by the lead escort through careful manuvering and exploiting holes in the escort screen. Lots of fun after that, including sinking my first troop ship. What I found interesting after that was how easy it was to get away. As long as I went deep and moved in the exact opposite direction of the convoy, the escorts never even came near me. Look what a little bit of knowledge can do!
