@Young:
Lets face it… A.S.D.I.C was not deployed as early in the war as oob suggests, 1940 encompassed the “happy days” of German U-boat dominance in the Atlantic, which in turn forced the Allies to develop anti-submarine equipment.
A variation of this idea that you might want to consider is having a system in which the ASW capabilities of destroyers
go up and down – perhaps randomly – during the course of the game, and perhaps only for certain countries. This would be more accurate historically, and perhaps more interesting from a gaming point of view.
The situation you’ve described – at its most fundamental, “the Germans had the advantage during the first half of the Battle of the Atlantic, and the Allies had the advantage during the second half” – is basically correct, with the turn-around point being mid-1943. In finer detail, however, the advantage actually swung back and forth between the two sides several times during the war. And it wasn’t a case of “before ASDIC, the Axis had the advantage; after ASDIC, the Allies had the advantage.” ASDIC already existed when the war broke out; the real problem was that the British had tested it in unrealistic conditions in peacetime, and had an overly-optimistic opinion of how effective it was going to be in actual combat and in rough weather. Moreover, ASDIC was only one part of a very long (1939-1945) and large and complex battle involving multiple technologies and tactics on both sides. These technologies (like centimetric radar, high-frequency direction-finding, and acoustic torpedoes) and tactics (like British hunter-killer groups and German wolfpacks) kept evolving during the war; intelligence and code-breaking also played a huge part, as did the weather.
A second point to keep in mind is the following one: the Battle of the Atlantic involved German U-boats (and Italian ones if we include the Mediterranean theatre) and British / Canadian / American naval surface forces. Its course does not reflect the capabilities of subs from other countries and other theatres, nor the ASW capabilities of destroyers from other countries and other theatres, so it would be misleading to automatically apply to all other countries a house rule that reflects the situation that existed specifically between British destroyers and German submarines. You might consider, instead, some sort of selective application. The Pacific theatre equivalent to the Battle of the Atlantic was the US submarine campaign against Japanese convoy routes. The rough parallel in this case is that, initially, US subs were hampered by faulty torpedoes. Once the US fixed that problem, however, Japan steadily lost shipping capacity to US subs because the Japanese gave inadequate attention to convoy protection throughout the war.