The reich designed and built the Type XXI U-boat in response to the heavy losses of May 1943. "Black May" was a wake up call; a new sub was needed to function under conditions of allied air superiority--to the mid Atlantic--and better overall ASW assets.
But the Type XXI program turned out to be worse than futile. Consuming vast resources, it availed nothing since the war was over just as the XXIs were becoming operational.
But what if the setback of March 1941, in which the ubootwaffe lost its three best skippers, was taken as the wake up call to build a better boat? What if the Type XXI became operational in large numbers beginning in May 1943 (note in the OTL it also took about two years after the decision to build/mass produce for craft to be operational in significant numbers)? What might a Type XXI fleet have achieved from May '43 onward?