Leaving aside the flaws in design (hydraulic components outside the pressure hull and so impossible to repair submerged) and workmanship (hull sections from different builders not fitting, poor quality welding etc.) cited by Blair in Hitler's U-Boat War, Type XXI fans tend to be so enamoured by the technology they give no thought to the difficulties involved in their effective operation.
Diesel-electric boats are still submersibles rather than true submarines. The Type XXI sprint speed was certainly impressive (post-war tests indicated more 17-18 knots rather than 20 but SH3 is a U-Boat sim after all) but its sustained maximum cruising speed was much lower. This made intercepting convoys on the high seas a huge problem.
How big? Try intercepting an 8-knot convoy on the surface with a 12-knot Type II. That small 4-knot speed advantage means that in many cases you will be unable to achieve an effective firing position. Without the sustained high-speed transit capabilities of a nuclear boat, convoy operations become dependant on the luck of the draw and favorable geometry. Type XXI's only had a couple of hours at 12-knots and the vast majority of interceptions would have been conducted by passive sonar without the benefit of signal processors, waterfall displays and high-speed computers. The problems were technologically unsolvable in 1944-45.
Even at the end of the war the problem was less attacking the convoys than it was finding them. Patrol lines of Type XXI's could not talk to each other or BdU without exposing their masts and transmitting on the HF bands. Keeping a plot of HFDF intercepts would allow convoys to be routed around lines of Type XXI's as they were routed around the VII's and IX's during the convoy battles of 1941-43. The big difference being that the transit speed of the XXI's was much less and so concentration against any convoy would be more difficult. To be sure a single XXI could make a successful slashing attack on a convoy but its chances of a re-attack on the same convoy would be minimal in most cases.
If you want to see where Type XXI's may have been effective, look at the Soviet submarine doctrine during the Cold War. The nuc boats operated on the high seas but the diesel boats were confined to strategic choke points and shallows where the targets could come to them. This solved much of the locating and interception problem but also put the boats where ASW assets were strongest. This is a good model to use though, since Soviet boats before the Kilo's were largely based on the Type XXI design. The Red Banner Fleet also had the priceless advantage of satellite intelligence and and secure SATCOM to minimize exposure of the boats at sea, operational aids unimaginable for the u-boot waffe.
It is perhaps significant that the patrol area for KK Schnee and U-2511's first combat patrol was off New York, where targets could come to him and no BdU control was required. Also if he could survive there and rack up any sinkings it boded well for lesser Type XXI captains functioning in an intense ASW environment. Initially the XXI's would have come as a nasty surprise but the Allies would have adapted and the results would have been exactly the same. One would be wrong to assume that everything would have gone the German's way and that the Allies just would have folded and collapsed in panic or not developed effective counter-measures.
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