This is part of an excerpt I wrote to a friend in this matter:
The problem with an idea that type XXI would be feasable in 1943 is multifold.
As a matter of fact, the XXI benefitted by it´s late arrival (beeing introduced to production by late 43 and early 44) in one or the other form from advances in ASW technology and operation conducted by the allies meanwhile.
This allowed a significant number of technological responses to be incorperated in the design. Let me list some of them here:
 The Snorkel.
It seems simple but with an earlier introduction date, the need to ship Snorkels sinmply wouldn´t be there. The Snorkel was about refitted to german U Boats in the same time as it was incorperated to the XXI design. Without Snorkel, the submarine would have to surface to recharge it´s batteries.
The Snorkel greatly reduced the exposure to radar detection, it´s principal reason of existence. If an XXI would be avaiable in 1943 it means introduction in service 1941 latest, and there isn´t much Radar in play yet (particularely airborne search radar)
 The high degree of silentness.
The XXI was designed for 18kts submerged speed. The actual design as executed was somehow slower, beeing 16.25kts fast at the 1 hour discharge rate. This had three reasons: (a) a reduced high current discharge capacity of the choosen batteries, the choice of the number of flooding slits, and (c) a change in the prop design, which wasn´t as efficient at high rpm but allowed delayed cavitation at lower rpm (close to surface no cavitation up to 122rpm -which is roughly 6kts). This is a rather critical tradeoff as it allowed to operate more silently in all conditions. The RN rebuildt one S-class submarine for high underwater speed trials in 1944 and the listening trials suggested to the RN that an effective counter to high submerged speed submarines may be the use of hydrophone listening due to the large amounts of noise generated by HMS SERAPH at high underwater speed. SERAPH was equipped with special high rpm efficient props to allow for the high underwaterspeed which weren´t quited. It was suggested that XXI beeing substantially larger than SERAPH would be even noisier so that passive sound detection should in principle work and correspondingly, much of the wartime counter procedures were incorrectly centered on a wrongly anticipated noisyness of this type. This may have been the case had a faster XXI with 17-18kts at the 1 hour discharge rate appeared in 1943.
Historically, and as executed, type XXI submarines were drastically more silent both, at low and high underwaterspeed, rendering this "response" entirely ineffective.
When U-2513 was trialed in 1946, it was found to be as noisy at 15kts as a BALAO-class fleet submarine was at 8kts. In top of that, while running on creep engines (6kts), it couldn´t be detected at all with the period USN sound listening gears. This silentness is what frustrated so many approaches to the problem formulated by the advent of XXI-type submarine but it´s not clear that the very high degree of silentness would also have been present in an early XXI, particularely because it also evolved out of the need from experiences os U boats facing ever increasing levels of allied ASW.
Attached is a graph from Urich, R. Principles of Underwater Sound, p.338.
It´s not from wartime data but from 1946 trial data on U 2513, but so be it. As can be determined, at 100m operational depth, the boat may run 10kts at the same relative noise level as 6 kts at the surface, which would be close to undetectable. The noise optimised prop was a great deal in this types capabilities.
 Radar and radar detectors
Historically, XXI were fitted with a complete set of radar and radar detectors which weren´t yet avaiable in 1943.
 active/passive horizontal lot for FC
While the passive sound detection GHG array was -more or less- standart equipment in all U boats, active S devices were, unlike in USN boats uncommon prior to XXI. The horizontal active device Nibelung is broadly comparable with the attachment "SWORD" in 1945 RN ASDIC, but having up to 8 times the effective range of it. It was precise enough to attain firecontroll solutions against surface or submarine vessels with three pings, which could be detected but not measured due to the choice of amplitude modulation with 1945 equipment. This very powerful active set would not be avaiable in 1943.
HF/DF was effectively countered by using Kurier type burst encrypts with sending times in the ms range. It would not be possible to HFDF these short messages with the period technology. Kurier was introduced in march 1945 and wouldn´t be avaiable in 1943.
On top of that, what is an XXI even in 1943 supposed to do against russian tanks in the East? Removing Hitler from command of the Army would be more effective than any XXI ever could hope to be in that regard.
Additionally, I remain sceptical that the XXI is a good submarine platform for the confined waters of the Channel. The XXIII is probably a more reasonable platform, less technoclogically challanging (no S-device, no radar and radar detectors) and by the virtue of it´s tiny size stealthy enough.
addendum: Contrary to what You may read in many secondary sources, the He-162 didn´t had poor range or endurance by design. It´s endurance was quite good compared to other contemporaries on internal fuel (same as P80, slightly worse or better than Meteor III (depending on time and cleared capacity) and considerably better than P59, MiG-9, Yak-15 and Vampire F1). However, operational shortage of fuel meant that it was never flown with full capacity fuel load and it´s mission profile called JG1 to execute low level sweeps where fuel consumption is worst.
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