Further, none of the old What If threads discuss the RN's late-war and Cold War exercises/research re fast submarines, as discussed by Malcolm Llewellyn-Davis (retired RN, MA/PhD of War Studies, King's College London) in works such as "The Royal Navy and Anti-Submarine Warfare, 1917–49." (hereinafter "RNASW49"). https://books.google.com/books/about/Th ... Nw1H3CJ1wC
This book explodes some AHF conventional wisdom that T21 could have been effectively countered in WW2:
Given T21's top submerged speed of 17.2 knots, WW2 escort groups likely had "practically zero" killing rate against it.
Of course technology continued to advance:
Nonetheless, asdic usable at higher speeds was only experimental in 1946 and asdic capable of faster search rates envisioned only for 1953.The exercises in 1948–49 with Madden’s 6DF had confirmed that anti-submarine
ships had, as anticipated, a limited capability against a fast submarine. Tactical
procedures were needed, therefore, to take advantage of every fleeting detection.
As for the ships’ systems, while the existing Type 144 Asdic and Squid combination
was reasonably efficient against submarines whose speed was less than 12 knots,
against faster submarines it would only achieve a kill in very favourable
circumstances.85 The main limitations of the system were, firstly, the speed at
which ships could operate the asdic effectively was too low, secondly (and directly
related to the slow ship’s speed), the rate of asdic search was too slow and, finally,
the accuracy of the fire-control solution was constrained by the limitations of the
asdic and the existing ahead-throwing weapons. The ship’s asdic operating speed
was critically dependent on the level of underwater self-noise, which in turn, was
dominated by the interference from the ship’s propellers, the design of the asdic
dome, and the ship’s motion in a heavy seaway. Research was underway into
improved propeller designs as well as ways of silencing existing propellers by
surrounding them with an artificial shield of bubbles. A new experimental asdic
dome had already been fitted to HMS Scorpion during 1946, which allowed asdic
operation at speeds of 25–28 knots, that is, seven to 10 knots higher than was
previously possible.86 Improving the asdic search rate was partly dependent on the
ship’s speed and partly on the scanning rate of the asdic itself. This had been
appreciated for some time and work was in hand to develop an all-round scanning
asdic, but this would not become available for many years due to the technical
complexity of the equipment, though a set capable of scanning sector-by-sector
was expected to be available by 1953.87
Note also that ASW technological development had continued after Germany's defeat, owing primarily to the Soviet threat and an expectation that she would capture German submarine technology:
RNASW49 at 147.There was not, however, a sudden schism between the wartime German threat and the
new Cold War Russian menace, at least at the political level, and this helps to
explain why the Royal Navy was developing its future anti-submarine doctrine
against a ‘generic’ threat.
So can we estimate some range of projections for T21's impact on shipping losses and sub-shipping exchange ratios in the BofA?
Llewellyn summarizes a late-war internal analysis summarizing the then-new submarine threat:
So ~1/9th of Type VII/IX U-Boat contacts resulted in a torpedo hit, whereas the new boats would expect (1) rarely to be stopped from converting contact into attack and (2) to have a higher rate of success with attacks. If only half of T21 contacts scored a hit, that would be a 4.5x increase in kill rate per contact. That T21 could operate submerged at around the same speed as T7/9 on the surface so analyzing their contact/attack dynamics similarly seems broadly appropriate. T21 can't maintain high speed for as long, but it needs far less time engaged to launch more torpedoes.McCrea pointed out that during the period of surface operations by U-boats, roughly
one third of contacts led to an attack, and about a third of these attacks resulted in
a torpedo hit. Most of the failed approaches were due to interceptions by the convoy
escorts. Some of the failures were due to the navigational problem of achieving a
firing position, given the relatively limited engagement envelope of the torpedoes
then in use. For the future, the situation would be different because a fast deep
U-boat would be very difficult to intercept, unless there were revolutionary developments in escorts. Moreover, with pattern-running and homing torpedoes there was
‘… practically no problem of reaching a firing position other than merely getting
within range.’ In addition these torpedoes could be fired without use of the periscope
and each salvo fired was expected to claim a greater number of casualties. In the
future torpedoes were likely, McCrea thought, to have a much longer range, so
that, together with improved underwater performance, future U-boats were not
expected to have much difficulty in converting contacts into attacks
So that's the contact-dependent side of the attrition equation. What about the Uboat losses side? RNASW49, as cited above, saw "practically zero" success rate for escorts against subs as fast as T21 even in 1949. But let's be super-generous to the RN and say that escorts have nearly half the kill rate per contact. Even so, the T21's shipping-sub exchange rate would be 10x higher than T7/9. So if T7/9's were sinking 50,000 tons per sub lost in '42, T21 would sink 500,000. (assuming all sinkings are by escorts. This wasn't true of course but, after the Schnorkel, aircraft sinkings declined dramatically in share of Uboat kills).
It's not all about the exchange rate except on an infinite time horizon. In terms of tonnage sunk per sub-day, the delta would be closer to that calculated for the per-contact kill rate. Still, over the course of a multi-year BoA, strategic analysis converges on the shipping-sub exchange ratio.
In addition, we'd have to estimate the number of contacts per sub in an "earlier T21" scenario.
We could - and hopefully will - do a more detailed analysis of the scenario. For now, it's sufficient to say that an earlier T21 would have been extremely disruptive to Allied war plans.
One will sometimes hear on AHF that T21 could not have come earlier due to its advanced Sonar. That's not true. The T21's "Balkon GHG" was deployed on older subs in January 1943. https://www.cdvandt.org/GHG1996.pdf
For pre-'43 periods, we have to understand Balkon's utility: it allowed a sub to use its sonar when running near the surface (.e.g. when schnorkelling). While this is a useful feature for making contacts, it's not essential to T21's survivability against escort attack, which is primarily based on its speed.
Do I think T21 could have won the war alone? No. At least not unless Germany starts with dozens of them and forces Britain to peace before invading the SU, which seems far-fetched. The Allies could have built gobs of 25-knot merchant ships that would largely evade contact with T21's, for example (though many of these would still be lost). That would have been enormously expensive and, depending when T21 appears, many OTL operations would have been impossible until a massive high-speed merchant fleet became available. So the war's course changes probably but not its outcome unless T21's earlier entry is combined with some narrative that changes other aspects of the war.
A more realistic outcome-changing narrative, IMO, would be something like my "better Barbarossa" ATL's in which Germany plans for strategic defense in the air/sea until after beating all European powers. In that case, earlier/higher focus on U-boat warfare means earlier deployment of T21 in, say, 1943.