120 Squadron RAF was setup in July 41 - but the close blokade of the uk had already been defeated by Coastal Commands other shorter ranged aircraft - it was only the 'Black Gap' a fairly large area in mid Atlantic where land based MPAs could not cover where Uboats could operate with impunity and so that is where the battle moved - however 120 Squadon with a handful of VLR Liberators could cover much of this area but this remained the only unit thus equipped operating in the Black Gap until early 43. US Units thus equipped where carrying out ASW patrols everywhere the Uboats were not!
It was only when increasing numbers of U-Boats bagan to surge this area and losses increased in late 42/early 43 that more VLR Liberators where prised away from bomber command in March 1943 and those USAAF squadrons where assigned at the same time, that any further VLR Liberators where made available.
A mere 50 additional VLR Liberators by May 43 helped turn the tide (along with an alignment of additional escorts, the RCN and USN escort groups coming of age, improved tech and intellegence methods).
So an earlier upsurge of Uboat numbers and effectiveness is going to elicit an earlier and more robust response from the allies because they cannot lose the BotA and throwing aircraft at the problem is a relatively easy and effective response for them to make at the expense of a small % drop in heavy bombers reaching the Strategic Airforces.
Also snorkles while allowing a submarine to remain submerged while using the deisels are not the be all and end all to that particular problem
Firstly in anything but calm weather the snorkle can become swamped and when this happened the engines sucked air from the submarine - a highly unpleasent ear popping experiance for the crew
Secondly while submariners are tough and all that - travelling at periscope depth at speed for any period would be exhausting for the crew as the surface wave effect would be moving the boat all over the place in anything but calm conditions.
Thirdly unless designed as a underwater sub its hull form would be less efficient than motoring on the surface and so use up its fuel and speed was limited to about 6 knots anyway because anything faster would deform and damge the snorkle.
Lastly - you cannot see anything - visabile horizon from a perisope is 2-3 miles - although depending on speed of the boat and conditions hydrophones could probably hear further than that?
A surfaced boat would be able to travel faster and see further than a submerged one using a snorkle.
The snorkle only makes sense as an 'improvement' in improving a submarines ability to traverse an area where enemy ASW airpower is strong - so until that happens why bother?
I realise that any answer to the challenge must be realistic and that your scepticism is probably founded on this, but I feel you are starting to argue against the thread rather than the realism of the posts. The challenge is for the "best possible", not the winning performance of the KM in the BotA. Having said that you could argue that the best possible is none. Although you would have to do that, rather than just shoot down other posters.
Germany had gone to war in September 1939 with 57 U-boats, but only 26 of them were long-distance ocean going boats. That OTL circumstance will not successfully blockade the British Isles or reduce merchant tonnage to the point that blockade is moot. The test here, I feel, is to get closer to that point. More U-boats both for training and ocean going blockade bring up that tonnage, if they can engage successfully. "Happy Time"s reflect periods of the U-boat war where the KM were engaging with some degree of success. So an increase in U-boats available, of the correct sort, at those times, might be thought of as increasing performance.
Had Germany gone to war with 200+ ocean going boats, with the manufacturing that implies, and the training fleet and air forces to support a campaign on that scale, it is far more likely that they would have been successful. Although in WW1 far more ships were hit without capitulation, so look for a huge
early improvement before a tipping point is reached.
Many of the counter-measures wouldn't have time to enact before a tipping point in the fighting capability of the British Isles would be reached. Anticipating some of these counter measures only puts back the deadline for reaching that tipping point. It seems to me that 1939-1941 with sustained losses of over 5,000 ships per year might do it. How many U-boats would that require, given counter-measures in place during the early war and proportional response to losses?
Any measures later than March 1941 start to fall foul of the counter-measures improving to a deadly degree.By 1945 the Allies are running out of targets.