A couple of unsolicited comments on snorkel techniques in GWX 3 follow.
There are a number of schools of thoughts regarding when to snorkel. Boats working the vicinity of the British Isles are always under threat and need to use extreme care when snorkelling but if you're careful, the main threat in the patrol area ceases to be aircraft but becomes surface ships and mines, the former probably summoned by aircraft. This is actually pretty accurate historically and may very well be something of a tribute to the GWX Team in getting the historical balance correct.
A snorkel patrol is essentially conducted in two phases, the transit to and from the patrol area and the patrol itself. There does not appear to be any hard and fast snorkel doctrine and Commanders seem to have been pretty much left on their own.
When transiting submerged you basically have two choices, snorkel consistently in the absence of any threats or proceeding fully submerged, snorkelling one or more times per day to recharge and air out the boat. Using the former you should be able to cover 150-200 km per 24-hours but running at 3-knots and snorkelling only twice per day reduces this to 70-90 km in 24-hours. With the distance from Bergen to the North Channel some 1400 km this means that once you decide to proceed submerged it may take a long time to reach station. Snorkel patrols were generally less than 50-days and a Type VII might have a ration endurance of perhaps 55-60 days. I plan 50-day patrols with a 10% (five-day) fudge factor.
Once on station, you can snorkel when required whenever it is safe to do so. Patrolling at 1-2 knots preserves your battery but airing the boat once or better yet twice a day is a good idea. Before you get your mast-mounted RWR in September or October, daytime provides the best time since you can see aircraft coming. If using H.Sie's hydrophone fix, you will not hear surface contacts so the historical technique of shutting down the diesels every now and then and listening is a good option. Otherwise your hydrophone operator will advise when a ship is detected and drop your TC to x1. Even with Map Contacts ON an aircraft will not drop your TC and you have to do so manually. Not sure how this works for those using the stealth meter.
I generally start thinking about battery charges when the box gets down to 90% but this is a captaincy decision and there's no "right" answer.
I spend most time when not snorkelling at about 25-metres, at this depth the crews can use the heads and Cookie can dump the gash down the BOLD tube.
Often you will detect contacts but the geometry will be wrong for an interception. My loadout generally consists of and Gnat or Falke, a G7E III Electric and two FaT's in Tubes I-IV and a Falk/Gnat in Tube V with an electric reload. Reloads are usually 2 x FaT electric, 2 x Fat air and 2 x G7E III forward and one aft. Launching a pair of Fat's into a convoy from outside the screen is sometimes effective and done with minimal risk. Save the air torpedoes for night operations, convoys may have air cover and I would swear that the planes in GWX can see the torpedo wake.
Boring in at speed makes noise and drains your battery so weigh the risks vs benefits. Having only hydrophones and occasional short looks through the periscope often means that you may lack the big picture and even with Map Contacts ON, hunting in 1944-45 is a challenge.
If you're careful and reasonably conservative in pursuing contacts your biggest fear should be mines, a threat that the GWX Team was not shy in deploying around British waters. If you use the external camera, you can see and avoid them. I don't and so have had several promising late-war careers terminated in a minefield.
What did the Real Skippers do?
Much of what data out there is contradictory which might indicate that it was left to the individual commanders, as noted above. One of the snorkel pioneers was KL Karl-Heinz Marbach of
U-953. In Harald Busch's
U-Boats at War he discusses his snorkelling experiences and tricks and I have pretty much adapted these in SH3 and they are explained above in game terms.
Herbert Warner in
Iron Coffins writes as if he invented many of the U-Boot Waffe's snorkel techniques but given that he inherited
U-953 from KL Marbach in August and probably the best snorkel-trained crew on the Atlantic at that time; like many aspects of Werner's book, this seems unlikely.
Probably the most successful snorkel boat commander was KL Hartmut Graf von Matuschska in
U-482. On his first patrol he sank four merchants and an escort for 32,621 GRT. All the merchants were in convoys when sunk, an incredible feat in August and September 1944.
U-482 was at sea for 42-days and spent some 37-days submerged. KL Matuschka was killed on his second patrol when
U-482 was sunk by aircraft in the Shetland-Faroes gap.
KL Graf von Matuschka
KL Marbach
Good Hunting