Some final thoughts.
As noted by many, the He 111 was borderline pointless - I never used a single spawn, unlike the Catalina, which I used if my DD got sunk. I get that the arctic convoys were attacked by German torpedo bombers, so the inclusion of the 111 was a fair call. In retrospect - and given the power of the PBY in this event - maybe the battles over the Bay of Biscay could have provided better inspiration: the Germans employed Ju 88C heavy fighters to counter allied Liberators, Sunderlands, Wellingtons which were hunting U-boats in transit to/from the mid-Atlantic. (Eventually the RAF deployed Beaufighters and Mosquitoes to counter the Ju 88s taking a toll on the ASW aircraft. See Bloody Biscay by Chris Goss). Given the effectiveness of the PBY in this event a Ju 88C heavy-fighter counter might have been more useful than the He 111.
Damage models.
Within 500m or so I found DDs almost invariable sustained ‘unrepairable damage’ from surfaced U-boat 37mm guns - at least I hope it wasn’t the 20mm guns… C’mon, 6”, 8” cruisers hardly ever do that! In return, 5” salvoes hitting surfaced U-boats rarely caused the ‘unrepairable damage’ that really should be standard in those cases.
Once submerged that ‘tankiness’ disappeared. Maybe that’s why so many event U-boats did what no RL U-boat would choose to do, by surfacing and going head-to-head with a DD? It’s hard to tell from 3PV, but I reckon lethal DCs can be at least a submarine length away (67m for a Type VIIC); I’ve been killed by a bomb the moment it hit the water and I was at 30-something metres, heading deeper.
Paper from 1935, Lt Cdr Doughty USN, re WW1 experience.
Admiral Jellicoe concluded post-war that a DC needed to explode within 14’ (4.5m) to instantly kill a sub; within 28’ (9m) to disable; and within 60’ (19m) to ‘demoralise the crew.’ At least 1000 DCs were dropped for every U-boat kill.
NavWeaps also has a lot of details, eg, the magnetic pistol on the US Mk 8 DC was set to detonate between 20’ and 25’ from a U-boat (6 or 7 metres). That contained a 270 lb charge.
Wikipedia’s Depth Charge page is pretty much in line with these figures. The UK Mk VII (290 lb amatol) was estimated to be lethal at 20’ (6m) and would force a U-boat to the surface at twice that; in 1942 when the amatol was replaced by torpex the distances increased to 8m and 16m. The USN Mk 4 & 7 needed to be within 15’ (4.6m) to rupture the pressure hull (kill). “Most U-boats sunk by depth charges were destroyed by damage accumulated from an extended barrage rather than by a single charge, and many survived hundreds of depth charges over a period of many hours, such as U-427, which survived 678 depth charges in April 1945.”
If we’re generous with the distances quoted above we could say DCs could insta-kill within 10 metres (pretty much the height of the Type VII). However, I feel they’re way more deadly than that. Aside from sinkings at PD or slightly deeper, where I’m sure DCs exploded within 10m, I don’t think I was ever sunk by a DC that was as close as the height of the sub. Insta-death was standard, with less serious damage being rare and often almost pointless as it could be fixed in a couple seconds. Death was almost always instant and not the result of accumulated damage.
DDs didn’t need the enhanced DC lethality given how much hand-holding they got in every other way. The skill for the DDs was knowing torpedoes come out the front of the subs and mines come out the back. Locating and tracking subs just happens for you. Just be careful about how you approach the subs and you can’t go far wrong, as most DC attacks will result in a kill.
Edit: and finally… BR3.7 for the typical WW2 sub? So facing ASW vessels like HMS Blackpool with Bidder passive homing torps and Limbo mortars capable of firing DC patterns from 400 to 1000 yds all around the ship, or Soviet sub-chasers with active/passive SET-40 torps. Might need a rethink on BR.