Well, DDs appear at a rate that is maybe an order of magnitude higher than RL.
Really, to apply real world sinking rates for subs to the game, you first need to at least have the mission layers in the right ballpark as RL. The stock layers are so absurdly loaded with warships it's no wonder we get sunk more than we should.
So I'd say that if you are basing the effectiveness on the chance per patrol of being sunk by a DD, you need to weight things if you are using the standard layers. Weight the sinkings to the number of unique DDs you have seen on each patrol. A single SH4 patrol probably sees more DDs than a real sub saw in the entire war. That or weight the sub sinking rate to tonnage sunk. Right now my layers are gutted of escorts. None of this early 1942 crap with 8 ships escorted by 4 DDs as the convoy norm. Smaller convoys (now with mcoca's tool, they even zig-zag) early war (and fewer of those by a long shot), and they likely have a single DD escort at more, with Minesweepers or Subchasers as the other escorts (with a %, so escorts are not always there). I also axed the contact report generation, so the only way I see stuff 95% of the time is to spot it myself.
I did a patrol last night, Salmon Class, start of war out of Manila. Patrol area was west, not the invasion zone. I saw nothing for over a week. I stayed in the general western approaches instead of going where I knew I'd find targets. I finally got a contact report after a few days, but it was way too far away (I increased the contact report range to closer to default since I have now reduced the reports at the source). The next reports were spread out, but in a week I got maybe 3. Finallyu got a close one for a task force, and I was not well-placed to end around it. Made a best intercept before it got away (my task forces now all move at 15-17 knots). Sank a CA, damaged a CVE in heavy seas. Had to turn and fire all 4 aft tubes to damage the CVE. Interesting that Taiyo doesn't sink with 1 torpedo like the CVs. I then went to Surabaya without seeing another ship en route. This is my new machine, and I haven't put the hardcore torpedo failures in yet. If those were in I'd have come home empty handed I bet.
So I have found that if I act aggressively but realistically I needn't worry too much about getting killed. When I get bored and do stupid things that no one would likely have done, THEN I get killed. In the above example, I reloaded a save game from the time I got the contact report, and dashed in at flank on the surface since the seas were rough as a test. I sank a CA, a CVE, and 4 DDs (all DDs with deck gun), damaged another CL. I was riddled with holes from DD guns, even one of the CLs threw shots at me and hit. Eventually, one hit destroyed my engines. Oddly, a little after that I got a death screen even though I was in the process of sinking 2 more DDs with the deck gun and the boat was afloat without being hit after the engine killer hit. Perhaps you never get the screen shake of the hit that kills you?
So making super subs because we have way way way too many DDs around can result is the junk we see now, like me not being afraid in the least of getting in gun duels with multiple DDs on the surface. I'd rather tone down the layers, then see what happens.
The only way to keep the experience realistic in that sense is to CHOOSE to play as if the DDs were actually dangerous IMO. So I don't use the deck gun, I submerge and go deep if spotted (since I know at PD if I get rammed I'll sink the rammer), etc.
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