This came up in SH4 in the context of modding the fish.
I was surprised to find that the number of torpedoes it took in RL was—contrary to anecdote—more with german torpedoes than American fish. The caveat being actual hits, when you include failures to detonate, prematures, etc, then the US fish suffer greatly.
The average number of hits to sink an allied ship was more than one fish. That means that many, but not all took 2. I checked all the Hog Islander attacks (and similar targets attacked by fleet boats in the PTO), and I checked all the Liberty Ship sinkings, along with ships of similar tonnage in the PTO.
The US fish were grossly more likely to sink the target with 1 fish, BTW.
For the PTO data I did not use US claims of tonnage, since they were so often wrong. I only used attacks where the specific, named ship was confirmed by japanese records (along with the actual tonnage). It's interesting to not that my records use claimed hits (the notes sometimes even correct these from japanese records). This means that likely I overestimated the number of hits to sink, since it's entirely possible that some claimed hits in a spread were prematures. This would mean that RL mk14s were even more effective.
When I checked for warship attacks, I found US and German fish t be effectively identical in terms of hits per sinking of a given tonnage.
From a post I made on the subject (Hog Islander attacks up to 1943):
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Actually, the hog island stats he posted earlier are telling. I checked (as I posted above), and 88% of the ~5k ships hit with 1 fish by US subs sank with just the 1 hit. When I picked ships in the US stats, I took them from 4800 to 5500 tons, too, so bigger ships, and STILL more effective.
I just broke the 32 hog island attacks on uboat.net into individual attacks by reading the accounts.
LOL.
50% of the Hog Islanders sank after 1 hit. 73% after 2 hits (I counted coup de grāce here in the 2 fish hits, including the couple that were finished off with 30 and 60 DG rounds respectively treated as a single added torp (the only alternative to the DG)). 83% of them hit with 3 sank (one not sinking out of 6 attacked).
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For Liberty ships:
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BTW, I just checked the Liberty ships. I got bored after doing all of 1944, but for every attack in 1944, 18.18% of attacks where 1 torp hit resulted in a sinking.
81.82% with 2 or more hits sank.
As a reality check, 79.17% of USN attacks on ~7k AKs with 1 fish resulted in a sinking. For the 7K data I used ships between ~6800 and 7800 tons (I had to flip through a book, and hand enter all the data, so I decided to err on larger rather than smaller shipping—I entered data for over 80 attacks). 83.64% hit with 2+ sank.
So, for super clarity:
Liberty (~7k t) vs U-Boats, 18.18% sunk after hitting with 1 torp (1944)
7k t jap vs Fleet Boats, 79.17% sunk after hitting with 1 torp. (entire war)
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(edit, that last "entire war" should read " '44 to end of war," my bad).
tater