OK, let's suppose the United States decides to fight the IJN with submarines and only submarines. How long does it take to win the war?
Changing nothing else, or allowing English & Nimitz to be a bit smarter on dispositions & priorities? The first is OTL. The second, there are a number of additional PODs. One, basing only in Hawaii. Two, concentrating on the best patrol areas. Three, fixing the Mark 6. Four, getting the
maru code broken sooner. Fifth, changing the priority on tankers.
The first alone adds about 25% in sinkings (more boats on station, but bad dispositions). The second roughly triples the Hawaii boats' sinking rate, so maybe 60% overall (fewer boats, but much greater success
per boat). The third alone, add about 20%. The fourth alone, over 70%. Changing the tanker priority hurts a lot, but I can't find a number in Blair that would nail it down. All of them? Japan is deeply screwed...
So, in terms of tonnage? (Relying on the JANAC numbers in Blair, & being a tiny bit generous rounding up...)
- Up from 725000 to 910000 in 1942, from 1.5 mil to 2 mil in 1943, 2.7 mil to 3.5 mil in 1944
- Up from 725000 to 1.2 mil in 1942, from 1.5 mil to 2.4 mil in 1943, 2.7 mil to 4.25 mil in 1944
- Up from 725000 to 870000 in 1942, from 1.5 mil to 2 mil in 1943, 2.7 mil to 3.5 mil in 1944
- Up from 725000 to 1.3 mil in 1942, from 1.5 mil to 2.75 mil in 1943, 2.7 mil to 5 mil in 1944
If we accept the impact on Japan's war-fighting ability directly correlates with imports, & accept Japan's economy was in crisis by Jan '45 (IMO, it was), you can do the math for yourself... The first takes six or seven months off; the second, about a year; the third, seven months; the fourth, about two years. All five? The war's over before it starts...


Okay, I know, that's a speculative exercise... How else can you quantify the impact? IDK. That also leaves out the influence of improved aggressiveness as lessons are shared more readily, & thanks to changes in Japanese ASW air coverage (compared to SWPA); of tech improvements, like radar, being fitted more rapidly; of the ability to do more patrols
per boat, thanks to decreased transit time (compared to SWPA, tho the distance from Pearl, or even Midway, may make that a wash); & of changes in command leading to unproductive skippers being replaced more readily.
All that said, can you take a year or more off the end of the war this way? Yes, fairly easily.