The primary mission of the US Pacific Submarine Force during World War II was to eliminate Japanese merchant shipping.Large warships such as carriers or battleships if encountered were of course a target you did not let go but as said, merchant traffic(includes tankers) were the main focus.Tankers became a priority because without the oil and fuel, the warships could not fight without fuel.I read that the Yamato and Musashi stayed put for most of the war because of their bad fuel economy and many other big warships did because fuel was scarce.
So say you run into a convoy with 3 large merchants and one mid sized or even small tanker, tanker is your main target.Now you can attack more than one target at a time, esp with bow tubes, say two fish to the small tanker and 4 to a big merchant.Obviously if you run into a TF with a carrier or BB, carrier is your priority followed by battleship and on down.Also obvious is if the carrier is a long shot and battleship is closer, go for the BB.
The Japanese had a completely different doctrine, they believed in svaving torpedos for warships and only attacked merchants when they happened to stumble upon them.They did not deploy specifically to interdict merchant/tanker traffic.Thus the US was able to run supplies, troops and oil all over without a real concentrated submarine threat from the IJN.May have had a U boat in the Atlantic type situation early in the war had they done this and who knows given the weakness of the pacific fleet early in the war if could have overcame the I boats as we did U boats in the Atlantic.
So to sum it up once more...
tankers
merchants
large warships...CV, BB, CA
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