Actually I thought one was used as a freighter.
Which was about par for the course for IJN subs. You really have to wonder what they were thinking? Here they were with this very large submarines force, with the largest boats in the world. With the best torpedo's in the world. With some of the best range/performance in the world. And yet they accomplished so little.
Yes they did sink some capitol ships. But what did that really mean against a USN alone that was operating 1,200 combat vessels? Against a Yank component alone that launched over 130 CVs and CVEs during the war, including over 30 huge super carriers? With their escorts. With their BBs,CGs,DDs,LSTs,LHDs,LSDs, oilers, supply ships. All with the latest in radar,sonar,fire control. And then with the huge Allies maritime air forces of both sea and air?
I'm glad the IJN didn't use their boats better but the history buff in me see's it as a shame. The Allies had long supplies routes that were vulnerable and the IJN had the weapon to interdict them. But not only in their heads did they only see the one great epochal battle scene that would either pave the way to victory or glorious death. I think too the power centers of their Navy and Military looked down on submarines they way they did tanks. If you died on Yamato or Musashi you did with honor. If you died on a submarine you just died. This was a pompous, feudal bunch running the show over there. Even in their Navy, which was the most modern of their armed forces.
Made even worse by the example of the German boats, or, the Allied ones which were slowly strangling their supply routes.
Of the 56 boats over 3,000 tons, in the world, in WW-ll, the IJN built 52 of them. Their boats had the longest range, the fastest underwater speeds, of any boat in operation. They were armed with a version of their famous Long Lance torpedo. Yet they only sank 184 merchant ships compared to Germany's 2,840. Americas 1,079. The RNs 493.
They started the war with 63 fleet boats and in the first 2 years, when the Allied navies were challenged, they surely could have done better then they did. The I-400 was a fascinating boat but in the end, like their super-battleships, they were simply a waste of steel.