The US Balao-class and Gato-class submarines had enough provisions to stay on patrol for 75 days under normal operating conditions. Information on other long-range submarines has proved hard to find, although this site mentions that British O, P and R class submarines had a range of 20,000 miles, but there is no mention of actual or possible duration at sea without being resupplied. What I am looking for here is the longest period submariners went without being able to set foot on (friendly) land.

The US navy had submarine tenders (supply ships, nod to justCal's comment) which could supply submarines at sea, which opens up the possibility of greatly increasing the aforementioned period of 75 days for some US subs. The Germans also had at least one submarine tender, the SS Saar, which was operational throughout the war. Thus, submarine crews could have remained at sea for extended periods during WWII without being able to set foot on land for some R & R.

What is the most number of days that a submarine remained at sea during WWII without going to a port to be resupplied (but which may have been resupplied at sea by supply ships)?

Also, were there any submarines of other navies (British, German, Japanese, Russian) which could stay at sea longer than this (75 days) without being resupplied? By 'could', I mean under relatively normal operating conditions (i.e. without the crew resorting to such extreme measures as drawing of straws / cannibalism, as suggested by Scott's comment).

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    Quick source concerning resupply without ports: Submarine Tender
    – justCal
    Commented Nov 13, 2017 at 14:27
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    @Bregalad Current-day US Sailors stay out on patrol for 3-6 months on some classes of submarines - submerged the entire time. No easy task, needless to say.
    – SnakeDoc
    Commented Nov 13, 2017 at 18:49
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    @SnakeDoc My god, I'd certainly be driven crazy if I was locked into a submarine more than a couple of days personally ! Also the Franck family were locked into a house for more than 1 year, and you can feel they become gradually crazier and crazier by reading the journal.
    – Bregalad
    Commented Nov 13, 2017 at 18:53
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    @Bregalad My understanding is the missile boats stay out for 3 months, and attack boats stay out for 6 (both stay submerged the entire time after leaving port) - Then the crew spends a few months back on shore once the patrol is over, doing training, etc. It's a rough life - submariners are considered some of the toughest sailors in the navy, and get some extras such as Hazard Duty Pay and better chow (food) for the conditions they're in.
    – SnakeDoc
    Commented Nov 13, 2017 at 18:58
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    Regarding resupply, the German navy had the Type XIV "Milchkuh (milk cow)" submarines that could extend the patrol range of their attack boats. en.wikipedia.org/wiki/German_Type_XIV_submarine Commented Nov 14, 2017 at 12:55

3 Answers 3


Not a "hard" answer, but more than a comment.

The German Type VIIC was the most common type of U-boat, but with limited range and endurance. Its fuel bunkers could keep the diesels going for 20-35 days non-stop at a speed of 10-12 knots (own calculation, using the various "range X at speed Y" data points given here and here).

The Type IXC/40 would, by the same token, have fuel for 57 days at 10 knots, non-stop.

We know from various sources (e.g. "Das Boot", Lothar-Günther Buchheim) that U-boats would "patrol" their designated areas at much slower speed, to conserve fuel. Stopping the boat for maintenance and repairs would further extend the solely fuel-based equation, while combat operations like closing in on a convoy (when a boat would run at high speed) would decrease range drastically.

Records for the longest war patrols were 97 days for a Type VIIC (U 552) and 225 days for a Type IXD2 (U 196). After that 225 day patrol, U-196 returned to Bordeaux on October 23rd, 1943. Apparently the crew was sane, or at least fit to serve, enough to leave, after 143 days in harbor, for another war patrol that lasted 147 days.

However, it is difficult to ascertain whether a boat was resupplied at sea on any given patrol. (U 196 was part of a group of similar boats operating in the Indian Ocean, with a tender operating as supply ship and mobile base, so it's fairly safe to assume they were resupplied in one way or another.)

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    @LarsBosteen WW2 submarines -- being diesel powered -- were "surface vessels that could occasionally dive". After all, the Type IX didn't carry a cannon mounted on deck for the jollies. en.wikipedia.org/wiki/10.5_cm_SK_C/32_naval_gun IOW, submariners would go out on deck, and could get fresh ait if the weather was good.
    – RonJohn
    Commented Nov 14, 2017 at 17:47
  • @RonJohn: Definitely not as in "lounging in the sun", as the boat would have to be ready to dive at all times. There is the watch on the tower, but there was preciously little time to be had on the deck itself, and I doubt that the mere sailors would get any.
    – DevSolar
    Commented Nov 14, 2017 at 17:58
  • @DevSolar I bet German sailors got to go out in shifts (mostly at night, but some during the day early in the war; not at all when the mid Atlantic "black hole" was finally filled) when the weather was good. Ditto Americans in the Pacific.
    – RonJohn
    Commented Nov 14, 2017 at 18:10
  • @RonJohn: I am not so sure about that, and asked it as a question.
    – DevSolar
    Commented Nov 14, 2017 at 18:48
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    Please revert to the previous form in that case! I added it as it seemed to be one of the first questions in the comments and the info improved on the data regarding that cruise in your answer.
    – gktscrk
    Commented Jun 17, 2020 at 10:15

Japan built several of the I-400 class submarines, two of which, I-400 and I-401, were operational in early 1945. These enormous submarines had a 37,500 nautical mile range, over three times the range of a Gato submarine, so one can presume that crew stores would be present to keep the crew functional for a voyage of several months. There seems little point in building that sort of range into a submarine if the crew can't last as long as the fuel.

However, by the time the I-400's were operational, the war situation had changed to where the planned attacks on the Panama canal, Los Angeles, Washington DC and NYC were no longer considered to be practical.

  • I just found out that the I-400 class you mention here could stay at sea for 120 days with a range of 1 to 1 1/2 times round the world (according to ww2pacific.com/i-400.html). Commented Nov 14, 2017 at 12:56
  • Ah, thanks Lars - I kept looking for an endurance figure, just didn't look far enough.
    – tj1000
    Commented Nov 14, 2017 at 14:56
  • You're welcome to edit your answer to include this info if you want. Commented Nov 14, 2017 at 14:59

The Japanese built the largest submarines in World War II (52 out of the 56 submarines) of over 3000 tons). These were also able to stay underwater the longest, over 100 days, and carried the best torpedoes. Some 41 subs carried aircraft. The reason for these innovations was that the Japanese were operating in the Pacific, the largest ocean in the world.

Despite these advantages, the Japanese submarine fleet had two problems. The first was that the Japanese built onl 174 of them. The second, and more important reason was that they were used to fight warships, not merchant ships, and also to resupply isolated Japanese garrisons, so 128 of the 174 were lost in combat. The survivors were mostly new or training ships.


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