Due to its heavily-dependent electric design, the Type XXI series was also known as the 'Elektroboot' ('Electronic Boat').

This allowed the boats to produce more power submerged than the vessel could create when on the surface - a decided advantage when compared to the diesel-electric-powered counterparts of the day with their closed-cycle turbine engines. The Type XXI was designed specifically for lengthy submerged operation and this centered on high-powered electrical machinery with density cells built directly into the hull. The submarine became the first to operate faster under water than on it and opened the doors to a new chapter in submarine warfare and design - one that would set the roots of submarine work in the Cold War period (1947-1991). (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.The Type XXI series of U-boats developed by the German Navy during World War 2 (1939-1945) were revolutionary on many levels and were looked upon by the service as its war-winning design - intended to take back control of the vital Atlantic Theater. However, it is difficult to ascertain whether a boat was resupplied at sea on any given patrol. 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. After that 225 day patrol, U-196 returned to Bordeaux on October 23rd, 1943. Records for the longest war patrols were 97 days for a Type VIIC ( U 552) and 225 days for a Type IXD2 ( U 196). 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. 'Das Boot', Lothar-Günther Buchheim) that U-boats would 'patrol' their designated areas at much slower speed, to conserve fuel. The Type IXC/40 would, by the same token, have fuel for 57 days at 10 knots, non-stop. 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 German Type VIIC was the most common type of U-boat, but with limited range and endurance. Not a 'hard' answer, but more than a comment. without the crew resorting to such extreme measures as drawing of straws / cannibalism, as suggested by Scott's comment). 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)?Īlso, 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. 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. The Germans also had at least one submarine tender, the SS Saar, which was operational throughout the war. 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. What I am looking for here is the longest period submariners went without being able to set foot on (friendly) land. 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. The US Balao-class and Gato-class submarines had enough provisions to stay on patrol for 75 days under normal operating conditions.

World war 2 submarine drawing