They incorporated many batteries to increase the time they could spend submerged, to as much as several days, and they only needed to surface to periscope depth for recharging via a snorkel.

However, the design was also flawed in many ways, with the submarines being mechanically unreliable[5] and, according to post-war analysis, vulnerable to combat damage.

The Type XXI submarines were also rushed into production before design work was complete, and the inexperienced facilities which constructed the boats were unable to meet necessary quality standards.

These include the Soviet Whiskey, American Tang, British Porpoise, and Swedish Hajen III classes, all based on the Type XXI design to some extent.

[8][9] The Type XXI was also much quieter than the VIIC, making it more difficult to detect when submerged, and the design eliminated protruding components that had created drag in earlier models.

[10] The ability to outrun many surface ships while submerged, combined with improved dive times (also a product of the new hull form), made the Type XXI much more difficult to pursue and destroy.

The Type XXIs also had better facilities than previous U-boat classes, with much roomier crew berths, and a freezer to prevent food spoilage.

[15][16] A post-war assessment of the Type XXI by the United States Navy concluded that while the design had some admirable features, it was seriously flawed.

[17] The submarines' engines were underpowered, which limited the surface speed and increased the time required to charge the batteries.

[11] One of the reasons for these shortcomings was that sections were made by companies having little experience with shipbuilding, after a decision by Albert Speer.

[17] It was planned that final assembly of Type XXI boats would eventually be carried out in the Valentin submarine pens, a massive, bomb–hardened concrete bunker built at the small port of Farge, near Bremen.

[22] Due to the combination of design and construction problems, historian Clay Blair judged that "the XXI could not have made a big difference in the Battle of the Atlantic.

[17] The FuMB Ant 3 Bali radar detector and antenna was located on top of the snorkel head.

[23] During 1957, U-2540, which had been scuttled at the end of the war, was raised and refitted as research vessel Wilhelm Bauer of the Bundesmarine.

However, Western intelligence believed the Soviets had acquired several more Type XXI boats; a review by the U.S. Joint Intelligence Committee for the Joint Chiefs of Staff during January 1948 estimated the Soviet Navy then had 15 Type XXIs operational, could complete construction of 6 more within 2 months, and could build another 39 within a year and a half from prefabricated sections, since several factories producing Type XXI components and the assembly yard at Danzig had been captured by the Soviets at the end of World War II.

The bunker has since been filled in with gravel, although even that did not initially deter many souvenir hunters who measured the position of open hatches and dug down to them to allow the removal of artifacts.