The second group was ordered under the war supplementary emergency programme after the original boats proved useful. Of its twelve completed boats, nine were lost. The hull shapes developed through the groups with increasing attention to underwater performance. The first group had four internal and two external 21-inch bow tubes, carrying ten torpedoes with reloads. Groups two and three carried four internal tubes and four reloads, a total of eight torpedoes.
Chapter · 6:33:30-6:35:30
How later U-class groups refined the original design
Later U-class groups developed the hull shape and carried four internal bow tubes with four reloads, compared with the first boats' six tubes and ten torpedoes.