From their first sea trials, the three U boats demonstrated excellent handling and manoeuvrability, which, combined with ease of production and low cost, made the design particularly successful.
In 1939, realization of the inevitability of war and that the small size of the U boats made them particularly suitable for North Sea and Mediterranean operations, prompted the Admiralty to put the class into quantity production. Twelve identical vessels were ordered, but of these only four were eventually fitted with six bow tubes. The two external tubes, and the bulbous bow they formed, were removed from the remaining boats because the notable bow wave the bulge created when running at periscope depth made it difficult to keep the boat trimmed longitudinally.
