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.
Under the 1940 and 1941 War Programmes, a further 41 U boats were ordered, but only 34 were completed. This second group of U Class submarines did not differ substantially from the first, but an approximate increase of 5 feet in the stern gave them a more streamlined shape aft and improved the flow of water over the propellers.
In addition to their four 21-inch stern tubes and three 0.303-inch portable machine-guns, the U boats were fitted with a 3-inch gun forward of the conning tower. However, as this was an afterthought to the original design, no separate hatch was fitted for the gun crew or ammunition. This resulted in the conning tower becoming extremely crowded before and after gun action and, if the gun crew was employed, rapid crash-diving was impossible.
The limited offensive potential of individual U boats was compensated for by the considerable number that were commissioned in a short period of time, and these small and nimble vessels became one of the most important operational classes in the Second World War, with a record that can fairly be described as heroic.
The wartime submarine fleet relied almost entirely on the S, T and U Classes. This policy was very different to that of the First World War (when some 12 new classes had been developed), and was pursued so as to cause the minimum interference with the shipbuilders' production programmes.
