Advanced O 21 boats and Dutch experiments with Allied types

The O 21 class belongs in the debate over the world's most advanced submarines in 1939, combining eight torpedo tubes with substantial gun armament and effective Dutch crews. Whether it outranked contemporary foreign designs depends on which measures of advancement receive the most weight. O 21 attacked an Italian tanker and Japanese merchant, army cargo, and transport shipping, so the design's reputation rests on combat results as well as specifications. Germany commissioned boats captured while incomplete, although this assessment holds that their service did not use the design to its full potential. In exile, the Dutch acquired one British U, S, and T boat before favoring the T class, while the former HMS Sturgeon survived through cooperative Dutch service and training roles.