The S138 series began near 684 tons fully loaded and eventually reached roughly 824 tons. Its first boats were already 140 tons heavier than the 544-ton S90. Batches built between 1906 and 1911 changed machinery and armament during a period of rapid naval development. Later boats carried four 500 mm torpedo tubes in place of the early three 450 mm tubes. One class name therefore covered boats with different performance and fighting capability.
Chapter · 35:35-44:34
The S138 label concealed a design revolution
Successive S138 batches grew, adopted turbines and oil firing, and changed armament so extensively that one class name obscured several distinct designs.