Successive S138 batches moved from three 450 mm torpedo tubes to four 500 mm tubes. The progression included a brief 460 mm stage. Both calibre and tube count increased, producing a substantial rise in striking power. That growth reflected the increasing size of the ships the torpedo boats were expected to stop.
Notable Point · 47:46-48:19
S138 torpedo growth followed larger targets
The S138 evolution increased torpedo calibre and tube count as the intended targets grew larger.