 During the sailing season of 1941 the Italian torpedoes "Vesihiisi" used (Finnish designation T/40) proved to be unreliable.During 1942 "Vesihiisi" was upgraded with new 12-hydrophone listening arrays and equipped with depth charge rack capable of carrying 4 depth charges.The submarine was further modified by streamlining the tower and moving the 20 mm gun up to the tower.