On 5 August 1941 "Vesihiisi" attacked a convoy transporting supplies to besieged Soviet garrison at Hanko. According to Finnish sources, the convoy consisted of a transport vessel of "Molotov" class () escorted by pair of large minesweepers and group of patrol boats. According to Soviet sources, there were only four minor ships: the 489-ton freighter "Hilde", built in 1894, one T58-class minesweeper, and two MO-class patrol boats. The submarine penetrated the escort screen and launched two torpedoes at a range of while being between the target and the escorts. Neither of the torpedoes exploded and the escorts forced "Vesihiisi" to dive to the depth of . Though the submarine did not suffer severe damage from the depth charges the repairs on the dock lasted for a week.