Finnish submarine Vesihiisi Vesihiisi was a Finnish 500 - tonne Vetehinen class submarine that was constructed in the early 1930s . The vessel served in the Finnish Navy during the second World War . German Design " Iku - Turso " , " Vesihiisi " , " Vetehinen " . While preparing the design of the " Saukko " , the Germans also prepared a design for a seagoing submarine for the Finnish Navy . Three submarines were built to this design , and like the " Saukko " , they were fitted for mine - laying , the mines being supplied by the Germans . Being designed for use against Russian bases ( never very far from the Finnish bases ) , radius of action was not of prime importance to this design , and only 20 tons of fuel oil were carried ( as opposed to the 67 tons carried by the German Type VIIa based on this design . Combat operations . Winter War . " Vesihiisi " was alerted with " Vesikko " to Hanko region on 30 November 1939 as several Soviet surface combatants were headed towards the area . Submarine however failed to arrive to the area in time to intercept the Soviet cruiser " Kirov " and its escorts . " Vesihiisi " run aground in early December 1941 and had to be docked for a few days for repairs . On December 27 " Vesihiisi " laid 16 mines off the Soviet naval station of Paldiski but soon after the ice forced the submarine to stay in port . Continuation War . " Vesihiisi " started the Continuation War on June 22 by laying 20 mines in Estonian waters ( controlled at the time by the Soviet Union ) . She laid 18 more mines on June 24 at Ruuskeri SSW from Gogland and further 18 mines on June 26 SE from Bolshoy Tyuters . After this the mining operation was postponed and " Vesihiisi " laid the 18 already loaded mines finally on August 2 east of Osmussaar . On July 2 , 1941 " Vesihiisi " while patrolling east of Gogland encountered an escorted freighter heading east . Torpedo attack failed and the submarine suffered light damage from the escort ' s depth charges . On August 5 , 1941 " Vesihiisi " attacked a convoy transporting supplies to besieged Soviet garrison at Hanko . Convoy consisted of a transport vessel of " Molotov " class ( Iosif Stalin class passenger ship ) escorted by pair of large minesweepers and group of patrol boats . Submarine penetrated the escort screen and launched two torpedoes at range of 700 m while being between the target and the escorts . Neither of the torpedoes exploded and the escorts forced " Vesihiisi " to dive to the depth of 75 m . Though submarine did not suffer severe damage from the depth charges the repairs on the dock lasted for a week . In December 1941 after the Soviets had evacuated Hanko the Finnish submarines were docked for the winter . During the sailing season of 1941 the Italian torpedoes the " 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 . Submarine was further modified by streamlining the tower and moving the 20mm gun up to the tower . On 9 August 1942 , the " Vesihiisi " was deployed along with her two sister ships to Mariehamn . Their mission was to conduct anti - submarine and escort operations in the Sea of Åland . In the evening of 21 October 1942 she torpedoed and sank the Soviet S class submarine " S - 7 " , near Lågskär in the Sea of Åland . The captain ( Sergei Lisin ) and four of the crew of the " S - 7 " were then captured . On 4 July 1944 " Vesihiisi " laid 20 mines between Moshchny Island ( Lavansaari ) and Seskar ( Seiskari ) . Boat was attacked with depth charges by two Soviet minesweepers but was able to escape without any damage . Submarine laid further 18 mines north of Moshchny Island on 6 July 1944 .