Vetehinen was a 500-tonne that was constructed in the early 1930s, which served in the Finnish Navy during the Second World War. The submarine was the first ship of its class of three submarines. The name vetehinen means "merman".
See also: Winter War and Naval warfare in the Winter War. The Finnish Navy was informed that the 8,800 ton Soviet icebreaker would be arriving in the Baltic Sea from the Atlantic. Vetehinen was sortied to intercept the icebreaker off the port of Liepāja on 4 December. However, the submarine ran aground before reaching open sea and had to be pulled loose by tugs before it was able to start its voyage. Vetehinen arrived at the site on 6 December and started patrolling off the buoy on the port entrance. On 7 December the submarine sighted Yermak but was unable to reach it. While on station, Vetehinen encountered a couple of unknown submarines which were suspected to be of German origin. On 8 December Vetehinen began the voyage home and was attacked by the Soviet, but the attack failed and the Finnish submarine crew did not even notice the attempt.
On 12 December Vetehinen sortied again to intercept Yermak, this time on its voyage from Liepāja to Tallinn. However once again before reaching the open sea Vetehinen ran aground and had to be inspected before continuing on its mission the same day. Upon reaching the area the submarine was unable to sight the icebreaker and was recalled on 14 December.
See also: Continuation War and Baltic Sea campaigns (1939–1945). On 22 June 1941 Vetehinen laid 20 mines north of Kunda, and a further 17 mines on 28 June between Suursaari and Tytärsaari.
On 3 July 1941 Vetehinen encountered a freighter west of Suursaari but due to distance was only able to damage it with its 76 mm gun.
On 3 August 1941 Vetehinen sighted a Soviet convoy consisting of pair of large minesweepers and a group of guard ships escorting a group of Soviet submarines. One torpedo was launched at a distance of 1500m (4,900feet) to no effect.
On 8 August 1941 Vetehinen launched two torpedoes at a 1500-ton freighter, but neither of the torpedoes exploded.
The Italian torpedoes (Finnish designation "T/40") proved to be unreliable in service despite functioning during test range firings. It was later discovered that these torpedoes were unreliable when fired from a submerged sub because the increased water pressure caused the seals of the torpedo detonators to fail. For this reason a report that the Finns received from the besieged Hanko that a Soviet freighter had arrived there with two Finnish torpedoes sticking out from its hull became more believable.
On 30 November Vetehinen sortied to intercept a convoy but was unable to reach it.
On 3 December 1941 Vetehinen sighted a Soviet convoy headed due east from Hanko and engaged it. The submarine attacked on the surface but the poor quality of the night sight and the activity of the escort prevented torpedoes from being fired with any accuracy. Vetehinen fired both bow tubes and after turning around both aft tubes at the convoy without any effect, at an estimated distance of 2500m (8,200feet). A mere 15 minutes later the submarine sighted another Soviet convoy, this time headed due West, but had no loaded tubes and was unable to engage it.
During 1942 Vetehinen was upgraded with new 12-hydrophone listening arrays and equipped with a depth charge rack capable of carrying four depth charges. The submarine was further modified by streamlining the tower and moving the 20 mm gun up to the tower.
On 5 November 1942, Vetehinen rammed the in the Sea of Åland and sank it. According to Vetehinen crew members, she was on a night patrol searching for Soviet submarines, which stayed underwater during daytime but usually came up during the night to recharge their batteries. A contact was found, and after confirmation of an enemy contact Vetehinen launched a torpedo, which missed probably due to being fired at too short distance. Vetehinen then opened fire with its deck guns. A second torpedo also missed, but the deck guns managed to damage the Soviet submarine which by then had started an emergency dive. The Captain of Vetehinen, determined not to let the submarine escape, ordered his submarine to ram the other vessel which at last was a success – the "teeth" on the bow of the ship ripped open the Soviet submarine's hull and caused it to sink. Vetehinen suffered a minor leak from the impact but managed to return to its home port.
On 5 July 1944 Vetehinen laid 17 mines near Koivisto islands.