SF Hydro SF " Hydro " was a Norwegian steam powered railway ferry that operated on Tinnsjø in Telemark . The ferry operated between Mæl and Tinnoset between 1914 and 1944 , connecting the two railways Rjukanbanen and Tinnosbanen . The railway was used to transport raw materials and fertilizer from Norsk Hydro ' s factory at Rjukan to the port in Skien . She was the target of a Norwegian operation on 20 February 1944 , when resistance fighters sank the ferry in the deepest spot of Tinnsjø in order to prevent Nazi Germany from sending heavy water to Germany . Usage . The railway ferries operated a route connecting Tinnosbanen and Rjukanbanen . Transport included both railway car s , carrying primarily fertilizer , potassium nitrate and ammonia from Norsk Hydro , as well as passengers . " Hydro " was the second ship delivered for the service ; the initial ship , SF " Rjukanfos " , had been delivered in 1909 but soon proved too small for the service . " Hydro " was ordered from Akers Mek . Verksted on 19 July 1913 on a bid of NOK 268 . 000 ; in the end she cost NOK 334 . 293 . She was launched on 10 December 1914 , but rebuilt and relaunched on 5 June 1915 as the original configuration used too much coal . In 1929 " Hydro " was supplemented with a third ship , SF " Ammonia " ; all three operated the route until the sinking of " Hydro " . Specifications . " Hydro " was larger than her predecessor , at 53 meters ( 174 ft ) with a capacity of 493 . 60 gross tonnes . Like all the railway ferries she had two parallel tracks , which merged at the front to allow a single track to enter the ship ; total track length was , allowing twelve wagons weighing 300 tonnes as well as 120 passengers . She was equipped with two steam engines , each at 190 kW ( 250 hp ) , giving her a cruise speed of . Sinking . The German occupation of Norway ( 1940 – 1945 ) during World War II made Rjukanbanen the area for a crucial struggle between the Norwegian resistance movement and the Third Reich . In February 1940 , before the occupation , the entire Vestfjorddalen and the docks were closed to foreigners . On 4 May 1940 German troops reached Rjukan , a month after the invasion of Norway had started . The ferries were camouflaged and by January 1941 lack of coal resulted in the steamships being fired by wood . One of the byproducts at Rjukan was the production of heavy water — a key component in nuclear weapon s , and necessary as a moderator in German nuclear designs . The hydrogen plant at Vemork was the first mass producer of heavy water , and in 1939 the German company IG Farben , owning 25 % of the shares in Norsk Hydro , asked for permission to import five liters of heavy water into Germany , later denied due to a lack of an export licence . In 1939 – 40 production at Vemork was 20 kilograms , by 1942 production had increased to five kilograms per day . The first attempt to halt the production from the resistance movement was Operation Grouse in October 1942 , which failed when the Germans caught the plotters . As a consequence passenger transport after 7 April 1942 from Ingolfsland Station to Rjukan was only permitted for soldiers , police , workers at the plant and schoolchildren . All filled ammonia wagons were stored indoors in a tunnel under heavy guard . On 16 November 1943 the United States Army Air Forces bombed the hydrogen plant ; the attack killed 21 civilians but failed to damage the heavy water plant as it was located under seven stories of reinforced concrete . The Germans decided to cancel production of heavy water at Rjukan and move the remainder of the potassium hydroxide — from which the heavy water was distilled — to Germany . The resistance movement was aware of this plan , and considered blowing up the train at various places , but instead chose to target the ferry " Hydro " . To minimize the civilian losses , Kjell Nielsen at Norsk Hydro delayed the tapping of the potassium hydroxide one day to allow the shipment to be carried on a Sunday . On Saturday 19 February 1944 the plant director Bjarne Nilssen informed the railway that a wagon with potassium hydroxide would be sent with train number three the following day departing from Rjukan at 8 : 55 and connecting with the ferry from Mæl at 9 : 45 ; the shipment would arrive at Tinnoset at 11 : 35 . NSB car Lf4 no . 32628 was loaded with 39 barrels , of which five barrels were to be unloaded at Notodden . The wagons were set up at Rjukan on the Saturday , but failed a weight check , and part of the load was transferred to wagon L - 84 . The wagons were stored overnight with a single guard . That same evening two civilians , Jon Berg and Oskar Andersen , were guarding the " Hydro " . The saboteurs Alf Larsen , Knut Lier - Hansen , Rolf Sørlie and Knut Haukelid waited a few kilometers from Mæl and broke into the ferry quay by cutting through a fence . They entered the ship , but were discovered by one of the two guards ; Lier - Hansen indicated that he was a worker and wanted to sleep on board , in the end convincing the guard . Sørlie and Haukelid went below deck to the keel where they spent two hours placing of plastic explosive ; they placed it in a circular formation long . The explosion would blow out one to two square meters ( ten to twenty square feet ) of the hull . The saboteurs left the ship unseen ; Larsen and Haukelid left for Sweden while Sørlie left for Hardangervidda . According to Anthony Cave Brown in " Bodyguard of Lies " , Haukelid concluded after a trial run that the explosives would be most effective if placed in the bow . If holed near the bow , the ship ' s screws and rudder would quickly be lifted out of the water , leaving her captain and crew without control . Haukelid also determined that " [ t ] he explosion had to be big enough to sink the ship , but not so severe as to cause casualties among the passengers and crew . " He carried the bomb , made from eighteen pounds of Nobel 808 plastic explosive and two fuses fashioned from alarm clocks , on board in an old sack . The timing was set to cause the ship to sink at the deepest part of the lake , but close enough to shore to allow any survivors a hope of rescue . The weather was calm ; the temperature was . On 20 February 1944 just before reaching the lighthouse at Urdalen the bomb exploded ; the ship immediately headed for land . The ship ' s crew failed to loosen all the lifeboats , and there were no instructions available for using the lifebelt s . By the time the crew left the bridge , the ship had tilted so much that they could walk down the side . At 10 : 30 " Hydro " sank , settling on the bottom at depth . Farmers from across the lake were soon in their boats and came to the rescue of the crew and passengers . In all , eighteen people were killed ; twenty - nine survived . The number of dead included eight German soldiers , the crew of seven , with the remainder being passengers . Some of the Norwegian rescuers felt that the Germans should not be saved , but this attitude did not prevail and four German soldiers were saved . All but one of the twelve passengers below deck did not survive . Eight days after the incident SF " Rjukanfos " went out to the place of the sinking for a memorial service . Post war interest . In 1948 the film " The Fight Over the Heavy Water " was made , depicting the various sabotage actions including the sinking of SF " Hydro " , featuring some of the original saboteurs . In 1965 a British - American film , " The Heroes of Telemark " depicted the same events , though in a less accurate way . Post war , a monument in memory of the dead has been raised near the location where the ship sank . In the early 1990s the wreck of " Hydro " was found by a mini - submarine . 600 kg of heavy water was also found on board , leaving no doubt that " Hydro " was indeed carrying the heavy water the day it was sunk . Two of the barrels have been salvaged , and one of them can be seen at Norsk Industriarbeidermuseum at Vemork , Rjukan . Analysis of the contents of one recovered sealed barrel ( No . 26 ) confirmed that they contained heavy water .