SF Hydro was a Norwegian steam powered railway ferry that operated in the first half of the 20th century on Lake Tinn in Telemark.

The combined track and ferry service was primarily used to transport raw materials and fertilizer from Norsk Hydro's factory at Rjukan to the port in Skien.

It was the target of a Norwegian operation on 20 February 1944, when resistance fighters sank the ferry in the deepest part of Lake Tinn to prevent Nazi Germany from receiving heavy water.

Transport included both railway cars, carrying primarily fertilizer, potassium nitrate and ammonia from Norsk Hydro, as well as passengers.

[3] 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.

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.

The resistance movement was aware of this plan, and considered blowing up the train at various places, but instead chose to target the ferry SF Hydro.

[7] 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.

[9] 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.

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.

[10] 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.

[13] In 1948 the film The Fight Over the Heavy Water premiered, depicting the various sabotage actions including the sinking of SF Hydro, featuring some of the original saboteurs.

In 2017, a new investigation of the ferry was featured in National Geographic Channel's Drain the Oceans, "Nazi Secrets" (Season 1, Episode 1).