Larsen arrived in Shetland with the boat M/B "Motig I", on 11 February 1941. After training with "Kompani Linge" in England and Scotland, Larsen returned to Lerwick in the "St Magnus" on 19 August 1941. He did his first Shetland Bus tour with M/B "Siglaos", skippered by Petter Salen, on 14 September 1941. After the loss of the minelayer "Nordsjøen", where Larsen was second in command, he became a skipper and could choose his own crew. His first crew was Palmer Bjørnøy, Leif Kinn, Arne Kinn, Kåre Iversen, Karsten Sangolt, Nils Nipen and Otto Pletten. His first boat was M/K "Arthur", the boat that he had "requisitioned" on his escape from Norway, after the wrecking of "Nordsjøen". On 8 November 1941, Larsen sailed from Shetland on his first tour as skipper. On their return to Shetland, they ran into a storm and Sangolt was blown overboard and drowned. Larsen made several tours with the "Arthur" but he also skippered other boats, like M/B "Siglaos" and M/B "Feie". In October 1942, he had to scuttle the "Arthur" in Throndheimsfjord after a failed attempt to attack the German warship "Tirpitz". He and the crew escaped to Sweden but a British agent, A.B. Evans, was arrested and later shot.