Researching a Norwegian Sailor

Hi, I am trying to find more information on Simen Andersen born 1859 at Eidanger Parish and Prgj to father Anders Simensen and mother: Gunhild Svenungsdatter. On the 1875 Census he is listed as a "Sailor" age 16. I have him arriving in Australia in 1881 on board the "Loch Naw" he is a mariner and is listed on a few vessels sailing around the Pacific. How do I find out when he became a sailor, are there registers of merchant sailors? The Loch Naw was a vessel that was domestic in Sydney Australia so while he claims that is the vessel he arrived in Australia on I'm not sure. He put this information on his Naturalisation papers. Hope someone can help. I have his marriage and death date and a picture of his grave if anyone else is reaching him. Many thanks Margaret Weston