Norwegian Merchant Navy Seaman records pre 1920

I am looking to trace information on my Norwegian Great Grandfather. This is all the information I have: Name- Jon Poulsen He was in Leith, Edinburgh around 1917 He is noted on my Grandfather's birth certificate as his father but I have no other information. My Grandfather never knew his father and the rest is a mystery. Is it possible to search seaman records? Thanks for any advice or help in advance.