Kammala jumla mai zuwa: An haifi Cort Sivertsen a Brevik, Norway, ɗan mai jigilar kaya. A lokacin da yake da shekaru goma sha biyar ya shiga aikin soja tare da rundunar jiragen ruwa na Holland; a 1639 ya yi yaƙi a ƙarƙashin Lieutenant-Admiral Maarten Tromp a
Yaƙin Downs. A shekara ta 1642 ya kasance na farko a kan "Grote St. Joris", wani jirgin ruwa na Dutch wanda jirgin ruwan Venice ya yi hayar a matsayin "San Giorgio Grande". Sivertsen ya kira kansa "Adelborst" a wannan lokacin, sunan Yaren mutanen Holland wanda ke nufin "cadet". A cikin 1645 ya zama kyaftin na "San Giorgio" kuma ya shiga cikakken sabis na Venetian. A shekara ta 1650 "San Giorgio" ya zama babban jirgin ruwa kuma Sivertsen ya zama kyaftin din. A cikin wani aiki a kan rundunar jiragen ruwa na Turkiyya a ranar 13 ga Mayu 1654 kusa da Dardanelles ya karya tare da jirginsa layin jiragen ruwa na Turkiyya kuma ya nutsar da goma sha biyar daga cikinsu; washegari sojojin Turkiyya na Tenedos sun mika wuya a gare shi. Don wannan an ba shi lambar yabo a cikin Order of Saint Mark; Majalisar dattijai ta Venetian ta ba shi lada tare da shekara-shekara na 1400 zinariya ducats. A cikin 1660 an sanya shi Mataimakin Admiral.