cort sivertsen was born in brevik, norway, the son of a shipper. at the age of fifteen he took service with the dutch navy; in 1639 he fought under lieutenant-admiral maarten tromp at the battle of the downs. in 1642 he was first mate on the "grote st. joris", a dutch ship hired by the fleet of venice as the "san giorgio grande". sivertsen called himself "adelborst" in this period, a dutch name meaning "cadet". in 1645 he became captain of the "san giorgio" and entered full venetian service. in 1650 the "san giorgio" became a flagship and sivertsen therefore flag captain. in an action against the turkish fleet on 13 may 1654 near the dardanelles he broke with his ship a line of turkish galleys and sank fifteen of them; next day the turkish garrison of tenedos surrendered to him. for this he was knighted in the order of san marco; the venetian senate rewarded him with an annuity of 1400 golden ducats. in 1660 he was made vice-admiral.