After his return in 1665 Cornelis fought in the Battle of Lowestoft; in July he became captain with the Admiralty of Zealand. In 1666 Cornelis was captain of his father's flagship "Walcheren" during the Four Days Battle. During the first night he witnessed his father's death, the Lieutenant-Admiral being cut in two by the parting shot of the escaping "Henry". He also fought in the St. James's Day Battle where his uncle was killed.