High Quality Content by WIKIPEDIA articles! A second mate (2/M) or second officer is a licensed member of the deck department of a merchant ship holding a Second Mates Certificate of Competency, which is issued by the administration. The second mate is the third in command (or on some ocean liners fourth) and a watchkeeping officer, customarily the ship`s navigator. Other duties vary, but the second mate is often the medical officer and in charge of maintaining distress signaling equipment. On oil tankers, the second mate usually assists the chief mate with the tank-cleaning operations.