Between 1840 and 1860 “mate” was also the naval rank now known as Sub-lieutenant. For other meanings, see mate (disambiguation). A mate is a deck officer aboard a merchant vessel, such as the chief mate (first mate), second mate, or third mate. One of the mates is always the watch keeping officer, unless the master takes that responsibility. Each mate also has other duties, such as making the passage planning, overseeing loading and unloading and personnel management.No tags for this post.