Staff

A Second Mate (参謀長) is a position held by a member of a crew. He is the second highest ranked officer on a crew outside the captain, the first being the First Mate. Typically, it was not essential to have a second mate on a ship, however captains of larger ships and crews may recruit one to have a stronger grip on his crew. On the crew he answers only to the Captain and the First Mate.

The Second Mate's duty is similar to the first mate and often he will share the jobs at hand between him and the First Mate. He is the next in line to take over as captain after the First Mate. The second Mate usually is (but not necessarily) in charge of navigation and typically stands the 12-4 navigation watch. That is, the Second Mate will stand watch from 1200 to 1600 at noon and again from 0000 to 0400 in the nights. The other duties of this position often depend upon the type of ship worked aboard. On oil tankers, the Second Mate usually provides the First Mate with assistance in tank cleaning.

In the United States system, it takes 360 days of sea service as a licensed Third Mate before one can become a Second Mate. This role is generally considered a "made-up" role, though it is sometimes used in fiction and by non-commercial/military persons.

Second Mates within One Piece
Cabaji is the second mate on Buggy's crew. Oddly enough, he seems to pull more favor with the captain and pull more weight with the crew then Mohji his first mate. Cabaji is the first and by far the only confirmed second mate to appear in the series.

Translation and Dub Issues
Due to a mistranslation within the VIZ Media, Pearl of the Krieg Pirates was mislabeled as a Second Mate, but in truth he is the commander of the second division of the crew.

Trivia

 * Among the Straw Hat Pirates, some people believe Sanji, the chef of the crew, is the second mate; as he sometimes share some duties with Zoro, who is often considered the first mate. It should be noted that duties are shared with other crew members as well. Both Nami and Usopp have given orders and had them followed. Luffy has not appointed anyone to the station, but the Straw Hats also don't officially have a "First Mate" to begin with.