Staff

A Second Mate is a positaution 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 essenital 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 Firstmate.

The second mate's duty is simulair to the first mate and often he will share the jobs at hand between him and the firstmate. He is the next in line to take over as captain after the firstmate.

One Piece
The only reference to a second mate comes from Buggy's crew. Cabaji is his second mate. Oddly enough, he seems to pull more favour with the captain and pull more weight with the crew then Mohji his Firstmate.