Here I'll be describing the various possible positions on a pirate crew. And not just the ones the Straw Hats have.
These positions are almost essential to have on a pirate ship. Almost all of them are filled by characters of the Straw Hat crew.
The Captain commands the entire crew as its leader. In the world of One Piece, he or she is usually the strongest member of the crew. This position in the Straw Hats is filled by Monkey D. Luffy.
The First Mate is the second-in-command of the pirate crew. He or she usually oversees the work of the other crew members, and answers only to the Captain. Despite never being stated as such in the story, Roronoa Zoro is unofficially thought of as the First Mate of the Straw Hats.
The Navigator charts the ships path and keeps it on course. Nami is the Navigator of the Straw Hats.
The Cook prepares the crew's meals and makes sure the kitchen is stocked. Sanji is the Cook of the Straw Hats.
The Doctor treats everyone's injuries and handles the sick. Tony Tony Chopper is the Doctor of the Straw Hats.
The Gunner mans and maintains the cannons. He or she must be an expert marksman and usually wields a gun or some type of projectile-based weapon. Usopp fills this position on the Straw Hats as the ship's sniper.
The Shipwright fixes and maintains the ship. Originally, Usopp unofficially filled this position by fixing the Going Merry with his carpentry skills, but the position was officially taken up by Franky later in the story.
The ship is just as much a member of the crew as the people on board, as evidenced with the Going Merry. When a ship is cared for, loved dearly, and treated like a nakama, it can develop it's own sense of self. The type of the ship needs to suit the crew. Larger crews will need larger ships. And they can't all be made of Adam Wood. Adam Wood is insanely rare and impossibly expensive. Generally, only marine ships are made of metal. But a pirate ship can still be made of metal, it's just a bit rare. Few pirates are willing to be compared to the Marines by sailing metal ships.
These positions are more like optional positions, as a ship can easily survive with out them, or another member of the crew can have abilities that branch into this area, rendering the crew with no need for the official position. They're useful if you want a larger crew, but don't want to create characters with no positions.
The Second Mate is the third-in-command, answering only to the Captain and the First Mate. The position is normally only present on larger crews.
The Helmsman steers the ship. This position can easily be filled by any of the other crew members, but someone with the official position of Helmsman will be much more skillful with steering the ship than a member of the crew with a different official position. Jinbe is the Helmsman of the Straw Hats.
The Lookout mans the crow's nest and uses a telescope or binoculars to gaze out at the horizon, ever alert for approaching ships and land masses. With smaller crews like the Straw Hats, this position can be leisurely filled by any member of the crew. But a crew member with the official Lookout position will probably practically live in the crow's nest. He or she will likely heavily personalize the space due to spending so much time there, such as by adding a hammock or sleeping bag, or stocking it with books to read and snacks to eat.
The Rigger handles the ship when docking and shoving off. Usually he or she unties the ship from the dock when setting sail, and ties the ship to the dock when docking. The rigger will also raise and lower the sail, and may also raise and lower the anchor. This position is easily filled by any member of the crew, as it is with the Straw Hats.
The Blacksmith repairs and maintains all of the weapons, armor, and equipment of the crew. They also make new weapons for the crew. Usopp and Franky both unofficially fill this position on the Straw Hats. They even have separate workshops on the Thousand Sunny.
The Tailor mends and sews up the damaged clothing of the crew members. Among the Straw Hats, Nami unofficially fills this position, as shown when she mended Luffy's hat after Buggy tore it.
The Scholar is the researcher and general knowledge expert of the crew. He or she will often provide the crew with information on locations and things they come across. It also helps for the scholar to have knowledges of languages, should the crew encounter people who speak a different language. This position is filled on the Straw Hat crew by Nico Robin as the Archaeologist. "Scholar" is just a broader term for her position, and should be used when categorizing characters.
The Chronicler records the crew's adventures in their log book. They normally don't take part in the major activities and goings-on so as to properly chronicle the events. Chroniclers are usually historians who have noticed the various changes and epic events that tend to happen when the pirate crew gets involved, and thus have joined up with them so as to "experience history as it's being made" and record it accurately.
The Merchant keeps record of the crew's inventory and spending. It is usually the Merchant who will purchase new supplies, exchange treasure for Beli, trade with the people of every port town, and hand each of the crew members their spending money. This position is unofficially filled by Nami on the Straw Hat crew.
The Musician plays music to entertain the crew. They are essential on long sea voyages, where pirates will have trouble keeping themselves entertained. It is officially filled on the Straw Hat crew by Brook.
The Janitor is in charge of cleaning the ship and making sure it does not get dirty. The ship can easily be cleaned by any member of the crew, so the only reason for this position is to lessen the work load of the other crew members.
The Cabin Boy or Girl performs the grunt work and menial chores on the ship. Considered to be junior or apprentice pirates, they are usually kepped busy so as to teach them the lessons of being a pirate. When there is no work to be done, they are usually forced to polish cannonballs.
Pirates often keep pets, so to speak. Cats are often kept on ships to hunt rats. And dogs are always faithful companions. And despite what some may think, pirate parrots weren't just a myth. Historically, pirates really did keep parrots as pets. The talking birds kept pirates entertained during long voyages. And since they were considered exotic, they often sold for high prices among the wealthy at port towns.
The Quartermaster is essentially the jack-of-all-trades of the crew. He or she has a wide range of skills to cover many areas, but nowhere near the amount of skill of the person who actually fills that position. He or she has a wide variety of tasks to perform on the ship. This position is often the least essential of a crew, and is normally only present on trade ships and large crews. On large crews, the Quartermaster is essentially a member of the infantry who is a cut above the other infantry.
Infantry are the members of the crew that have no position. They are usually unskilled in combat and easily taken down by officially positioned members of an enemy crew. They are usually only present on very, very large crews.
The Cooper is a barrel maker. He or she makes containers to keep gunpowder dry, food free of pests, and water and spirits, including various necessities from leaking or rotting by perserving the longevity of the crew's products from any types of damages. They were considered extremely important, so every ship needed a cooper—a man skilled in making and maintaining barrels.
Other Type of Positions