User blog comment:GrilloSama/Jinbe For 1st Mate/@comment-2A02:587:741F:3A00:E151:51B8:F255:E3A8-20170925140359/@comment-2602:306:CFE2:F510:1DCC:415A:3C94:B811-20170926035429

In the beginning, Luffy listed off the positions he needed and wanted in his crew so he has thought of at least positions. Everyone even acknowledges their role. Luffy kept saying they needed a musician almost every arc until Brook. It would lead to reason that he wants a form of structure in his crew. They all follow Nami's instructions whenever it is about navigation or weather. If food is needed they all listen to Sanji, Chopper for medicine. Basically while I have also thought about Jinbe as first mate, he will most likely be helmsman. There isn't a set First Mate, it feels more like a floating title among whoever needs to wear it along with their other title.