@blue-san:
No, Nami is the navigator and that's that.
Are you telling me Franky is not the shipwright and Brooke is nor the musician? Or were you responding to that idiot who thinks Nami > Zoro?
As for the rest of your post, I doubt Luffy is going to fight Sentoumaru. I don't think Sentoumaru is weak enough to be treated as any sort of fodder by Luffy, so any fight between them would probably drag out too long since they seem to be trying to leave Shabondy asap.
@semil:
Sorry, I didn't really explain well. I meant they don't have a hierarchy to their positions. Their positions are what they do, but unless its a task that pertains specifically to a crewmate's specialty nobody ranks higher than anyone else.
**There is a very clear hierarchy in the crew, it's just not emphasized much because none of the Strawhats let the power get to their heads. They act like a bunch of friends most of the time, but when the need arrives a hierarchy presents itself. Zoro is always the one that holds the authority when Luffy isn't around, and if he isn't around it's Sanji. The only exceptions are if they are doing something specific to a crewmember's forte, such a navigating. And those times are usually held in a less serious light than when Zoro or Sanji are in charge.
Anyway, the point is that Zoro is the first mate even if Luffy didn't officially appoint him. He was the first crewmember, is closest in terms of strength to Luffy, has always taken change when Luffy isn't around, always fights the second strongest, and was flat-out called the second in command by Urouge. Even if Luffy hasn't appointed him as the first mate, his actions and the position/authority he holds in the crew make him the first mate.**