^Right. So I think Roddenberry's intention was probably that Kirk commanded the "destroyer-equivalent" vessel as a commander and the Enterprise, a heavy cruiser, as a captain. What we should've been seeing all these years were commanders in charge of smaller ships like the Grissom and maybe even the Reliant (which has often been classed as a frigate by fan blueprinters). But I guess the producers either didn't know the naval practice or thought it would be too confusing to have officers of commander rank addressed as "captain." DS9 was the only show that ever did that, with Dax when she commanded the Defiant.
It was also done in the Abrams films, with Spock being referred to as "captain" once Pike left the Enterprise.
Which was, I believe, an incorrect use of the process. Sulu calls himself Captain at one point too, I think. The usual practice is for the permanent commanding officers to be referred to as Captain when of Commanders rank or lower (using Captain as position rather than rank) and not when left temporarily in command of the vessel.
In Spock's case, I don't think anyone was expecting Pike to come back. He was essentially handing over command, which why he promote Kirk to XO.
It depends on the nature of the situation. Sulu's purpose was more one of deception, rather than an appropriating that rank, since he did not want to betray the retrieval team's efforts. Secondly, and to @BillJ 's point, Pike was not expecting to come back, hence his reaction to Kirk when they do actually rescue him. Which means, Spock was field promoted to the position of "Captain" as was Kirk, due to the chain of command. All of it would be pending official review by Starfleet Command, but the chain of command exists to preserve lawful command, and field promotions continue that chain. That is obviously a simplified explanation for the sake of the story in Star Trek, but it isn't as out of place as it sounds.
Although this is true, it should be noted when we did see Paul Rice (who took command of the Drake after Riker turned it down) he did have four pips on his uniform, meaning he was an O6 Captain, or Captain by rank. So one can presume that had Riker accepted command of the Drake, he would have indeed gone from Lt. Commander to Captain. The Abrams movies definitely screw the practice up with them apparently permitting whoever has command of the bridge to be addressed as "Captain" which is complete nonsense. Even in situations like Pike or Robau being captured by Nero, just because Spock or George Kirk are left in command does not mean they are the Captains. Sulu being left in command while Kirk was on an away team in STID certainly does not make him Captain, and then there's the stupidest example, though thankfully it's from a deleted scene. In Beyond, the bridge's night watch officer is actually addressed as "Captain" by Kirk himself. The thing everyone seems to forget with Dax being addressed as Captain when she commanded the Defiant is that for that time she really was the official Starfleet-assigned Commanding Officer of the Defiant (Sisko having been reassigned to Admiral Ross's staff) and that is why it was permissible to address her as Captain. Curiously, though the Defiant was essentially Worf's command during the rest of the series, he was never addressed as Captain. It does seem Starfleet does not follow the tradition of the CO of a space station being addressed as Commander regardless their actual rank, though IIRC that is mainly a NASA thing. Which should be worth considering in the is Starfleet a military arguments when the Isn't a Military crowd try to say Starfleet is a futuristic NASA, they don't follow NASA traditions but they do follow military/naval ones.
But, Pike did essentially a field promotion, as I mentioned before. Not sure why Spock wouldn't be addressed as "Captain" since he was moved up in duty and responsibilities. Secondly, that scene with Kirk and the night watch, that sounds more like Kirk funning with the officer rather than being serious. Finally, Starfleet only observes naval traditions when it suits the plot
He wasn't really made the ship's commanding officer though, he was just left in command and ordered to the Laurentian system, at which point a more experienced officer from the fleet could and probably would have been officially assigned as Captain. Or more accurately when Nick Meyer or Ron Moore are involved.
Also is STID - when Sulu calls down to the Shuttlecraft Bay and orders the ship captured in the "Mudd incident" be made ready, Sulu says: "This is acting-Captain Sulu..." When Sulu sends the 'targeted transmission' to Khan - he omits that because that's what Kirk ordered him to do. It's part of the ploy Kirk is running.
So... he didn't call himself "Acting Captain" when he was just acting that he was the captain? I'm so confused!!!
What about Kim being captain in the nightshift? Also, we shouldn't assume Starfleet copied over the US-American naval rules book. I bet Andorians, British, Tellarites, Vulcans and Co. had some input, too.
He was just the watch commander. In "Warhead," when Kim was in command on the night shift, he introduced himself in a hail as Ensign Kim, not Captain.
And yet, Starfleet ships use USS as the prefix for their ship names. The Andorians, British, Tellarites, Vulcans and Co already gave ground in this major area rather than using their own naming prefixes or a new neutral one that didn't originate from any one specific race or nationality.
Yes, it does, in real life. And the choice of Star Trek's creators to give the same prefix to Earth (later Federation) vessels, even if they came up with a different phrase for it to stand for as a feeble excuse (I mean, United Star Ship, seriously? What does that even mean?) shows how unthinkingly (or consciously?) America-centric their view of humanity's future was.
Well, it was an American show made primarily for American audiences... Seriously, as I recall, TOS made a lot of things up as it went along, and it seemed like in the earliest shows, when that "U.S.S." prefix was made, the working assumption was that the Enterprise was an Earth-only ship, not part of a interstellar confederacy. (On the other hand, "U.S.S." is no so engrained, that honestly, to me at least, anything else -- U.F.S., for example -- just sounds wrong.)