Last Voyage of the U.S.S. Constitution

Last Voyage of the U.S.S. Constitution is a side quest in Fallout 4. The Sole Survivor has been drafted by Ironsides to assist him and his robot crew in their quest to return the USS Constitution to the ocean.

Detailed walkthrough
Upon approaching the USS Constitution, the Sole Survivor is greeted by Lookout, who tells them to meet with Captain Ironsides on the ship's main deck. Finishing this conversation starts the quest.

Enter the bank building and turn north, up the clutter ramp to the second floor and up the stairs to the third floor. Once there, turn south, head through a broken wall and to the ship's bow, and enter the ship through the hatch located there. Once inside, head up two sets of stairs and meet the First Mate on the third deck, upon which a small conversation takes place. After this, the main deck can be entered through either of the hatches, at the stairs behind the First Mate.

Speaking with Ironsides reveals that the captain requires assistance with repairing the ship's systems. Asking for a reward and passing the easy persuasion check, allows the Sole Survivor to deduct a 100 cap upfront payment from the terminal called Ship's Purser, which is sitting on some crates at the north end of the main deck. They are then tasked to speak with both the bosun and Mr. Navigator about the repairs. As the conversation with Ironsides ends, the ship gets attacked by scavengers. They can be dealt with in any way possible, firing the ship's cannons by flipping the circuit breaker on the starboard side is entirely optional. Note that hitting robot allies with stray fire may result in turning the entire crew hostile and therefore failing the quest; however, the cannons are safe to fire as they do not affect the robots.

Repairing the ship's systems
Talk with the bosun, who is located on the third deck in front of the captain's quarters, and wants to have three power cables either repaired (requires Intelligence 3+) or replaced with new power cables, which can be taken from a nearby metal box. The cables in need of fixing are stored in yellow utility boxes, whose lid must be opened first. One is located on the third deck, on the ground near the western ship hull. The other two can be found one deck below, one on the ground near the stairs to the first deck, and the second on the western ship wall.

Once the cables are fixed, return to the bosun and receive the next task, which consists of fixing the power relays for the ship. This can be done by either repairing an eroded power relay coil (requires Intelligence 5+) or by installing a replacement coil. The fuse box housing the relays is located at the western wall on the third deck. After power is restored, return to the bosun and receive praise by him.

Next, find and speak with Mr. Navigator, who needs the ship's guidance chip retrieved from the scavenger's outpost, which is located a small trip west. Once there, talk with Mandy Stiles, who tries to persuade the Sole Survivor to turn on Ironsides and his crew and instead plunder the ship with her and her scavengers. At this point, one can either side with Ironsides or with the scavengers, with different effects on the quest's progression:
 * If helping Ironsides, the chip must be stolen from the scavengers' camp, doing so turns the local scavengers hostile, even if the theft is undetected.
 * If helping the scavengers, they give up the guidance chip for free, while requesting that any repair parts for the ship are being brought to them first, so that they can sabotage them.

Regardless of which side was chosen, once the chip is acquired, return to the Constitution 's main deck and install the guidance chip into the core guidance system at the bow. After doing so, Mr. Navigator appears for another short talk. One is then tasked with either repairing (requires Intelligence 9+) the guidance radar or retrieving the Poseidon radar transmitter and installing it afterwards. After the radar is repaired, speak again with Mr. Navigator and receive instructions to talk with Ironsides again.

Ironsides then gives the player the key to his quarters, as he requires them to retrieve and install the FLL3 turbopump bearings there. They can be retrieved from a specific location, with known locations being the Corvega assembly plant, Fort Hagen, and the General Atomics factory. With the bearings in their possession, the Sole Survivor must now decide if they side with Ironsides or the scavengers.

Siding with Ironsides
Upon retrieving the bearings, head back to the USS Constitution, and install them at the turbopump in the captain's quarters on the second deck. Head back to the main deck and speak with Ironsides, after doing so, another wave of scavengers attacks the ship, later even making their way through the lower decks. If they reach the main deck, the quest fails. After eliminating them, return to Ironsides, who awards the Broadsider and asks the player character to complete a final task of firing up the auxiliary power to launch the ship. Head down to the street and enter the second building on the southern side of the road. Head to the attic, open the lid, flip the circuit breaker, and watch the ship take off and subsequently crash into a skyscraper in the distance.

Afterwards, head over to Faneuil Hall, find the tower to the north into which the ship has crashed, and use two elevators in the tower to access the ship. Speaking with Ironsides will award the Lieutenant's hat and conclude the quest.

Siding with the scavengers
Once the FLL3 turbopump bearings are retrieved, take them to Mandy instead of Ironsides, where Davies will sabotage them so that the ship will explode when the rockets are fired up. Then install the sabotaged turbopump bearings in the captain's quarters on the second deck and talk to Ironsides again. Go ahead and agree to turn on the auxiliary power. Head down to the street and enter the second building on the southern side of the road, where the player can optionally meet with Mandy, but she only tells them to start up the power. Head to the attic, open the lid, flip the circuit breaker, and watch the ship's rocket fail, as it remains landlocked. Ironsides and his crew turn hostile, and all that is left to do is to storm the ship with the Scavengers and defeat all the robots.

Meet up with Mandy after claiming the ship, she has relocated to the nearby Cabot House. She reveals that the scavengers held a vote and are not going to split the cut, and instead decided to kill the Sole Survivor. As there is no way to resolve the situation peacefully, the two scavengers as well as Mandy and Davies need to die in order to complete the quest.

Behind the scenes
This quest was created and designed by Ferret Baudoin. He performed research related to rockets and how to tamper with them, which he later joked about, saying that he was likely put on a watchlist.

Bugs

 * If before the quest starts, the robots are hostile, typing will add the player to the faction, thus making the robots friendly, and allowing the quest to begin normally.
 * If the Lookout refuses to talk to you, type, which will start the quest, and allow the Lookout to speak again.
 * During the scavengers' first assault on the ship, the game's music may be stuck in an infinite battle loop even after the battle is over. One can fix this by saving the game, closing the application, then rebooting Fallout 4. The music should be back to normal upon resuming the game. Fast travel should also work.
 * It is possible that the scavengers will not attack the ship after the player has talked to Ironsides, making the quest uncompletable. A possible workaround for PC is typing in the console, which will continue the mission by setting the scavengers to attack. Typing  will set the quest to the very last stage.
 * When completing the quest and returning to the USS Constitution after its second crash, Ironsides will give the player full access to his captain's quarters after promoting the player to lieutenant. Despite this, the items are still listed as "owned" and can't be taken without stealing them. Ironsides will also be stuck in the same dialogue loop of allowing the player access to the captain's quarters.
 * After repairing the guidance system and being instructed to talk to Ironsides by Mr. Navigator, Ironsides may become bugged and not respond to the player by talking. There are several workarounds for this bug: One workaround is to save and reload; another workaround is to enter and leave the USS Constitution; a third workaround is to ask a companion to inspect the captain, after which he will return to normal; the last workaround is to spawn another Ironsides nearby with the console command and talk to the duplicate, which will initiate a conversation with the original Captain Ironsides in which one will have to turn to him to continue the quest.
 * Mandy Stiles may be hostile when first met, leaving only the option to side with Ironsides.
 * The player may not receive any quest marker for the FLL3 turbopump bearings and they will not spawn at any of the three possible locations at which they are normally found thus rendering the quest impossible to complete. On PC, the item can be added with the console command.
 * Placing the turbopump bearings may not complete the quest conditions, leaving Ironsides in a looped dialogue about them.
 * After the scavengers' second assault on the ship, when the quest asks the player to speak with Ironsides, it may be possible for Ironsides to get stuck in a sentry bot's "steam out" condition, and will remain there, making the quest unable to be completed. This can be fixed by reloading a save from before the second assault. Attacking him will eventually turn the crew hostile, though interestingly enough will not always fail the quest.
 * When the scavengers are all finished, a glitch may happen in which the defending mission continues, with no possible way to finish the quest on consoles. On PC one can use the console command to finish the mission and solve the glitch.
 * During the scavengers' attack, some scavengers can make their way inside the ship and may get stuck in an infinite loop of exiting and entering the deck, causing them to appear and disappear instantly. Killing them as soon as they appear is possible with a rapid-fire weapon. This may be a cause of the above bug.
 * When the player is tasked with retrieving the guidance chip, it may not be present inside the file cabinet and the quest marker may direct them to the garden patch in Covenant, thus making the quest impossible to complete.
 * After speaking with the bosun and completing his task, the mission will not update. If having the companion check out Ironsides does not work, and spawning in a new Ironsides does not work, the player can leave the ship, shoot a scavenger, and the mission quest will update afterward.