The Yangtze-31 (Chinese: 扬子-三十一;), is a Chinese nuclear-powered ballistic missile submarine commanded by Zao. When first discovered by the Sole Survivor in 2287, it is lodged in the shallow waters off the coast of the Commonwealth south of Boston Airport and northeast of the Four Leaf fishpacking plant. The submarine is not marked on the map until the Sole Survivor swims out to it. Upon discovery, the quest Here There Be Monsters will automatically be started if it hasn't been received already from Donny Kowalski near the Shamrock Taphouse.
The Yangtze is a unique design for a ballistic missile submarine. It has three large propellers located on either side and under the reactor control room, as well as a smaller propeller at the rear of the sub with the rudder. The sub's design is unique as all of the facilities and machinery are located fore of the conning tower, which sits halfway on the sub, with only a small propeller and the rudder being behind the conning tower.
The first room the Sole Survivor comes to when entering the submarine is the control room/bridge where Zao is located.
Down the stairs is the reactor control room with a radio room on the port side (left when facing forward). There are two doors on either side of this room that go to a small room with a cage wall and a large machine behind which is presumably the propeller drive shaft.
Behind the reactor room and under the bridge is a room with the machinery and instrument panels. On the port side of the room is Captain Zao's quarters and on the starboard side is the officers mess.
Forward of the reactor control room is the upper reactor level and contains the reactor access hatch and a stairway, on the port side, leading to the lower level. The lower level contains reactor machinery and two doorways, at the rear, leading to the engine room for the lowest of the large propellers.
Past the reactor room on the upper floor is a small room that contains two decontamination showers and a locked door with a stairway to the lower deck. There is also a science lab on the port side of the room.
Forward of that room (still on the upper level) is the sub's mess room, galley, and food storage (the room opposite the galley). Forward of the mess is the crew quarters, containing 80 beds (configured in 8 stacks of three and 14 stacks of four), the sub's lavatories and a laundry room (port side), as well as the ships med-bay/infirmary (starboard side).
At the front of the quarters are two doorways that lead into each other at the front of the sub, then lead into a room on the lower deck that contains machinery with a workshop on the starboard side and a storage room on the port side. The next room contains a doorway on the starboard side leading to the sub's armory, and a hallway at the end leading to the silo room.
The silo room contains the 6 missile silos with 3 along each side. Each silo has an access hatch that allows the crew to access the SLBM inside. All are closed except the one with the missile that failed to launch. The next room is a small room before the reactor room which contains a locked door to the upper level.
Unlike the only other Chinese submarine the player character can personally encounter in the series, SSN-37-1A, the Yangtze is in fact a full-scale combat vessel with extensive crew amenities and visible missile and torpedo tubes. It is shaped very differently to any real-world submarine, unlike the SSN-37-1A, which bears a very conventional (albeit small) design.