This is where the ship's boilers would have been. Instead, we can see the iron framing. The outside was two layers of teak. There are only three surviving ships that have this composite construction. One is in Australia, the other is up the Thames. You can read about that one in our tour: Cutty Sark: A tour of 147 years of sailing history.
Still in use today, the massive ropery building is over 1,100 feet/340 meters long. It plays tricks on the eye, since rarely will you see a building this narrow be this long. There's a separate tour of this building, including how to make rope, if you have time.
As gorgeous as the outside of the No 3. Slip is, the view inside is even better. The mezzanine, where I took this photo, was built around 1904, some 66 years after the initial construction. There are over 400 windows in the ceiling.