By the 1790s the Blackwall shipyard, on the north side of the Thames downstream from Greenwich, was the largest merchant shipbuilding yard in the world, mainly building and maintaining East Indiamen and building ships on contract for the Royal Navy. It was run by the Perry and Green families, with other partners, and in 1789-90 John Perry built the new Blackwall basin for the mooring and refitting of East Indiamen. It had the distinctive Blacwall masthouse (essentially a crane for masting and demasting) on its western side, which was only dismantled in 1862. This can be seen in the background here. The fact that the Union flags shown do not have the red St Patrick saltire (diagonal) cross shows this image dates to before 1801, when it was added to form the modern Union Jack. It may however be an early 19th century painting based on an earlier source, since it is not accurate in various ways for a 1790s date