Lappin says that "The Post Boy was built in 1820 and designed with a shallow draught to allow her to sail from Glasgow at any state of the tide.She provided connections at Dumbarton for the Loch Lomond steamer Marion and at Greenock for some of the larger packets that could not easily navigate the upper river." {i}  A Post Boy was one of a relay of riders delivering the Royal Mail. The speed of Royal Mail deliveries probably influenced the choice of name.

 {ii}

Reference:
{i} An article by Graham Lappin published in "Clyde Steamers" magazine and given to the curator by the author.
{ii} Alistair Deayton. Scottish Loch & Canal Steamers (Stroud, Tempus Publishing, 2004) p13