HMS Porpoise was the former mercantile quarter-decked sloop Lord Melville, which the Royal Navy purchased in 1804 to use as a store-ship.

After voyages to the West Indies, the Cape of Good Hope and North America she served as a harbour ship at Woolwich and Sheerness.

She then returned to mercantile service under her original name and made one voyage transporting convicts to New South Wales, and a second to Van Diemen's Land.

[5] On 28 January 1806 she left Portsmouth for New South Wales, escorting the transports Lady Madeleine Sinclair, Fortune, Alexander, Elizabeth, and Justina.

[7] On 14 May Porpoise, the storeship Woolwich, and the brig Rolla detained and sent into the Cape of Good Hope the Danish packet ship Trende Sostre (Three Sisters).

In April 1808 Lieutenant William George Carlile Kent (acting), replaced Symons,[2] who had discharged himself (that is, deserted) from the vessel and returned to Britain.

The court martial exonerated Kent, saying that he had tried to carry out his duty for "the good of His Majesty's service" under "extreme and extraordinary difficulties".

[2] On 29 February 1809 Bligh boarded Porpoise after being held under house arrest for over a year following a revolt by the New South Wales Corps, known as the Rum Rebellion.

For a little while he blockaded the port with the idea of capturing the convict transport Admiral Gambier, but changed his mind and sailed for Hobart.

[2] Lord Melville, built at Shields and of 400 tons (bm), reappears in the Register of Shipping for 1816 with Weatherall, master, Bell & Co., owner, and trade London–Botany Bay.