Locko was originally the French East Indiaman Modeste, built in France. The Royal Navy captured Modeste in 1778, while she was on the return leg of her maiden voyage, whereupon individuals associated with the British East India Company (EIC) purchased her. She entered the EIC's service in 1780, then performed three voyages for the EIC. On the first voyage she was present at the Battle of Porto Praya, and led an inconclusive attack by five merchant vessels on a French frigate. The second and third voyages were much less eventful. Lockos owners disposed of her on her return in 1788 from her third voyage.
Capture
The French East Indiaman Modeste was returning to France from China in company with three other Indiamen when a gale on 25 September 1778 or so parted them. On 29 September, , Captain W. C. Finch, was on her way from Lisbon to England when she captured Modeste in the Bay of Biscay. Modeste was returning from China and richly laden. Her cargo, which consisted primarily of tea, was valued at £300,000, half of which was insured with English underwriters.
British East Indiaman
Captain Patrick Lawson purchased Modeste, had her lengthened and superbly fitted out, and renamed her Locko. Lawson had been captain of Lord Holland when, on his return to Britain in 1779, a crew member had denounced him for smuggling. Lawson lost his goods, and his command, which he had purchased from Lord Hollands previous captain, Fasham Naire. Lawson enlisted Robert Williams, who had been principal managing owner of Lord Holland on Lawson's voyage, to become principal managing director of Locko.
The EIC took Locko into service in 1780.
EIC voyage #1 (1781-83) Captain Patrick Lawson sailed Locko from Portsmouth on 13 March 1781, bound for Bombay and China. Locko was part of a convoy of Indiamen accompanying a British squadron under Commodore George Johnstone.
Locko and the convoy reached São Tiago on 10 April. Six days later, Locko was present at the Battle of Porto Praya, when a French squadron under the Bailli de Suffren attacked Johnstone. Both squadrons were en route to the Cape of Good Hope, the British to take it from the Dutch, the French aiming to help defend it and French possessions in the Indian Ocean. The British convoy and its escorting squadron had anchored at Porto Praya (now Praia) in the Cape Verde Islands to take on water, when the French squadron arrived and attacked them at anchor. Due to the unexpected nature of the encounter, neither fleet was prepared to do battle, and the result was an inconclusive battle in which the French warships sustained more damage than did the British. The French did capture the Indiamen (recaptured the next day), and Fortitude, and the victualer Edward, and gained a strategic victory, because Suffren beat Johnstone to the Cape and reinforced the Dutch garrison before continuing on his journey to the Ile de France (now Mauritius). Still, Johnstone went on to capture five Dutch East Indiamen and destroy a sixth at the battle of Saldanha Bay on 21 July.
Locko, reached the Cape region on 22 July. She then sailed to Johanna, which she reached on 2 September. The convoy left Johanna on 21 or 22 September, but ran into an adverse monsoon that drove the vessels to the coast of Arabia, and especially the four Indiamen , Latham, Locko, and were driven further, to Kissen Bay, near the Bab-el-Mandeb. The Indiamen were carrying the 2nd Battalion of the 42nd (Highland) Regiment of Foot. They remained there for some six weeks as the indigenes were friendly and the vessels could procure fresh food. The Highlanders entertained the indigenes with bagpipe music, and danced the Highland Fling; the indigenes reciprocated with a war dance involving shields and spears. The Indiamen arrived at Mocha on 4 December. They then sailed out of the Red Sea and reached Bombay on 6 March. A voyage that would take a coppered vessel not facing adverse winds about two-and-a-half weeks had taken the Indiamen five-and-a-half months.
Locko left Bombay on 8 August in company with the Indiamen Asia, , and Osterley, and the country ship Shah Byram Gore. As the squadron was sailing through the Strait of Malacca, on 9 September the ships encountered the 38-gun French frigate Pourvoyeuse, which was under the command of Lieutenant de Lannuguy-Tromelin. After an engagement the next day that lasted some two-and-a-half hours and in which Osterley had two men killed, and Locko and Essex had several men injured from exploding cartridges, Pouvoyeuse withdrew.
On 4 October Locko reached Whampoa anchorage. She was at the Second Bar on 20 December.