After some time in North America, where Duckworth became involved in a court-martial after an accident at Rhode Island on 18 January 1777 left several men dead, the "Diamond" was sent to join Vice-Admiral John Byron's fleet in the West Indies. Byron transferred him to his own ship, , in March 1779, and Duckworth was present aboard her at the Battle of Grenada on 6 July 1779. Duckworth was promoted to commander ten days after this and given command of the sloop-of-war . After cruising off Martinique for a time, he was promoted to post captain on 16 June 1780 and given command of the 74-gun . He returned to the "Princess Royal" as flag-captain to Rear-Admiral Sir Joshua Rowley, with whom he went to Jamaica. He was briefly in command of , before moving into in February 1781, and returned to England with a trade convoy. In the years of peace before the French Revolution he was a captain of the 74-gun , lying at Plymouth.