After the Royal Exchange experiment, Duncan received command of the nearly new HMS Valiant in February 1761. Serving as Keppel's flag captain made him a full captain in a post ship rather than merely an officer addressed as captain. That distinction mattered because command of a rated ship established the posted seniority from which future admirals were selected. The appointment was both a return to the mentor who valued Duncan's communication skills and a substantial institutional correction to his career path. Valiant then carried him through the Belle Ile expedition and onward to Havana.
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Why HMS Valiant transformed Duncan's career
Appointment as Keppel's flag captain moved Duncan from an awkward commander's posting into a rated ship whose service counted toward eventual flag rank.