Complex sentence: A year later, on 20 November 1811, he received a significant elevation to Flag Lieutenant on HMS "Arethusa", the flagship of his uncle, now Vice-Admiral and Commander-in-Chief of the Jamaica Station.
Split into simpler sentences: A year later, on 20 November 1811, he was transferred to "Arethusa" as Flag Lieutenant, a significant promotion. The "Arethusa" was the flagship of his uncle, now Vice-Admiral and Commander-in-Chief of the Jamaica Station.