"Roebuck" returned to American waters by 5 December 1779, when she captured the American privateer, "Lady Washington". Then, with five ships-of-the-line, the 50-gun , the 44-gun , four sixth-rate frigates and two 20-gun sloops, she accompanied transports, carrying 7,550 troops for an attack on Charleston. Under the command of Admiral Marriot Arbuthnot, the ships left New York on 26 December and in January 1780, in need of repairs, called in at Savannah, captured by the British the previous month. From there, the convoy proceeded to the North Edisto River where the army disembarked on 10 February. The troops marched the overland and occupied James Island, while the ships sailed to the entrance and effected a blockade of Charleston harbour. Some of "Roebuck"s company were among the 450 seamen and marines later sent to lay siege to the town. The 64-gun and 74-gun ships-of-the-line, being too large to be of any use in the shallow waters around the harbour, left for New York in March 1780, leaving "Renown", "Romulus", , , , , and "Roebuck", to which Arbuthnot moved his flag. These ships were lightened while they awaited a high enough tide and favourable conditions to carry them over the sandbank which lay across the entrance to the harbour.