American Revolutionary War
Stewart was not given another command for a considerable period of time after this, choosing instead to focus on his political career. This situation changed with the start of the American Revolutionary War, and he was appointed to command the brand new 74-gun ship of the line HMS Berwick on 27 December 1777. Serving in the English Channel, he fought at the Battle of Ushant on 27 July the following year. Stationed in the vanguard division of the British fleet, Berwick was heavily engaged during the battle and had ten men killed. Afterwards Berwick continued to serve in the English Channel, and was part of the fleet that relieved Guernsey in September 1779.
While Berwick was still at sea disagreements between the two commanders of the British at Ushant, Admiral Augustus Keppel and Vice-Admiral Sir Hugh Palliser, had led to political controversy. The First Lord of the Admiralty at the time, John Montagu, 4th Earl of Sandwich, specifically noted that:
Captain Stewart behaved exceedingly well in the action, is a very good officer and has kept himself clear of disputes of every kind, and shown no particular attachments. With his ship having received a refit in December, Stewart was sent with Berwick to the West Indies on 8 April 1780, in a force commanded by Commodore Robert Boyle-Walsingham. Serving at Jamaica from 12 July, Berwick was one of many ships to be heavily damaged in the October Great Hurricane of 1780 and Stewart ran from the storm with his ship jury rigged, reaching Britain in December.
Berwick was repaired in a refit that was completed in February 1781, and Stewart was subsequently appointed a commodore and Commander-in-Chief, North Sea, on 27 March. Based out of Leith, Berwick and the 32-gun frigate HMS Belle Poule captured the French 32-gun privateer Calonne off the Firth of Forth on 16 April. Stewart's force was however not large enough to defend against the squadron the Dutch were forming to oppose it, and Vice-Admiral Hyde Parker was sent out with reinforcements in July. In Parker's force was the 64-gun ship of the line HMS Bienfaisant whose commander, Captain Richard Brathwaite, was senior in rank to Stewart. This forced Stewart to relinquish his position as commodore, but direct command of Berwick continued with Stewart's ex flag captain, Captain John Fergusson, and Stewart was only able to stay on in Parker's fleet as a volunteer.
Stewart was present as such at the Battle of Dogger Bank, fought on 5 August, where Berwick led the British line of battle against the Dutch and was heavily damaged. Afterwards Parker resigned his command in the North Sea and Brathwaite also left the station, leaving Stewart able to re-hoist his broad pennant as commodore in September. Employed in the blockade of Dutch ports, towards the start of 1782 he failed to stop a Dutch convoy from escaping him while in the Downs, and after criticism of his actions he resigned his command in around March.
Stewart was then given command of the 80-gun ship of the line HMS Cambridge, in the Channel Fleet, on 17 June. His crew from Berwick were also transferred for this. The fleet was subsequently sent to relieve the Great Siege of Gibraltar, which it did as escort to a convoy. After this it fought the Battle of Cape Spartel on 20 October, with Cambridge serving as one of the seconds to Vice-Admiral Mark Milbanke's flagship the 90-gun ship of the line HMS Ocean. Stewart's ship had four men killed during the engagement, after which the fleet returned to England.
