He returned to active service again during the summer of 1778, and was present with Barrington's fleet at the Battle of St. Lucia on 15 December 1778, serving aboard Commander James Richard Dacres's 18-gun . Two days later the "Ceres" was chased and captured by a squadron under the comte d'Estaing. He was exchanged and appointed to serve as lieutenant aboard Vice-Admiral Barrington's flagship, the 74-gun . In May 1779 he was briefly ordered to be master and commander of the storeship , but had returned to the "Prince of Wales" by 6 July, when he took part and was wounded in the Battle of Grenada. Knowles returned to England with Barrington in October 1779, and by December had joined Admiral Sir George Rodney's flagship, the 90-gun , as a volunteer for the Relief of Gibraltar.