Knowles had been promoted to commander of the 40-gun in 1732, but the position appears to have been for rank only, as he did not become post-captain until 4 February 1737, when he was appointed to command . He was ordered to reinforce Admiral Edward Vernon's West Indies fleet in 1739, as the War of Jenkins' Ear developed. Knowles rendezvoused with the admiral at Port Royal, having captured two Spanish ships en route, one of which was a register ship carrying 120,000 pieces of eight, and clothing for 6,000 men. Knowles was unable to sail with Vernon when he commanded the fleet to Portobelo, but arrived there on 27 November, five days after Vernon's victory in battle. Vernon gave Knowles the task of destroying the Spanish forts. Because of their solid construction, this work took three weeks and 122 barrels of gunpowder. When the task was completed the British withdrew, and Knowles had impressed Vernon with his competent command of land operations.