By October 1757 Rowley had been given the task of commissioning the 60-gun fourth-rate . Once launched she joined Admiral Henry Osborn's fleet of 14 ships of the line in the Mediterranean. Osborn was at the time blockading the French under Admiral La Clue in the Spanish city of Cartagena preventing them from joining the fleet off Louisburg in Nova Scotia. French command had ordered the Marquis Duquesne to break through the British blockade and reinforce La Clue and then with superiority of numbers break out of Cartagena and make their way to America. Osborn intercepted Duquesne and his three ships of the line and one frigate. The subsequent action became known as the Battle of Cartagena and took place on 28 February 1758. Osborn's squadron captured two of the French line of battle ships and, under the guns of the Spanish castle the 60-gun French "Oriflamme" was driven on shore by the "Montagu" and the . Whilst the battle was not particularly grand the annihilation of the forces under Duquesne had two distinct effects. Firstly, the battle restored much of the pride that had been sapped from the navy after several defeats including those at Toulon and Minorca. Secondly, the siege of Louisburg and its surrender led to the French being marginalised as a significant power in North America. The battle can therefore be considered by the British as one of the defining achievements of the Seven Years' War. Had La Clue managed to break out from Osborn's close blockade the modern map of North America might appear quite different.