The simple answer is that the British blockade was ineffective by the time Franklin left for France. Franklin sailed from Philadelphia on October 27, 1776. By that time, the British Navy had not had enough time to enforce a proper blockade across the massive coastline of British North America. Writing in December of 1777, John Montagu, Earl of Sandwich and Vice Admiral wrote that the navy had been poorly used, and that there was no direction or grand strategy for the British military. N. A. M. Rodger wrote in his biography of Montagu that it was not until December 1777 that Lord North and the King's Cabinet had agreed on a method to suppress the American rebellion - that was to have the army occupy key port cities, while a naval blockade would strangle the remaining international attachments. All this was over a year from Franklin's departure date from Philadelphia. Certainly Britain would have wanted a blockade earlier in the war to prevent people like Franklin escaping, but in October of 1776, this was not the case. Many of the King's ministers believed that if the rebellion in the North could be quelled, then the rest of the colonies would fall in line. With the defeat at the Battle of Long Island in August 1776, the British had every reason to believe their victory would be swift and brutal. A blockade was incredibly expensive and thought unnecessary by members of the Cabinet, even if men like Montagu disagreed. It is only with the benefit of hindsight that we can see that Montagu had the right view, which was to make the American war a Naval war from the start.