Suffolk, dated June 20, stating that the forces to be embarked were to be used for an attack against Philadelphia. When Cornwallis reached Portsmouth, he began embarking troops pursuant to Clinton's orders. On July 20, with some transports almost ready to sail, new orders arrived that countermanded the previous ones. In the most direct terms, Clinton ordered him to establish a fortified deep-water port, using as much of his army as he thought necessary. Clinton took this decision because the navy had long been dissatisfied with New York as a naval base, firstly because sand bars obstructed the entrance to the Hudson River, damaging the hulls of the larger ships; and secondly because the river often froze in winter, imprisoning vessels inside the harbour. Arbuthnot had recently been replaced and to show his satisfaction at this development, Clinton now acceded to the Navy's request, despite Cornwallis's warning that the Chesapeake's open bays and navigable rivers meant that any base there "will always be exposed to sudden French attack." It was to prove a fatal error of judgement by Clinton, since the need to defend the new facility denied Cornwallis any freedom of movement. Nevertheless, having inspected Portsmouth and found it less favourable than Yorktown, Cornwallis wrote to Clinton informing him that he would fortify Yorktown. Lafayette was alerted on July 26 that Cornwallis was embarking his troops, but lacked intelligence about their eventual destination, and began manoeuvring his troops to cover some possible landing points. On August 6 he learned that Cornwallis had landed at Yorktown and was fortifying it and Gloucester Point just across the York River. Convergence on Yorktown Admiral Rodney had been warned that de Grasse was planning to take at least part of his fleet north. Although he had some clues that he might take his whole fleet (he was aware of the number of pilots de Grasse had requested, for example), he assumed that de Grasse would not leave the French convoy at Cap-Français, and that part of his fleet would escort it to France as Admiral Guichen had done the previous year. Rodney made his dispositions accordingly, balancing the likely requirements of the fleet in North America with the need to protect Britain's own trade convoys. Sixteen of his twenty-one battleships, therefore, were to sail with Hood in pursuit of de Grasse to the Chesapeake before proceeding to New York. Rodney, who was ill,