The arguments against change seemed compelling: convoys would make a bigger target for submarines, and they would have to travel at the speed of the slowest vessel. According to international law, convoys could not be formed in neutral ports. Allied port facilities would be overwhelmed by the arrival of numerous ships at once. Merchant captains were not trained to sail in convoys. And, of course, there was an insufficient number of naval vessels to provide adequate escorts. Pointing to a shortage of escorts clinched the argument for many admirals. The most effective escort, due to its speed and armament, was the destroyer. But these vessels were essential for the operation of the Grand Fleet: destroyers provided the protective screen behind which the invaluable battleships traveled. Thus, the need for the Grand Fleet to be prepared for operations in the North Sea at a moment's notice clashed with the need to provide adequate escort vessels.