At the start of World War I a new and potent threat to Britain''s naval supremacy took shape in the form of the Kaiser''s Imperial German submarines, thanks to their recently acquired ability to submerge and stalk their adversaries. A submarine''s crew could not board and capture a merchant ship, however, and at first the German leadership was reluctant to order their U-boat captains to use gunfire or torpedoes to sink merchantmen - crewed by civilian seamen - because of the expected hostile reaction of neutral countries such as the United States. Instead, U-boat captains were ordered to surface, then check the manifest of merchantmen and allow their crews to take to lifeboats before sinking the cargo vessels, rendering the U-boat highly vulnerable to attack. This enabled the Royal Navy to counter the submarine threat with vessels whose outward appearance was that of a merchantman, but which kept hidden an arsenal of weapons that would spring to life if a U-boat surfaced - the Q-shi
