The mystery ship was not a specially constructed war vessel, such as a destroyer or cruiser, but merely a merchantman converted into a powerfully armed patrol ship, camouflaged to give the appearance of genuine innocence, but with masked batteries, hulls stuffed with wood to render them almost unsinkable, hidden torpedo tubes, picked gunners, a roving commission and a daring commander and crew. Their work was performed on the broad highways of the sea, and they hunted singly or in pairs, often fighting against overwhelming odds with certain death as the price of failure. [Illustration: INNOCENT LOOKING BUT DEADLY, H.M.S. "HYDERABAD" The famous "Mystery Ship," powerfully built to resemble a helpless merchantman. Sitting almost flat on the surface of the sea the torpedoes from U-boats ran harmlessly beneath her keel. _Thornycroft & Co., Ltd._] [Illustration: THE HIDDEN TORPEDO TUBES OF H.M.S. "HYDERABAD" _Thornycroft & Co., Ltd._] The number of these vessels was not large, possibly 180, but their operations extended far and wide. They roamed the North Sea, the Atlantic, the English Channel, the Mediterranean, the Arctic Ocean and even the Baltic, but until challenged were quite unknown to all other vessels of the Allied navies. Theirs was a secret service, performed amidst great hardships, with no popular applause to spur them on. As all "Q" boats--as they were officially called--differed from each other in size and armament, any description given here can only be taken as applying to one or more vessels with which the writer was personally familiar. Some of these so-called mystery ships were old sailing schooners, others fine steamships, while quite a number were converted fishing smacks, drifters and trawlers, the method being to give the prospective commander a free hand in the conversion of his ship from a peaceable merchantman to a camouflaged man-of-war, and many were the ingenious devices used. One vessel fitted out for this desperate duty at a Scottish base was a steamer of about 400 tons burden. She was armed with a 4.7 quick-firing gun hidden in a deck-house with imitation glass windows, the sides of which could be dropped flat on to the deck for the gun to be trained outboard by simply pressing an electric button on the steamer's bridge. Two life-boats, one on each side of the aft deck, were bottomless, and formed covers for two additional 12-pounder guns. A false deck in the bow shielded a pair of wicked-looking torpedo tubes, each containing an 18-inch Whitehead ready for launching; and the crew for each gun were able to reach their respective weapons, without appearing on deck, by means of specially constructed gangways and hatches. The very act of dropping the sides of the aft gun-house hoisted the White Ensign, and technically converted this unsuspicious-looking merchantman, which asked only to be allowed to pursue its lawful vocation on the high seas, into a heavily armed warship.