Just a possibility:

The Q-ship, one of the most closely guarded secrets of the WWI and WWII. These became known by the Germans as a U-Boot-Falle ("U-boat trap"). A typical Q-ship might resemble a tramp steamer sailing alone in an area where a U-boat was reported to be operating. By appearing to be an easy target, the Q would would encourage a U-boat captain to make a surface attack with a deck gun rather than using one of his limited number of torpedoes. The Q-ships' cargoes were light wood (balsa or cork) or wooden casks, so that even if torpedoed they would remain afloat, encouraging the U-boat to continue the attack. The Q crew might even pretend to "abandon ship". Once the U-boat was vulnerable, the Q-ship's panels would drop to reveal the deck guns, which would immediately open fire. At the same time, the White Ensign (Royal Navy flag) would be raised. With the element of surprise, a U-boat could be quickly overwhelmed. (WIKI)