Q ships – also known as mystery ships – were converted tramp steamers, colliers and sailing ships fitted with concealed guns and used by the Royal Navy as a lure for U boats.

The officers and men wore plain clothes and the ships sailed under the Red Ensign. The purpose of Q ships was to pose as defenceless merchant ships small enough for U-boats to attack. When the U-boat came to the surface, closing in to attack, the concealed guns would be brought out, the White Ensign would be hoisted and fire opened on the U-boat.

Overall Q ships were not a success and they were withdrawn in 1917 when U-boats began unrestricted warfare.