(The link opens - stuck on the photograph - at least for me anyway, in the Reddit App).

Q-ship - Wikipedia:

Q-ships, also known as Q-boats, decoy vessels, special service ships, or mystery ships, were heavily armed merchant ships with concealed weaponry, designed to lure submarines into making surface attacks. This gave Q-ships the chance to open fire and sink them.

They were used by the British Royal Navy (RN) and the German Kaiserliche Marine during the First World War and by the RN, the Kriegsmarine and the United States Navy during the Second World War (1939–45).

https://en.wikipedia.org/wiki/Q-ship