[R.W.Ratikal]

A Q ship is a regular tramp steamer or merchant cargo ship flying a neutral flag and with false decking and deck houses behind which are mounted 5-inch guns and machine guns. At the sight of the pirates the false sides are dropped and the pirates are blasted out of the water.

The Germans used them very effectively during WWII.

[Snardius]

Repetition is not our friend. Others here have said the ship was used against German U-boats. Which is it?