During the Great War the UK blockaded Germany and armed its merchant ships to help defend them against U-boats. A single stern gun, equivalent to what a submarine might carry, was mounted, and civilian captains were encouraged to flee and shoot back from their more stable gun platform. 766 civilian ships had been armed by December 1915, and rose to 2,899 by February 1917. Arming of merchant ships pushed Germany away from prize rules towards unrestricted submarine warfare.
NOTES ON GUNNERY FOR DEFENSIVELY ARMED MERCHANT VESSELS, TRAWLERS AND DRIFTERS 1918
Official R.N. Gunnery School handbook drawn up for the information of Masters and Trained Gunnery Ratings of defensively armed merchant vessels. Valuable information covering: General instructions; Instructions for drill and practice; Sighting; Care and maintenance; Drill for all guns; Single gun control; Single gun anti-aircraft control; Ammunition; Hints on shooting; Notes on the submarine.