Target practice aboard a merchant ship. Image copyright Imperial War Museum
From 1937 Merchant Navy defence courses had been organised to train officers in gunnery and convoy work and to teach seamen to be gunners. The fitting of guns on merchant ships started the day war began. By the end of 1940, 4,000 ships had been fitted with anti-submarine guns.
At first the job of manning the guns fell mainly to naval reservists of the DEMS (Defensively Equipped Merchant Ships). As the battle continued thousands of merchant seaman also became skilled in handling guns and other armaments. Later they were joined by anti-aircraft and anti-surface gunners of the specially created Maritime Regiment of the Royal Artillery.