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An armed boarding steamer (or "armed boarding ship", or "armed boarding vessel") was a merchantman that during World War I the British Royal Navy converted to a warship. AB steamers or AB vessels had the role of enforcing wartime blockades by intercepting and boarding foreign vessels. The boarding party would inspect the foreign ship to determine whether to detain the ship and send it into port, or permit it to go on its way.
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Origins
On 28 September 1914 Admiral John Jellicoe, commander-in-chief of the Grand Fleet, sent a telegram in which he pointed out that he did not have enough destroyers available to enforce the blockade. Furthermore, the weather was too severe of the destroyers. Although Jellicoe did not mention it, after the loss on 22 September of the cruisers HMS Cressy, Aboukir, and Hogue, he also did not want large warships making themselves sitting targets for submarines by stopping to examine merchant vessels.
The first request was for 12 vessels, all to be capable of 12-14 knots, be able to carry enough coal for five days at sea, have wireless, and have boats suitable for boarding parties to use. Each armed boarding steamer was to carry two 3-pounder guns (47mm/L50), and be under the command of an officer from the Royal Navy. These 12 vessels were requisitioned in October and completed by mid to late-November. Other vessels followed.
The Navy found that cross-Channel passenger vessels were particularly suitable because of their large cargo capacity. As experience with the programme increased, the armed boarding vessels received heavier armament. The Navy realized the need for heavier armament after the German auxiliary cruiser SMS Meteor attacked and sank the armed boarding ship HMS Ramsey on 8 August 1915. The Navy wanted to arm the boarding ships with some obsolete 14" torpedo tubes, and modern 4" (102mm) guns (possibly the BL 4 inch naval gun Mk VII); Meteor had sunk Ramsey using both a torpedo, and gunfire from two 88mm (3.5") guns.
The Navy pressed the vessels into other roles. Some carried depth charges for anti-submarine duty while escorting convoys. Still others, particularly in the Mediterranean, served as transports.