During this period she sailed the usual sea routes in the Channel and the Southwest Approaches, masquerading as a merchant ship and inviting attack by a German U-boat.

Great success was claimed[6] On 20 June 1917, under the command of Lt J Lawrie and in the guise of the French schooner Eider, she encountered a U-boat sailing west of Brittany, which approached and opened fire.

Mitchell carried out her role as a decoy, being hove to and abandoned until he U-boat was within 600 yards, when she returned fire scoring several hits.

On 3 August 1917 Mitchell had her third encounter, sailing south of Start Point in the guise of the French schooner Cancalais.

However the U-boat was too cautious, and after being shelled for fifteen minutes, Lawrie elected to clear away and close under engine power.

With her local crew she delivered slate to Hamburg and London until the First World War when, in 1916 she became one of the first "Q" ships or decoy vessels.

February 1998 The Mary B Mitchell is commemorated in Bangor, Wales, by a memorial plaque and a bronze weathervane which adorns the city’s new shopping precinct.

This reputation has persisted to this day, although post-war analysis showed neither of the U-boats she engaged had been sunk, and that her achievement had been overestimated.

One source regards them as greatly overrated, diverting skilled seamen from other duties without sinking enough U-boats to justify the strategy,[10] while another suggests Q-ships were all the more important in the early stages of the fight against the U-boat because so few methods had appeared to work,[11] though their effectiveness declined as the war at sea progressed.