Inscribed,as title, and signed by the artist, lower right. HMS 'Probus' was one alias of the brigantine 'Thirza', built in 1865 and owned in 1915 by the Whitstable Shipping Co. Ltd. She was requisitioned on 30 August 1915 by the Admiralty for use as a submarine decoy vessel (Q-ship) and was armed with two 12-pdr and two 6-pdr guns. She operated under the names 'Elixir', 'Probus', 'Q30', 'Ready' and 'Thirza'. As 'Probus' she was in action with a German submarine south-west of Start Point on 21 June 1917. The U-boat was disguised as a ketch, with masts and sails rigged, and at 14.30 she opened fire on the 'Probus' with her deck gun. The 'Probus' engaged in a gunnery duel with the U-boat until 15.35, scoring some hits, until an armed trawler arrived and the U-boat withdrew. Here the disguised submarine is in the left distance. The 'Thirza' was returned to her owners on the day the war ended, 11 November 1918.