During world war one a large programme of purchases trawlers to provide a cheap anti submarine, minesweeping and patrol service. Many trawlers were purchased and converted During December 1914 to April 1915, 10 military class 350 tons trawlers were purchased while on the stocks, and another 9 ex Portuguese were bought the following September. The Royal navy settled on three standard designs. these vessels were known as Mersey design. 438 tons, standard with a full load of 665 tons, the smaller Castle design, 360 tons and 547 tons loaded, and the Strath design 311 tons and 429 tons loaded.

Light armament were supplied. The Mersey's, Castles and Strath's all carried one 12- pounder gun, many of them varied slightly by having the addition of 3.5 to 7.5 inch "bomb Throwers" and a few had a 4-inch Gun. These trawlers were ordered in three batches In November 1916 a total of 250. and in 1917 another 150. and in 1918 a further 140 were ordered.

Trawler Blackwater was a Mersey Class trawler, pennant T04, with a displacement of 438 tons, armed with 2-3 inch guns and carrying a complement of 20 crew.