These First World War Flower-class sloops predated the name's later anti-submarine reputation. The Royal Navy ordered the first 12 on 1 January 1915 and the next 12 on 12 January because it urgently needed purpose-built minesweepers. John "Jacky" Fisher's prewar disposal programme had removed many older small ships earmarked for that work. Their merchant-like hull form allowed commercial yards to build them quickly, while a mast ahead of the bridge gave lookouts a high position from which to scan the water during minesweeping.
Chapter · 1:39-6:37
Why the Acacia class began as minesweepers
The Acacia-class order answered the Royal Navy's urgent mine-warfare shortage in early 1915, before the submarine torpedo threat became its defining concern.