(1) Fleet Sweeping.—Keeping clear the sea routes of the battle fleet.
(2) Exploratory Sweeping.—Searching the sea for isolated groups or fields.
(3) Routine Sweeping.—The daily or weekly sweeping of areas, channels and coastal trade routes, largely used by shipping.
(4) Clearing Large Mine-fields.—Big concentrations of ships to rapidly clear important routes temporarily blocked by large mine-fields.
(5) Special Shallow-Water Sweeping.—Such as that carried out off the Belgian coast by specially constructed shallow-draught ships, frequently with single-ship sweeps.
(6) Convoy Sweeping.—Precautionary sweeping in front of incoming and outgoing convoys. This was regularly done even if the fair-way was covered by routine sweeping.
(7) Harbour Sweeping.—Precautionary sweeping usually carried out by small craft at big naval bases such as Portsmouth (Spithead) and Rosyth (Firth of Forth) inside the submerged defences.
(8) Searching at Low Tide.—This was done by shallow-draught vessels of the M.L. type in order to locate badly laid mines which might project above the surface at low water. Several hundred were discovered in this way.
In order to carry out these duties efficiently the heterogeneous fleet of minesweepers was divided into small fleets stationed at the numerous anti-submarine bases, and these were again subdivided into units of ships especially adapted for the different classes of work. Each pair of vessels had to be more or less alike in size, draught, speed and man[oe]uvring ability to enable them to work efficiently in dual harness. Consequently there were complete units of vessels specially constructed for dealing rapidly with discovered mine-fields and for use with the battle fleets. Shallow-draught vessels of the motor launch type for work in the shallow water off the Belgian coast. Converted pleasure steamers of the usual Thames, Mersey and Clyde type for convoy sweeping. Motor launches for clearing fair-ways and for searching at low water. Flotillas of trawlers and drifters for the hard and monotonous routine sweeping on the important coastal trade routes. They comprised in all several thousand ships engaged solely on this work.
At each important base there was a Port Minesweeping Officer (P.M.S.O.), with one or more assistants, whose duty it was to administer, under the command of the S.N.O., the fleets in the attached area, and to furnish preliminary telegraphic and detailed reports to the Minesweeping Staff at the Admiralty, who issued a confidential bi-monthly publication to all commanding officers which was a veritable encyclopædia of valuable information regarding current operations, events and enemy tactics. Attached to this department was a section of the Naval School of Submarine Mining, Portsmouth, where all knotty problems were unravelled and appliances devised to meet all kinds of emergencies.