Each unit of ships was under the command of a senior officer, responsible for the operations of these vessels, and where big fleets were engaged a special minesweeping officer was placed in supreme command. Only by close co-ordination of effort from the staff at Whitehall and elsewhere to the units at sea could this gigantic work have been expeditiously accomplished. It frequently happened that any delay due to very severe weather in clearing a field or area meant complete stoppage or chaotic dislocation of the almost continuous stream of merchant shipping entering and leaving a big harbour, which, in turn, disorganised the adjacent harbours to which ships had often to be diverted. It disturbed the railway facilities for the rapid transport of the food or raw materials from the coast to the manufacturing centres, from the sugar on the breakfast-table to the shells for the batteries in France. One hour's delay in unloading a ship may mean three hours' additional delay on the railways, the loss of a shift at a munition works and a day's delay in a great offensive. It is a curious anomaly, made vividly apparent to those in administrative command during the past years of stress, that the more perfect the organisation the greater the delay in the event of a breakdown in the system.
There were various methods of minesweeping, but in all of them the object was to cut the mooring wire of any mine that came within the area of the sweep and so cause the mine itself to bob up to the surface, where it could be seen and destroyed by gun-fire. In order to encompass this many kinds of minesweeping gear were devised and given practical trial during the war. The one most generally used, however, was the original but vastly improved sweep. This consisted of a special wire extended between two ships and held submerged by a device known as a kite. This apparatus is best described diagrammatically ([Fig. 25]).
There was, however, another type of sweep used for exploratory work and also for sweeping in shallow water. It was a one-ship sweep (i.e. required only one vessel to drag it), and this can also be best described by a diagram ([Fig. 26]).
Fig. 25.—Diagram showing the form of apparatus principally used by British minesweepers. AA. Sweeping vessels. BB. Sweep-wire. CC. Wires holding kites. DD. Kites which hold sweep-wire at correct depth below the surface by their "kite-like" action when being towed through the water. E. Mine and mooring. F. Surface of the sea. G. Sea-bed.
It will be observed that in all cases the object is to drag a submerged wire through the water at an angle from the ship's course until it encounters the mooring wire of a mine. When this takes place it is the purpose of the sweep-wire to cut the mooring wire and allow the buoyant mine to float up to the surface free of its sinker (see [Fig. 27]). In order to effect this various kinds of hard wire with a cutting capacity were used as sweep-wires, and also numerous mechanical devices, all of which are more or less of a secret character; but the object remained the same—to find and cut the mooring wire.
Fig. 26.—Diagrammatic sketch showing principal parts of a single-ship sweep. A. Towing vessel. B. Tail wire. C. Kite holding sweep-wires D at correct depth below the surface. D. Light sweep-wires held at an angle by spars E and surface hydroplane floats F. The dotted lines show how either arm of the sweep swing towards the centre line when exposed to the pull of a mine. This movement of the hydroplane floats indicates to those on board the sweeping vessel that a mine has been caught. The mine H slides down the sweep-wire until the mooring is cut at G, and the mine floats freely to the surface.
The introduction of what became known as "delayed action mines"—weapons held down on the sea-bed, after being launched, for varying periods of time, so that sweeping operations might take place above them without their being discovered; then, when the time for which the delay was set had expired, they rose to within ten feet of the surface and became a great danger to shipping in places recently swept and reported clear—caused a new form of sweep to be devised and used in waters where these mines were likely to be sown.
