Submarine Warfare of To-Day

(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?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 encyclop?dia 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.

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.

^{Fig. 27.—Diagram showing mine mooring being cut by sweep-wire. A. Mine-mooring wire. B. Hard and cutting face of sweep-wire. The dotted lines C show how the mine floats to the surface by its own buoyancy when the mooring wire holding it down has been cut.}

This type of sweep was known as a “bottom sweep,” and generally consisted of a chain fitted into the bight of a sweep-wire and dragged along the sea-bed, the idea being to overturn the delayed mine and so upset its mechanism that it would either rise immediately to the surface or else remain for ever harmless at the bottom of the sea. In many cases the heavy chain passing over the horns of the mine would bend and make them useless, so destroying the efficiency of the mine even if it did eventually rise to the correct firing depth.

Into almost every operation carried out on or under the sea there enters the tide difficulty, and in all mining and minesweeping operations it is one of the most important factors to be considered. The effect of the tide on mine-laying has been dealt with in a previous chapter, and the same difficulties in reverse order are experienced when sweeping the sea for these invisible and dangerous weapons. It has already been shown that a vessel may sometimes pass safely over a mine at high water which would touch her sides or keel and explode if she passed over it at low water when the mine was nearer to the surface. All minesweeping vessels, therefore, need to be of comparatively shallow draught in order to reduce the risk of touching mines, but against this is the fact that shallow-draught ships, even if powerfully engined, have but little grip on the water and experience an undue loss of speed when towing a heavy sweep-wire. Such vessels can seldom operate in even moderately heavy weather owing to their rolling and pitching propensities. Therefore a vessel of medium—bordering on shallow—draught, with a fairly broad beam, is the best type. Here, again, is a difficulty. Minesweeping is a type of defensive warfare requiring a vast number of ships successfully to carry on against an enemy well provided with surface and submarine mine- layers, and not even the greatest naval power in the world could seriously contemplate maintaining a peace fleet of, say, 2000 such vessels in constant readiness. Therefore recourse has to be made, when war comes, to mercantile craft, which seldom possess all the desired qualities.

This is what actually occurred in every maritime country at war during the years succeeding August, 1914, and in order to meet the danger attending the use of passenger ships, trawlers and drifters, often with a considerable draught, minesweeping operations were, whenever possible, confined to the three hours before and