The Germans laid many thousands of these deadly and invisible weapons in the 140,000 square miles of sea around the British Isles alone in the face of over 2000 warships. To search for these patches of death in the wastes of water may well be likened to exploring for the proverbial "needle in a haystack." Yet the sweepers, whose sole duty it was to fill this breach in the gigantic system of Allied naval defence, explored daily and almost hourly, for over four years, the vast ocean depths, discovering and destroying some 7000 German mines, with a loss of 200 vessels of their number. The result of this silent victory over one of the greatest perils that ever threatened the Sea Empire was that some 5000 food, munition and troop ships were able to enter and leave the ports of the United Kingdom weekly with a remarkably small percentage of loss from a peril which might easily have proved disastrous to the entire Allied cause.

This, then, in broad outline, was the task which confronted this section of the naval service, and its successful accomplishment forged a big link in the steel chain encompassing the glorious victory.

Before passing on to describe the ships and the appliances used it is first necessary to give a more detailed account of the operations generally included under the heading of minesweeping. As it was impossible to tell exactly where mines would be laid from day to day, an immense area of sea had to be covered by what was known as exploratory sweeping. This consisted of many units of ships emerging from the different anti-submarine bases almost every day throughout the year and proceeding to allotted areas of water, where they commenced sweeping north, south, east or west, in an endeavour to discover if the areas in question were safe for mercantile traffic. If no mines were discovered that particular area would be reported safe, but if only one of these weapons was cut from its mooring by a sweep-wire the area would be closed to merchant ships until the sea around was definitely cleared of the hidden danger. This system of open and closed areas entailed an enormous amount of efficient administrative staff work apart from the actual sweeping, and its success was partly dependent upon the vigilance of the patrols employed to divert shipping from dangerous patches of sea.

When a mine-field was discovered which interfered with the free movement of a large number of ships a big concentration of sweepers from all the adjacent bases was ordered by telegraph and wireless. The area was isolated by patrols and the mines swept up. In one field no less than 300-400 mines were known to have been laid. Finally a further exploratory sweep was made, and if nothing further was discovered the area was again opened to traffic, and the sweepers turned their attention either to routine duties or to the clearance of another field.

The entrance to every important harbour was swept once or twice a day, and all convoys had sweepers ahead of them when they left or entered such confined waters. The seas adjacent to harbours and naval bases were searched at low water for mines which might be showing above the surface. Around the anchorage of the Grand Fleet in Scapa Flow a wide belt of sea was kept clear of mines so that at any moment the fleet could reach blue water without risk from these weapons. The same precautions were taken off the Firth of Forth for the benefit of the battle cruisers, and outside Harwich for Admiral Tyrwhitt's light forces.

A passage known as the "war channel"—about which more will be said later—extending from the Downs to Newcastle, was swept daily by relays of sweepers operating from the anti-submarine bases along this 320 miles of coast-line. This buoyed and guarded channel formed a line of supply for the great fleets in the north.

Each big fighting formation was provided with a special flotilla of fast fleet sweepers, which were capable of clearing the seas ahead of the battleships and cruisers moving at 20 knots. This was a separate organisation to what may be described as the routine sweeping of the trade routes. These vessels were always within call of the fleets they served.

It has been estimated that over 1000 square miles of sea were swept daily by the anti-mine fleets of the British navy during the four years of war. This may not sound a very stupendous figure compared with the area of the danger zone, but in practice it necessitated terribly hard work from dawn to dusk by several thousand ships and many thousands of men in summer heat and winter snow.

There was in addition to all this the clearing of British mine-fields no longer required in the positions in which they had been originally laid. This was not entirely an after-the-war problem, for although the great mine barriers were left until peace was assured, there were fields of minor importance which had to be cleared to meet new situations as the years of war passed swiftly by. A notable instance of this was the destruction of a big field of some 400 mines off the Moray Firth.

The foregoing refers only to the minesweeping in the principal danger zones in British waters, no account being taken of the work carried out by Allied vessels in the Mediterranean, off the coasts of France, Italy, Greece, Gallipoli, and in such distant seas as those washing the shores of New Zealand, Australia, Hong-Kong, Japan, Singapore, Bombay, Aden, the Cape of Good Hope, the United States, Eastern Canada, West Africa and Arctic Russia, in all of which mines were laid by surface raiders like the Wolfe, and afterwards located and cleared by Allied warships.

From the foregoing some idea of the gigantic nature of the task will be obtained, and we can pass on to a more detailed account of the actual work. Minesweeping may be divided into eight well-defined sections, as follows:—

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.

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).

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.

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.

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 the three hours after high water. Shallow-draught M.L.'s carried out the scouting for mines at low tide. It is difficult to see what would be the fate of a nation hemmed in by mines and devoid of a mercantile fleet sufficiently numerous to provide powerful sweeping units. The trawlers and pleasure steamers were a godsend to England in those years of intensive submarine warfare. This undeniable fact incidentally provides another example—if such is now needed—of naval power resting not entirely on fleets and dockyards, but on every branch and twig of maritime activity.

It is difficult to describe in small compass and non-technical language the various tactical formations employed in minesweeping operations. They were many and various. The Germans used their vessels in long lines, the ships being connected together by a light wire-sweep plentifully supplied with cutting devices, into which the mooring wire of the mine was expected to obligingly slip. This method suffered from the serious drawback that if any part of the sweep-wire caught on a submerged obstacle, such as a projection of rock, the whole line of ships became disorganised. There were also many other objections to this system, some of which will doubtless be apparent to the thoughtful reader.

The formation usually adopted by British minesweepers was that shown in Fig. 28, in which it will be observed that each pair of ships is actually independent of the others, but is acting in company with them, and that the pathway swept by one pair is slightly overlapped by the following pair. In the event of an accident to one ship the next astern can immediately let go its own end of sweep-wire and go to the rescue of any survivors. It may be apropos to say here that the smaller class of minesweeper is usually blown to pieces if she touches a mine.

The set of the tide is another important factor which has to be taken into serious consideration when plotting a sweep. This complication enters into every operation, and its salient points will be made quite clear by referring to Fig. 29.

The actual speed at which minesweeping operations are carried out depends greatly upon the engine-power of the sweepers themselves. In the case of trawlers and drifters it is seldom possible to drag the 300-600 feet of heavy wire through the water at a greater rate than 4 to 6 knots. M.L.'s can accomplish 8 knots with a lighter wire, while big fleet sweepers with engines of several thousand horse-power can clear the seas at 18-23 knots.

Sufficient has now been said to enable the reader to realise fully the arduous, exciting and often very hazardous nature of the work. Veteran sweepers listen for the loud hum of the wire which proclaims that a mine has been caught. Then comes an interval of a few seconds of suspense. Sometimes the mine bobs up within a few feet of the ship; at other times it is in the middle or bight of the wire, far astern, and half-way between the two sweeping vessels. When a mine is cut up a few shots from a 3-pounder, a shattering roar and the mine is destroyed. All that remains is a column of smoke reaching from sea to sky.

It frequently happened that the mine became entangled in the sweeping gear and was unknowingly hauled on board with the sweep. When this occurred the position was fraught with extreme peril. Any roll of the ship might cause an explosion which would shatter to fragments everything and everyone within range. Safety lay in lowering the sweep gently back into the sea—an extremely difficult operation on a rough day.

This carefully guarded fair-way consisted of a 320-mile stretch of sea, extending along the east coast of England from the Downs to Newcastle, which was marked on the seaward side by a continuous line of gigantic buoys, two miles apart. It was patrolled day and night by hundreds of small warships, and swept from end to end by relays of sweepers acting in conjunction with each other from the different anti-submarine bases along the coast.

The war channel formed a comparatively safe highway for all coastal shipping passing north or south through the danger zone, and vessels from Holland, Denmark, Norway and Sweden were able to cross the North Sea at any point under escort and proceed independently and safely along the British coast to whichever port could most conveniently accommodate them at the time of their arrival. It also relieved the terrible congestion on the railway lines between the north and south of England by enabling a coast-wise traffic to be carried on between the ports of London, Grimsby, Hull and Newcastle, as well as enabling the numerous Iceland fishing fleet to pass up and down the coast in comparative safety on their frequent voyages to and from the fishing grounds of the far north. From the naval or strategic point of view it more or less secured a line of supply for the Grand Fleet assembled in the misty north. Colliers, oilers, ammunition and food ships were able to proceed through the comparatively narrow section of the danger zone with a minimum of risk; and, had it been required, there was available a cleared passage for any squadron from the big fighting formations to come south at high speed to checkmate a bombardment or attempted landing on anything like a grand scale.

It may perhaps be wondered why this channel was not extended up the east coast of Scotland as far as Scapa Flow. In the first place, the North Sea widens considerably as the higher latitudes are approached, the coast of Scotland does not lend itself to a clearly defined channel and the heavy weather which prevails for so many months in the year made the maintenance of gigantic buoys and their moorings almost impossible. Secondly, there were various systems of mine defences in this area, and, although not defined by a chain of buoys, the passage north from Newcastle to the Scottish islands was, in actual fact, maintained by a vast organisation of patrols and sweepers, but over this section of sea supply ships for the Grand Fleet were nearly always under escort. The area from the Scotch to the German coast was looked upon more as a possible battle-ground for the fleets at war than as a route for merchant shipping, owing to the comparatively few big commercial harbours along the eastern shore.

Laying the moorings of over 150 gigantic buoys in fairly deep water, exceptionally prone to sudden and violent storms, was in itself a noteworthy feat of submarine engineering. The chains and anchors had to be of great strength, and the whole work, which occupied many weeks, was carried out in waters infested with submarines and mines.

The task of sweeping this vast stretch of sea almost continuously for four years was by no means either straightforward or without risk. The Germans, when they discovered the existence and purpose of this channel, sought to turn it to their own advantage by systematically laying mines around the moorings of the mark-buoys, where they could only be swept up with great difficulty, owing to the sweep-wires fouling the moorings of the buoys. This strategem had to be answered by the creation of "switch lines," or small sections of false channel marked by buoys, while the real channel was only outlined on secret charts. In this way the preservation of the war channel and its use for misleading and entrapping U and U-C boats became a semi-independent campaign, in the same way as that which surrounded the great mine barrages and other activities of the anti-submarine service.

It is an axiom of war that new weapons of attack are invariably met by new methods of defence. The mine was no exception to this rule, although up to the present time the various antidotes are in all cases only partial remedies. During the years of war, with the brains of a maritime nation focused on the subject, there were naturally many devices suggested and tried for protecting ships from mines. The great majority of these suggestions may be classified in two groups: (1) Those which sought to deflect the mine from the pathway of the ship; and (2) those which sought to minimise the result of the explosion. One method from each of these groups was adopted with various modifications to suit different classes of ships.

In the first group came the Paravane, which had as its basis the suspension of a submerged wire around the bow of a ship, which caught and deflected the mine-mooring wire before the horns of the mine itself could reach the sides of the ship. It also cut the mooring and enabled the mine to rise to the surface and be destroyed by gun-fire.

In order to understand this appliance it is first necessary to know what is the action of the majority of moored mines on coming in contact with a ship. It seldom happens that a vessel strikes a mine dead on the bow or stem-post. The cushion and dislocation of water formed by a big and fast ship around its bows is usually sufficient to cause the mine to swing a few inches away from the bow and to return and strike the ship several feet back on the port or starboard side. A careful study of Fig. 30 will show how this is prevented by the deflecting wires of the paravane.

The paravanes themselves are submerged torpedo-shaped bodies which hold the wires under the surface and away from the ship's side, deriving their ability to do this from the speed at which they are being towed, submerged, by the ship itself. A piece of string through the axle hole of a small wheel, which is then placed on the ground and pulled along, will give a good idea of the action of the paravane against the passing water.

It is not possible to give here the exact details of this highly ingenious device upon which so much scientific and practical attention was wisely bestowed, but sufficient has been said to enable the reader to form a clear conception of how the mine was caught and held away from the ship's side by the deflecting wire of the paravane.

This device, in one of its many forms, was fitted not only to warships, but also to many hundreds of merchantmen, and was known to have saved thousands of tons of valuable shipping and cargo.

Among those devices which had for their object the minimising of the result of a mine explosion may be mentioned the "Blister System" so successfully employed in the construction of monitors and other big ships, the idea being to surround the inner hull with an outer casing which received the effect of the explosion of either a mine or torpedo and left the inner or real hull of the ship water-tight. Its one weak feature was that it reduced the speed of the ship and the ease with which she could be man[oe]uvred. In future types of large and heavily armed ships this drawback will undoubtedly be largely overcome by an increase in engine-power made possible by the development of engineering science.

The "blister," although outwardly forming a continuous structure round the entire vessel, extending well above and below the water-line, tapered off towards the bows and stern, and was subdivided into different compartments. In this way an explosion against one section did not necessarily damage any other part. The British monitors which so successfully bombarded the Belgian coast and the fortifications of the Dardanelles were fitted with blisters, and more than one of them owed their salvation to this means.