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 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.
Fig. 28.—Plan showing the usual formation adopted by British minesweeping vessels. A. Three pairs of sweepers. B. Sweep-wires. C. A mine entering the sweep of the second pair. D. A vessel following the sweepers for the purpose of sinking by gun-fire the mines cut up.
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.