MINESWEEPING Methods of Sweeping.—At the outbreak of War, the British system of minesweeping was for two vessels to tow a stout wire between them, the wire being kept at the required depth by means of water kites, one being attached to each side and towed astern of each vessel.
The sweep comes into contact with the mooring rope of the mine which is generally cut in the process of dragging particularly when serrated wire is used. When the mooring rope is cut the mine comes to the surface when it can be sunk by rifle-fire. In some cases, however, the sweep strikes a horn of the mine, and explodes it. This occasionally parts the sweep and consequent delay is caused while a new wire is passed. The most serious drawback is that if the sweep fails to break the mine wire, the mine is towed along, more often than not unknown to the sweep ers. This latter trouble caused many of the early minefields in the War to be scattered far beyond the limits in which they were laid, and constituted a serious danger. The eventual safeguard was to heave the sweep slowly in, and to sight the whole wire and water-kites on the surface before slipping it, when a mine holding to the wire would be seen and reswept afterwards. The innovation of a serrated form of sweep wire eventually reduced the difficulty to a considerable extent. This simple method of minesweeping stood the test of the whole war, and its simplicity had the great advantage of reducing the training period which would have been necessary had a more complicated apparatus been used. Sweeping with this type of sweep can be carried out by any suitable pair of vessels. Trawlers and drifters are fitted with appliances for their ordinary fishing work which make them readily convertible into sweepers. They are also very good sea boats and as far as weather conditions are concerned, sweeping can be carried out by them as long as they can come together to pass the end of the sweep wire from one to the other. Where, however, mines have been laid at very shallow depths and there is little or no rise and fall of tide lighter draught vessels must be used as sweepers. When sweeping, marks are laid out to show the channel which has been cleared. That minesweeping is a most arduous and dangerous work is shown by the fact that throughout the World War an average of half of the crew of a trawler was lost when one was mined. For the first two months of the war, for every two mines
swept up one trawler was lost. Improved methods and greater experience later minimised the losses, until an average of one loss for every 8o mines was achieved in 1918.
The French towed from a single vessel two sweeps, one cn either side, each kept out by an otter similar to the usual otter used in trawling but regulated for depth by attachment to a large torpedo-shaped float, the depth at the other end being regulated by a water-kite. Along each wire was distributed a series of small mechanical and explosive wire-cutters. The "shooting" of these sweeps caused trouble unless the crews were adept, and the speed at which the apparatus could be towed was limited to six to seven knots. The spread of the British system was 400 to Soo yd. per pair according to the type of minesweeper, that of the French system not more than 200 yards. In the latter part of the war the "Americans adopted the French method, which had by then been somewhat simplified.
The German system was akin to the British, excepting that, in order to cover the route of their fleet more quickly, they insti tuted a very light form of sweep which could be towed at 20 knots. Directly this apparatus met with an obstacle it was auto matically slipped, thus giving notice of the obstruction, whereupon the slower type of sweepers were hastened to the spot to clear it. In practice this system constantly led the fast sweepers to report mines when they had only encountered wrecks or some other harmless obstruction, and little confidence was therefore placed in the reports of mines by this fast flotilla. Under ordinary cruising conditions, searches by sweepers were made whereby only a small percentage of the water was covered—sufficient, however, to allow a strong probability of the presence of mines being discovered in good time. The introduction of the "paravane," which protected the ship herself, very much reduced the mine danger; but it is an apparatus which, for assured efficiency, requires skilled handling. There are situations where its use is impracticable, and it has not replaced the necessity for the minesweeper.