When what was, perhaps, Hitler's boasted "secret weapon" was revealed in a new campaign of indiscriminate sinking of merchant shipping, the whole subject of minelaying and sweeping immediately became of paramount importance.
The sailors who man the minesweepers – small trawlers equipped with special gear or shallow-draught sloops, but a little larger – have what is, perhaps, the most dangerous and yet least spectacular job in the Royal Navy. Just how dangerous was emphasized when, soon after the sinking of the "Simon Bolivar" and other ships in Germany's new campaign of "frightfulness", H.M. minesweeper "Mastiff" was reported lost with seven valuable lives. Yet a few days later Grimsby fishermen queued up outside the Board of Trade office in answer to the Admiralty's call for men for the minesweepers.
In the last war, when the war at sea had reached its grimmest pitch, one sweeper was lost for every two mines swept up – and each time half the crew was killed or wounded. The enemy laid altogether 43,636 mines, and of these our sweepers found and destroyed 23,873; over 700 fully-equipped sweeping vessels were engaged in the work.
The work of a minelayer is equally dangerous and arduous. To enable their mines to be sown effectively, the Germans are thought to be using relays of U-boats. Even the smallest of these can carry up to a dozen "eggs", and in all probability specially-built submarine minelayers, with mine-wells in the bottom of their hulls, are now in service. A fast surface layer can put down more than 200 mines "at a sitting". Moreover, instead of the usual straight-line method of laying (which simplifies the sweepers' task), the U-boat commanders drop their mines in irregular zigzag fashion – say six here, five there, then another six farther on – forming a large area that may keep the sweepers at work for days on end before they can signal "all clear". Unlike a U-boat, a mine cannot be detected in advance by any apparatus, and the minesweeping crews put their wits and their lives in a warfare where chance may tip the scales against them. The principal feature of the submarine mine is the unpleasant-looking horns projecting from its steel casing. These are made of soft lead, and are filled with tubes of acid. Any vessel striking one of these horns causes the acid to detonate the deadly explosive inside the mine.
The mine, on a long mooring cable, is laid by dropping its heavy anchor or sinker to the sea-bottom after which it settles at the correct depth.
Minesweepers work in pairs, with each unit 300 to 500 yards apart. Between them, sometimes suspended from two sets of apparatus called Oropesa floats, is drawn the sweep wire, which has a series of steel cutters. Should this come into contact with a mooring cable, the mine will rise to the surface and it can then be destroyed by gunfire. The paravane (see page 119) is a form of mine protection hung in the sea from the bows of a warship when the presence of a minefield is suspected.
The tragic toll of the German minefield laid off England's East Coast in November called forth much speculation as to whether the enemy were using "magnetic mines" such as are described opposite. Some at least of these may have been dropped from aircraft, with parachutes attached to reduce the shock when they hit the water. If they are laid on the sea bottom, normal minesweeping methods are ineffective.
The extent of the German minelaying activity is illustrated by the fact that more than 200 mines were washed up on the Yorkshire coast, quite apart from those picked up by trawlers.