To understand this verse it is worth knowing about how World War I minesweepers worked.

Mines were held in position below the surface of the water by a sinker and a steel wire. The mine was actuated by ‘horns’ – protruding flexible spikes which, when struck by a passing ship, moved sufficiently to complete a battery-powered electric circuit which fired the mine.

Sweeping mines was done by dragging a serrated wire between two boats, which was kept down at the desired depth by a ‘kite’, which worked like a child’s toy kite, but upside-down. Hence this was a job particularly suited to fishing trawlers.

Once the wire broke the vertical mine mooring wire of the mine, the buoyant mine would then bob to the surface. Alternatively (i.e. for chains) the mine would simply be dragged upwards. It could be neutralized, usually by firing at it with a rifle to make holes in it; the casing would then fill with water, destroying its buoyancy, so that it sank, and was no more danger to shipping. If the minesweeper was lucky, and a rifle bullet hit a horn, then the mine would explode, settling the matter once and for all.