Minesweeping methods

Mechanical
During a sweep, a sweepwire is towed from the stern of the minesweeper at a depth well below the depth at which mine cases are expected to be encountered. Sweepwires may be streamed from one or both sides of the ship. They are diverted, or spread away from the sides of the ship, by an otter attached to their outboard end and supported by a float attached with a float pendant. The depth of the sweep is regulated by the length of the float pendant. To maintain the entire length of the sweepwire at approximately the same depth, a depressor is secured to the sweepwire near its inboard end. The depth of the sweepwire near the inboard end is determined by the length of the depressor tow wire. Sweepwires are armed with special cutting devices attached to their outboard end (end cutter) and at a breaking strength of 32,000 pounds is used as a sweep intervals along their length (intermediate cutters). When the sweepwire contacts a mine mooring cable, the mooring is cut by one of the cutters or, in some cases, by the sweepwire itself, allowing the buoyant mine case to rise to the surface, where it can be exploited or neutralized.

Acoustic
Ships under way produce varying amounts of sound. Over the years, mines that detonate in response to some of these sounds have been developed. Some of the mines detonate as soon as they detect a certain sound. Acoustic minesweeping equipment is designed to simulate the acoustic signature of a ship and to detonate the mines a safe distance from the minesweeper. Acoustic minesweeping equipment consists of waterflow-operated devices that are operated by the flow of water as they are towed. The acoustic device A Mk 2(g), commonly known as rattle bars, is a mechanical device that produces a chattering sound for countering acoustic influence mines. As the A Mk 2(g) is towed through the water, the flow of water between its parallel pipes and bars causes them to hit each other with great force, producing sound. The sound output depends on ship’s speed. Higher speed increases the output; lower speed decreases it. During turns, the device has a tendency to stop chattering unless its speed through the water can be maintained.

Magnetic
Most ships are surrounded by a magnetic field associated with their design and construction. As a ship moves, its magnetic field moves with it, causing a change in the Earth’s magnetic field in the vicinity of the ship. Magnetic influence mines are designed to recognize this change in the Earth’s magnetic field and to explode at the most appropriate time to damage the ship that produces the change. Minesweepers counter these mines by generating an electrical current and applying it through insulated electrical cables to create a magnetic field over the mines. This magnetic field is strong enough to “fool” the mines into thinking a ship is passing overhead and makes them react accordingly.