A minesweeper is a naval warship designed to counter the threat posed by naval mines. The dedicated, purpose-built minesweeper first appeared during World War I with the Flower-class minesweeping sloop.
Minesweepers are equipped with mechanical or influence sweeps to detonate mines. The modern minesweeper is designed to reduce the chances of it detonating mines itself; it is soundproofed to reduce its acoustic signature and often constructed using wood, glass-reinforced plastic (GRP) or non-ferrous metal, or is degaussed, to reduce its magnetic signature.
Mechanical sweeps are devices designed to cut the anchors of moored mines, and preferably attach a tag to help the subsequent localization and neutralization. They are towed behind the minesweeper, and use a towed body (e.g. oropesas, paravanes) to maintain the sweep at the desired depth and position. Influence sweeps are equipment, often towed, that emulates a particular ship signature, thereby causing a mine to actuate. The most common such sweeps are magnetic and acoustic generators.
There are two modes of operating an influence sweep: MSM (mine setting mode) and TSM (target setting mode). MSM sweeping is founded on intelligence on a given type of mine, and produces the output required for detonation of this mine. If such intelligence is unavailable, the TSM sweeping instead reproduces the influence of the friendly ship that is about to transit through the area. TSM sweeping thus clears mines directed at this ship without knowledge on the mines. However, mines directed at other ships might remain.
The minesweeper is distinct from a minehunter; the minehunter actively detects and neutralises individual mines. Minesweepers are in many cases complementary to minehunters, depending on the operation and the environment; a minesweeper is, in particular, better suited to clearing open-water areas of a large number of mines. Both kinds of ships are collectively called MCMVs (mine countermeasure vessels), a term also applied to a vessel that combines both roles in a single hull. The first such ship was HMS Wilton, also the first warship to be constructed from GRP.