USS Constant (MSO-427) in 1988
Class overview
Name	Aggressive class
Operators	United States Navy  Republic of China Navy  Belgian Navy  Royal Norwegian Navy  Royal Netherlands Navy  Philippine Navy
Subclasses	Onversaagd class
In commission	1953–Present
Completed	53
Active	1
Retired	52
Preserved	2
General characteristics
Type	Minesweeper
Displacement	853 tons (full load)
Length	172 ft (52 m)
Beam	35 ft (11 m)
Draft	10 ft (3 m)
Propulsion	4 × aluminum block Waukesha diesels, 2,400 bhp (1,800 kW) 2 × shafts 2 × controllable pitch propellers
Speed	14 knots (26 km/h; 16 mph)
Complement	7 officers, 70 enlisted
Sensors and processing systems	AN/SQQ-14 mine hunting sonar
Armament	1 × twin 20 mm gun 2 × .50 cal (12.7 mm) twin machine guns

The Aggressive-class minesweepers are a class of US-built minesweepers. They are designated as MSO (Mine Sweeper Ocean), distinguishing them from the smaller coastal MSCs and inshore MSIs. Besides the US Navy, this class of vessels has also been used by the Belgian Navy and the Norwegian Navy, among others.

Background

Minesweeping, or the disposal of naval mines, by these vessels is performed in different ways:

• Sweeping proper, with an underwater cable cutting the mooring cables of moored mines. The mines then come to the surface and are destroyed by gunfire.
• Acoustic sweeping, with a towed device producing noise to trigger acoustic mines.
• Magnetic sweeping, with a towed device producing a magnetic field to trigger magnetic mines. To protect the minesweeper itself against magnetic mines, the hull and superstructure of the ship are made of wood. Other components are made of non-magnetic materials, and any magnetic materials are strictly controlled.

Construction and disposition

Of the 53 constructed for the United States Navy, 10 were built at Higgins Corp., New Orleans, Louisiana, 9 at J.M. Martinac Shipbuilding Corp., Tacoma, Washington, 8 at Wilmington Boat Works Inc., Wilmington, California, 6 at Luders Marine Construction Co. of Stamford, Connecticut, 4 at Broward Marine Inc, Fort Lauderdale, Florida, 4 at Martinolich Shipbuilding Co., San Diego, California, 3 at Burger Boat Company, Manitowoc, Wisconsin, 3 at Colberg Boat Works, Stockton, California, 2 at Fulton Shipyard, Antioch, California, 2 at Norfolk Naval Shipyard and 2 at Seattle Shipbuilding and Drydocking Co., Seattle, Washington.

33 of the class were decommissioned before the mid-1970s. Four ex-USN ships were sold to the Republic of China Navy 1994 and re-classed as Yung Yang-class minesweepers. They were still in active service in 2012. USS Implicit was decommissioned 30 September 1994 in Tacoma, Washington and was the last Aggressive-class minesweeper in US Navy active service.

• 
  A crewman monitors a mine detection and classification sonar console aboard the ocean minesweeper USS Conquest
• 
  A crewman monitors a mine detection and classification console aboard the ocean minesweeper USS Conquest
• 
  A paravane used for mine sweeping aboard USS Conquest
• 
  USS Illusive lowers an acoustical device during an exercise.
• 
  USS Illusive lowers a different acoustical device for training.
• 
  USS Illusive deploys a mineweeping cable.
• 
  USS Constant deploying the "mag-tail", used to simulate the magnetic field of a vessel, are towed behind the ship to trigger magnetic mines.
• 
  A remote control mine-seeking system, Super Sea Rover, aboard USS Conquest
• 
  USS Esteem deploys a marker, for marking mines or mine fields.

Units