March 19, 2012: The U.S. Navy is sending four more Avenger class mine hunters to the Persian Gulf, along with four more minesweeper helicopters. This is a precaution in case Iran makes good on its threat to interfere with ship traffic in the Persian Gulf. Naval mines would be a very effective weapon for this sort of thing.

The Avengers are 72.3 meter (224 foot) long ships that draw only 4.8 meters (15 feet) of water, enabling them to operate close to shore. They are armed with two .50 cal. (12.7mm) machine guns, two 7.62mm machine guns, two 40mm automatic grenade launchers, and have a crew of 84. The four Avengers currently in the Persian Gulf operate out of Bahrain. Another four are based in Sasebo, Japan. The other six are based at San Diego, California.

The U.S. Navy recently upgraded the sonars on its Avenger class mine hunter ships. The new AN/SQQ-32(V)4 mine hunting sonar improves the ability of the sonar to spot mines on sea bottoms cluttered with other stuff (natural or manmade). In many parts of the world shallow coastal waters are used as a dumping ground for junk that won’t float ashore. This has been found to help hide bottom mines. The Avengers have also received new engines. The four original diesel engines in each Avenger have never been very reliable. With their new engines the Avengers can still move at up to 27 kilometers an hour. Normally, however, the Avengers move much more slowly (3-4 kilometers an hour) when searching for mines. The Avengers also got improved hydraulics and new mine destruction systems.

The upgrade is also part of an attempt to deal with delays in the new LCS class ships or at least the ones equipped for mine hunting. So for the last six years the navy has been hustling to refurbish its existing Avengers. The 3,000 ton LCS ships are designed for minesweeping (and a lot of other jobs) but the 1,400 ton Avengers specialize in minesweeping. Built mostly of wood and very little iron the fourteen Avengers entered the fleet between 1987 and 1994 and all are still in service. The upgrades will enable the Avengers to remain in service at least until 2016 and probably until the end of the decade.

The navy also had a dozen smaller Osprey class coastal mine hunters (900 tons displacement, crew of 51), but these were all given away to foreign navies and replaced by the LCS (which has been delayed) and new minesweeping helicopters (which are on schedule).