Ferries, submarines and sailboats are common on the Thames River, but not 510-foot guided-missile destroyers.

On Monday the U.S. Navy destroyer famous for helping foil the hijacking of an American container ship by Somali pirates last year was spotted turning around on the Thames near Electric Boat.

The USS Bainbridge (DDG 96) completed its mid-morning maneuver before heading back down river toward the open ocean.

The Bainbridge did not pull into the EB shipyard or go to the Naval Submarine Base or even notify the local Coast Guard station of its arrival, according to officials at those sites.

A spokesman for the Navy's Second Fleet finally solved the mystery — the Bainbridge was here to use the local degaussing range.

Degaussing is the process of removing or neutralizing a magnetic field. Housed in a building at the former Naval Undersea Warfare Center at Fort Trumbull, the local range measures the magnetic fields of steel-hulled ships.

Sean Sullivan, former commander of the Naval Submarine Base in Groton, said Monday that he was aware of the cables running beneath the channel when he was stationed at the base.

A ship's magnetic field "will influence the equipment on the ship adversely, or make it not work as well as it's supposed to, so the Navy maintains degaussing ranges," Sullivan said, adding that the local range merely measures magnetic fields but does not adjust them.

"To get rid of it, you have to put the ship in a dry dock," he said.

The Bainbridge, which is homeported at Naval Station Norfolk in Virginia, was off the northern Somali coast last April when the American container ship MV Maersk Alabama was seized by Somali pirates and the ship's master, Capt. Richard Phillips, taken hostage.

One of the pirates boarded the Bainbridge and demanded safe passage for himself and the others in exchange for Phillips' release. While he was aboard, Navy SEAL snipers shot and killed the remaining three pirates.

The Bainbridge was off the coast of Scotland last month to participate in Joint Warrior, a two-week multinational training exercise. It was nearby Monday as part of routine operations.

j.mcdermott@theday.com