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At 0200 hours on June 3, 1972, America’s last POW rescue attempt began when Lt. j.g. Mark Lutz guided a Mark 6 swimmer delivery vehicle (SDV) away from the submarine Grayback, submerged just outside the mouth of North Vietnam’s Red River. Lutz was to deliver two U.S. Navy SEALs, Lieutenant Melvin Dry and CWO Philip Martin, to a small island approximately 4,000 yards away to await the arrival of two U.S. POWs who were to escape their camp by boat. It was a daring secret mission made possible by the only U.S. Navy submarine capable of covertly delivering Marines or Special Forces to an enemy shore.

Originally designed to house the Regulus sea-to-surface missile, the Grayback was modified in 1967 to transport a SEAL team and their equipment, including the secret “Swimmer Delivery vehicle” (SDV). (U.S. Navy)

Commissioned in March 1958 as the lead unit of Regulus cruise missile carrying submarines, the advent of Polaris missiles led to Grayback’s modification and 1968 re-commissioning as an amphibious personnel transport. It received the designation LPSS-574 (for Submarine, Landing, Personnel). The alterations included lengthening the hull to accommodate two auxiliary fuel tanks forward of the engine room, extending the sail by 10 feet and converting the missile magazines on the bow into a dry deck shelter capable of embarking up to 67 troops and two SDVs. The shelter had a special bulkhead to enable underwater launch and recovery of the troops and SDVs. It also had a decompression chamber installed in the starboard missile hangar location. However, SEALs reported that the shelter’s designation as “dry” was at best an exaggeration as it was always damp and humid. Another negative aspect of its design was the air venting, which routed air from the sub’s head through the shelters when the submarine snorkeled to re-charge its batteries.

The hull expansion and alterations reduced Grayback’s speeds by about 2 knots. Although slow when submerged, it was extremely quiet when operating on batteries. Its comparatively small vertical size made it ideal for operations in the relatively shallow South China Sea and coastal waters off Vietnam.

Most of Grayback’s missions during and after Vietnam remain classified. It reportedly conducted its first covert mission of the war in 1971, but the recent release of details about Operation Thunderhead ensures that it will be best known for its key role in that POW rescue attempt, even though it failed. Unexpected currents and poor navigational information precluded Lieutenant Lutz reaching the island before the SDV’s batteries ran out. The SEALs had to abandon it and be rescued by helicopter, sinking the SDV before they departed. Lieutenant Dry was killed while trying to return to Grayback by helicopter, and the rescue mission had to be aborted.

Grayback remained active in Asian waters for another 12 years before its aging equipment and a lack of spare parts led to its 1984 decommissioning. It was sunk as a target off Subic Bay, Philippines, on April 13, 1986. However, its activities during and after the Vietnam War convinced the Navy of the need to retain Grayback’s capabilities. The development of Detachable Dry Deck shelters began in the 1970s, and the first Los Angeles-class nuclear-powered submarines were modified to carry them in 1987. Grayback may be gone, but its capability to support special operations forces lives on in the design specifications and construction of every U.S. Navy submarine built since the 1990s.

USS Grayback 

Max. speed (surfaced):  14 knots

Max. speed (batteries): 13 knots

Cruising speed:  4-6 knots

Surface propulsion:  Diesel-electric

Displacement (surfaced):  2,768 tons

Displacement (submerged):  3,638 tons

Crew:  12 officers/77 enlisted

Armament:  16 21-in. torpedos


First published in the October 2011 Vietnam magazine. Subscribe today!