In 1975, after conducting deep ocean surveys since 1958, USNS Michelson was declared unfit for sea and taken out of service. The reasons for this are obscure but it's likely the ship failed a Coast Guard inspection of the hull. The ship's mechanical systems could be repaired or replaced but the hull's integrity was another matter. Michelson had experienced cracks in the deck plating during the mid-1960s. A replacement for the ship was found but not placed into service until 1978. The new home for Oceanographic Unit Three was the C4-S-1sa vessel originally SS Canada Mail, renamed USNS H. H. Hess (T-AGS-38), named for geologist Harry Hammond Hess.

Launched in 1964, SS Canada Mail was longer, faster and had a deeper draft than the VC2-S-AP3 Victory vessels that became survey ships Bowditch, Dutton and Michelson. It also carried up to 12 passengers on voyages from Pacific Northwest ports to East Asia.

A "break bulk" vessel, SS Canada Mail had a short useful life, overtaken by the containerization of ocean freight. By 1974 it was in the "reserve fleet" at Suisun Bay in California.

Oceanographic Unit Three, previously aboard USNS Michelson, was transferred to H. H. Hess. Alterations resulting from the conversion were largely internal and below the waterline. Outwardly, the vessel appeared little changed.

Michelson's electronics including the SASS array sonar were installed aboard the ship. The first surveys were conducted in the Pacific in June 1978.