Photo: "Pipe Dreams"

These valves in "shaft alley" of USNS Northern Light, a commercial breakbulk steamship, controlled water flow into the propeller shaft to keep it cool.

"Breakbulk ships" were characterized by large open hatches and fitted with boom-and-winch gear or deck cranes and primarily used in ports. USNS Northern Light, originally known as SS Mormacbay, served in the commercial trade industry from 1961 until 1980, when she was acquired by the Navy and eventually moved to Suisun Bay.