Black Oil, also known as "Bunker C", was the fuel that powered the Navy from the days that the Navy converted its ships from coal-fired up through World War II. Then it was replaced with Navy Special Fuel Oil, which was a blend of Bunker C and somewhat lighter fuels.
Both Bunker C and NSFO were heavy fuels. They were hard to pump unless they were warm. So ships back in the day had steam lines running through their fuel tanks so the fuel could be warmed up prior to being transferred. The fuel was also hard to burn; the boilers had to have steam lines running to their burners, for the steam would be used to atomize the fuel in order for it to be burned. NSFO was not suitable for use by marine diesel engines; the Navy had to supply its new diesel LSTs with a different fuel, known as "Distillate Fuel, Marine" or DFM. DFM was also needed as the construction of gas turbine warships was on the horizon and NSFO was unsuitable for gas turbines.
In the 1960s, naval oilers had to be able to supply four types of fuel: Aviation gasoline for the A-1s, S-2s and E-1s, jet fuel (JP-5) for the rest of the airplanes and helicopters, NSFO and DFM. The S-2s were replaced by the S-3s and gasoline was also phased out, much to the relief of everyone who had to deal with it. The decision was made to phase out NSFO, and, if JP-5 jet fuel wasn't significantly more expensive, DFM probably would have also gone away. DFM didn't need to be heated, so the steam lines in the fuel oil tanks were capped off. It didn't require steam atomization, which saved on steam losses. DFM basically wasn't a heck of a lot different from the diesel fuel that came out of a truck station's pump. Gas turbine ships were more finicky about contaminants in the fuel; they had large fuel purifiers whereas the only purification done on steam ships was to make sure any water in the fuel tanks from ballasting operations had settled out and then had been stripped out.
The steam heating lines in the tanks, as I mentioned, were capped off. This had tragic consequences in 1983 aboard the USS Ranger, when a fire broke out in one of the engineering spaces. The fuel for the fire came from an old steam line in the fuel tanks that had corroded away from the inside. In essence, several fuel tanks were linked together by the rusted-out steam line, which fed fuel into the fire. Six sailors were killed in the fire and it became a priority to remove the steam lines during overhauls.
DFM is still in use.
In a future post, I will write about what it took to get a steam-powered warship ready to sail.