Originally Posted by Nyogtha
It's good to find folks willing to read still about these days.
Long haul trucking and railroads use diesel instead of gasoline engines for a reason - the higher density of diesel contains more energy per unit volume.
Marine diesel is not typically the same as highway diesel for this reason; the push for higher cetane ratings and lower sulfur in highway diesel reduces the aromatics content and therefore the density, but also decreases the resistance to ignition.
Marine fuel oil (and heavy fuel oil in general) has higher density - you can see some of the heaviest grades are "heavier than water" - if they were liquids at 15�C, and even the heaviest grade has a maximum pour point of 30�C / 86�F so it's still not really "tar / asphalt". There is a lot of energy per unit volume available in heavy fuel oils for propulsion across the oceans (or generating electricity in a fuel oil powered generator as another example).
FWIW, the term "bunker fuel" is a holdover from when oceangoing vessels had actual coal bunkers with men shoveling coal into the furnaces.
As an historic note, it's also worth mentioning that railroads-or at least out in the Western US-were once heavy consumers of Bunker C also. Of course, most of it went to steam engines(Union Pacific's surviving steam fleet is oil fueled) but there were a handful of notable other experiments that used Bunker C. The Union Pacific made several "oil turbines" that burned it.
The start-up procedure for a cold oil-fired locomotive-to me-is quite interesting. You have to have a small amount of steam in the boiler both to heat the oil to where is can be easily transferred from the tender to the firebox, and then to atomize it at the burner tip. I recall reading an account written by a volunteer at a tourist railroad of starting one-basically they stuck a kerosene torpedo heater in the firebox for a few hours to raise ~5psi of steam, which was enough to light the main burner. Back in the day, at least the pilot on the main burner would have been left on all the time, which would have been enough to maintain pressure overnight or even a couple of days. The boiler would have only been allowed to cool completely for major work, such as work on the boiler.