Bunker Fuel
Bunker fuel is technically any type of fuel oil used aboard ships. It gets its name from the containers on ships and in ports that it is stored in, called bunkers. Bunker A is No. 2 fuel oil, bunker B is No. 4 or No. 5 and bunker C is No. 6. Since No. 6 is the most common, "bunker fuel" is often used as a synonym for No. 6. No. 5 fuel oil is also called navy special fuel oil or just navy special, No. 6 or 5 may also be known as furnace fuel oil (FFO said ef-ef-oh), and its high viscosity (thickness) requires it to be heated, usually by a reticulated low pressure steam system, before it can be pumped from a bunker tank.

Typically motor ships (as opposed to steam ships) use diesel when entering and leaving port, and switch over to heavy fuel oil once on passage.