Bunker fuel is any fuel used on board a ship.
The most commonly used type is residual fuel oil bunker or Bunker C.
In addition, the bunker fuel which can be used in the Sulfur Emissions Control Areas (SECAs) must have a maximum sulfur content of 0.1%, which requires either using Bunker A or a Ultra Low Sulfur Fuel Oil (ULSFO). SECAs currently include the Baltic Sea, the North Sea, the English Channel, and the area up to 200 nautical miles off the North American coast plus a zone in the Caribbean.
From January 1st, 2020, bunker fuels used globally outside the SECA regions will be restricted to having a maximum sulfur content of 0.5%.
Grades of Bunker Fuel
Gas oil range bunker fuel, typically called marine diesel or marine gas oil.
Low-viscosity vac resid range bunker fuel. Typically cut with some lighter material (VGO) to reduce viscosity to the point that it will flow without heating.
The most common form of bunker. Composed primarily of vac resid range material, with a high viscosity that requires heating in order to pump. Typically sold at several viscosity specifications: 180 centistoke, 380 centistoke, or 460 centistoke, with 380 being the most common grade. The viscosities are measured at 50C, the typical heated temperature of the fuel.