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Bunker Oil - Marine Fuel Oil

Distillate Fuels, residual Fuels

What is Bunker Oil? - Description and Classification Overview

Bunker oil is generally any type of fuel oil used aboard ships. We can distinguish between two main types: distillate fuels and residual fuels.

Marine fuels are classified using the “Bunker ABC”:

  • Bunker A corresponds to the distillate fuel oil No. 2
  • Bunker B is a No. 4 or No. 5  fuel oil
  • Bunker C corresponds to the residual fuel oil No. 6

No. 6 is the most common oil, that's why "bunker fuel" is often used as a synonym for the No. 6 residual fuel oil which requires heating before the oil can be pumped. No. 5 fuel oil is also called “navy special”. No. 5 or No. 6 also furnace fuel oil (FFO).

In the maritime field another classification is used:

  • MGO (Marine Gas Oil): a distillate fuel oil (No. 2, Bunker A)
  • MDO (Marine Diesel Oil): a blend of MGO and HFO
  • IFO (Intermediate Fuel Oil): a blend of MGO and HFO, with less gasoil than MDO
  • MFO (Medium Fuel Oil): a blend of MGO and HFO, with less gasoil than IFO
  • HFO (Heavy Fuel Oil): a residual fuel oil (No. 6, Bunker C)

Marine fuels are traditionally classified according to their kinematic viscosity. This is a valid criterion for oil quality as long as the oil is produced by atmospheric distillation only. Today, almost all marine fuels are based on fractions from more advanced refinery processes and the viscosity itself says little about the oil's quality as fuel. Despite this, marine fuels are still quoted on the international bunker markets with their maximum viscosity set by ISO 8217 as marine engines are designed to use different viscosities of fuel. The density is also an important parameter for fuel oils since marine fuels are purified before use to remove water and dirt. Therefore, the oil must have a density which is sufficiently different from water.

Measurement data

Distillate Bunker Oils (ISO 8217)

Distillate FuelKin. Viscosity
at 50°C
at 15°C
DMX 1.4 ... 5.5
DMA 1.5 ... 6.0 < 0.890
DMB < 11 < 0.900
DMC < 14 < 0.920

Residual Bunker Oils (ISO 8217)

Residual FuelKin. Viscosity
at 50°C
at 15°C
RMA 30 < 30 < 0.960
RMB 30 < 30 < 0.975
RMD 80 < 80 < 0.980
RME 180 < 180 < 0.991
RMF 180 < 180 < 0.991
RMG 380 < 380 < 0.991
RMH 380 < 380 < 0.991
RMK 380 < 380 < 1.010
RMH 700 < 700 < 0.991
RMK 700 < 700 < 1.010


ISO 8217

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ISO 8217
Kin. vis. yes
Dyn. vis. no
Density yes
Multiple temperatures yes
Keywords Bunker oil, marine fuel, bunker C, heavy oil, viscosity, ISO 8217

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