Marine fuel is the fuel burned by oceangoing ships, ferries, and other large vessels to power their engines. It ranges from thick, tar-like residual oils to lighter diesel-type fuels, and the industry consumes enough of it to move over 90% of international trade. What makes marine fuel distinct from the gasoline or diesel you’d put in a car is its origin at different stages of the oil refining process, the international standards it must meet, and the increasingly strict environmental rules governing its sulfur content and greenhouse gas emissions.
Residual vs. Distillate Fuels
Marine fuels fall into two broad families based on how they’re produced at a refinery: distillate fuels and residual fuels.
Distillate fuels are the lighter, more refined products. They flow easily, burn relatively cleanly, and behave a lot like the diesel used in trucks and generators. Marine gas oil (MGO) and marine diesel oil (MDO) are the most common distillate grades at sea. They’re used in smaller vessels, in port areas where emissions rules are strictest, and as a backup fuel on larger ships.
Residual fuels are what’s left over after the lighter components have been distilled away. Heavy fuel oil (HFO) is the most familiar example. It’s extremely thick, almost like warm asphalt, and must be heated before it can flow through pipes or be burned in an engine. For decades, HFO dominated international shipping because it was the cheapest option: essentially the bottom-of-the-barrel byproduct that refineries needed to sell. Residual fuels can have a viscosity up to 700 mm²/s (measured at 50°C), compared to a maximum of about 6 mm²/s for a standard marine distillate. That difference is enormous and explains why large ships need onboard fuel-heating systems.
How Marine Fuel Is Standardized
All marine fuels sold internationally must meet the ISO 8217 standard, which sets limits on viscosity, density, flash point (the temperature at which fuel vapors can ignite), and contaminant levels. The standard defines a series of letter-coded grades. On the distillate side, grades like DMA and DMZ cover fuels with maximum densities around 890 to 900 kg/m³ and minimum flash points of 60°C. On the residual side, grades run from RMA 10 (a relatively light residual oil with a density cap of 920 kg/m³) up to RMK (a very heavy grade that can reach a density of 1,010 kg/m³).
These specs matter because a ship’s engine is designed for a specific fuel range. Loading the wrong grade can cause injector damage, incomplete combustion, or even engine failure mid-ocean. The flash point minimum of 60°C for most grades exists purely for safety: fuel stored in a ship’s tanks needs to resist accidental ignition.
The Sulfur Rules That Changed Everything
The single biggest regulatory shift in marine fuel came on January 1, 2020, when the International Maritime Organization’s “IMO 2020” rule took effect. It slashed the maximum sulfur content allowed in fuel burned outside designated Emission Control Areas from 3.5% down to 0.50% by mass. That one change forced the entire global fleet to switch away from traditional high-sulfur heavy fuel oil or install exhaust gas cleaning systems (scrubbers) to strip sulfur from their emissions.
The rule created a new category of fuel that now dominates the market: very low sulfur fuel oil, or VLSFO, blended to come in at or below the 0.50% sulfur threshold. VLSFO is typically a blend of residual and distillate components, giving it properties somewhere between old-style HFO and clean marine diesel.
In certain coastal regions, the limits are even tighter. Emission Control Areas (ECAs) cap sulfur at just 0.10%. Five ECAs are currently in force: the Baltic Sea, the North Sea, the North American coastal zone, the U.S. Caribbean Sea area, and, as of May 2025, the Mediterranean Sea. Two more were designated in 2024 covering the Canadian Arctic and the Norwegian Sea, with a proposal for the North-East Atlantic approved in early 2025. Inside these zones, ships typically burn ultra-low sulfur distillate fuels or use equivalent emission controls.
Alternative Fuels Gaining Ground
Conventional petroleum-based fuels still power the vast majority of ships, but the industry is under pressure to decarbonize. The IMO’s 2023 greenhouse gas strategy calls for net-zero emissions from international shipping by or around 2050, with checkpoints along the way: at least a 20% reduction by 2030 (striving for 30%) and at least 70% by 2040 (striving for 80%), all compared to 2008 levels. Zero or near-zero emission fuels are expected to supply at least 5% to 10% of the energy used by international shipping by 2030.
The leading alternatives each come with trade-offs. Liquefied natural gas (LNG) is the most cost-effective low-carbon option available today and is already used by a growing segment of the fleet. It produces significantly less sulfur and particulate emissions than oil-based fuels and reduces carbon output, though methane slip (unburned gas escaping the engine) limits its climate benefit. Methanol is gaining traction because it’s liquid at room temperature, making it easier to store and handle than LNG. Several major container lines have ordered methanol-capable newbuilds. Ammonia offers the deepest potential emission cuts. A life cycle analysis of a large tanker found that wind-powered ammonia produced in regions with strong renewable resources could cut well-to-wake greenhouse gas emissions by 70% to 80% compared to conventional fuels. Its drawbacks are toxicity, the need for entirely new fuel-handling infrastructure, and higher costs for now.
How Ships Actually Get Their Fuel
The process of delivering fuel to a ship is called bunkering, and it happens in one of three ways. The most common at major ports is pipeline bunkering (also called shore-to-ship), where fuel flows from large storage tanks on shore through pipelines directly into the vessel’s tanks. It handles big volumes efficiently and is the most tightly regulated method.
Ship-to-ship bunkering uses a smaller tanker vessel that pulls alongside the receiving ship, often at an anchorage or offshore. This gives flexibility to vessels that can’t or don’t want to dock, but it requires careful safety protocols to prevent spills. Truck-to-ship bunkering is the simplest approach: tanker trucks park on the quay and pump fuel aboard through hoses. It works well for smaller quantities but is too slow for large ocean-going vessels that may need thousands of tons at a time.
A large container ship can carry 10,000 to 15,000 tons of fuel, and bunkering costs often represent the single largest operating expense for a shipping company. The price of marine fuel fluctuates with crude oil markets, sulfur regulations, and regional supply, which is why the choice between fuel types, scrubber installations, and alternative fuels is as much a financial decision as an environmental one.