Marine Diesel Fuel -What You Need To Know

Modern marine

Compared to gasoline, diesel fuel has some advantages in the boating world, but it needs careful attention too. There’s been a lot written in recent years about the effects of ethanol-laced gasoline in the boating world, and all of the issues—both real and perceived—related to how it may impact fuel systems and the performance and reliability of marine engines using it. Not nearly as much much has been said about diesel fuel, which on a quieter level has undergone changes as well. First, the EPA has required significant reductions in the amount of sulfur in diesel fuel. We’ve learned over time that the amount of sulfur in the fuel is directly linked to the amount of pollution produced when the fuel is burned in an engine. Diesel engine exhausts emit soot and particulates, and oxides of nitrogen (NOx) which contribute to smog and acid rain. Additionally, there are some hydrocarbons, some carbon monoxide, and other small traces of hazardous air pollutants.

Modern marine diesels like this turbocharged Yanmar have a safety edge over their gasoline counterparts because of the lower volatility of diesel fuel. But diesel does come with its own set of challenges to be managed

Along with the reductions in sulphur in diesel fuel, diesel engines themselves have gone through some significant changes during the last 10 to 15 years. With the advent of technology like “common rail” ultra-high-pressure fuel injection systems and the addition of electronic engine controls and injection systems, diesels are emitting fewer emissions and performing better than ever.  The days of black smoke clouds belching from diesel exhaust pipes are rapidly coming to a close.



Walk down the “elixir aisle” at any major automotive or marine supply store and you’ll find a magic potion for just about any engine oil or fuel shortcoming. I’m frequently asked what I think of these additives. As far as gasoline is concerned, my answer is always the same: You really don’t need any additives if your engine coolant is changed as required, and if you’re using quality engine oils and fresh fuel. The exception would be to use a stabilizer in fuel that might be stored for over a month or so. Gasoline is more highly refined, and aside from issues with water contamination and ethanol-related woes, generally more stable than diesel fuel.

Diesel is a heavier, more viscous fuel, and needs some additives to maintain constant properties over the course of a year with four seasons. Also, diesel has a rather unique property that you never have to worry about with gasoline: It will support microbial growth, commonly referred to inappropriately as “algae.” Let’s end that misnomer right here and now. Algae is basically a plant that needs photosynthesis to exist and proliferate. That means you would need sunlight in your diesel fuel tank to create algae. Trust me, if you have sunlight getting into your diesel fuel tank, you have bigger problems than plant growth inside the tank.

Other microorganisms can exist quite happily in the dark however, and in fact will grow rather rapidly at the interface between any water in your fuel (which will always settle to the bottom of the tank) and the fuel itself. You can experiment, as I have, by adding water to fresh, clean fuel and leaving it in a clear glass jar on a shelf in a closet. Chances are that within three to six weeks you’ll see stringy black goo in the fuel. This is not algae, but it is microbial growth, and it will gum up the inside of a fuel injection system in short order.

A foul 30-micron element emerges from a Racor fuel filter. Note the almost black color of the fuel in the bowl. The microorganisms have been hard at work in this fuel system.

What’s needed to control this natural occurrence is a biocide additive and of course high-quality filtration.

Contributing to the issues with diesel fuel is the fact that it is more likely to be stored for longer periods of time than gasoline. Diesel is in fact often in storage for months before it reaches its final destination, your engine. Additionally, it may get moved from one storage tank to another, and pick up water and other contamination along the way. Temperature change also impacts the fuel; it will tend to thicken in cold weather without additives.

Also, as part of the sulfur content reduction already mentioned, the matter of the fuel’s “lubricity” comes up. Diesel injection systems are designed and built to extremely close mechanical tolerances down to the 0.0001 inch (0.00254 millimeter). This in part is how the high fuel pressures we see today can be achieved. Since the inside of your diesel fuel system is actually lubricated and cooled by the diesel fuel itself, the matter of lubricity is of significant concern. So, lubricity additives are a good idea.

Another property exclusive to diesel fuel is a phenomenon known as “waxing.”  In the US there are essentially two types of diesel fuel available, No.1, which is essentially kerosene, and No. 2, used for home heating and diesel engines. As seasons and average ambient temperatures change throughout the year, fuel distributors alter their blends and additive ratios to help address this problem. As the super high-pressure common rail injection systems have become mainstream in the marketplace, common practices of the old days, such as adding gasoline or more kerosene to the No. 2 diesel during winter months, are now strictly prohibited by the engine manufacturers. Thousands of dollars in damages can occur due to the lubricity issues already mentioned. The best advice here is to use only additives recommended by your engine manufacturer.



Over the last 15 years or so, biodiesel and biodiesel blends have become available in some parts of the US. It can be made from a variety of sources, including used cooking oils, rendered animal fat, soybeans, and peanuts. Green movement folks of course love the idea, while some studies suggest that the energy used to produce the fuel negates its claims for carbon footprint reduction. But what about your engine? Are there any real issues there?

Biodiesel acts as a bit of a solvent, and can actually loosen up any accumulated contaminants that typically settle in the bottom of your fuel tank. These in turn will get pulled into fuel filters and may necessitate more frequent filter changes until things get cleaned out. That said, there was initially a lot of worry among manufacturers that because of this solvent characteristic, and fears of premature injection system component failure due to a lack of lubricity were rampant. In the final analysis, most of the engine manufacturers have approved the use of biodiesel blends, but that said, it is of paramount importance to double-check your engine manufacturer’s requirements before use. Is the ratio 5%, 10%, or 20%? Be certain on this, and don’t assume that a manufacturer’s percentage limit on one engine is acceptable across their full line of engines. That could end up being a very expensive assumption.



Because of the enhanced probability of water ending up in your diesel fuel, a top quality water-separating fuel filter is extremely important for any diesel engine. Filtering particulate matter is important too. However, it’s also important not to overdo it here. Engine manufacturers will typically define the micron level they want for both primary and secondary fuel filters. Basically this defines the size particle the filter element can filter down to in thousandths of an inch. A first reaction to this with many boaters is that if a 10-micron level filter element is good, then a 5-micron element must be better.  Don’t do it! It's possible to create a scenario that will actually end up restricting fuel flow in your system, which will cause a “lean out” condition that can cause extensive damage. Follow manufacturer’s recommendations here, too.



My office is at a marina where some pretty high-end boats get stored, and every now and then I see the fuel polishing truck in the parking lot. Basically what we’re talking about here is a system whereby the fuel in your tank(s) gets force-pumped through a series of filters, and in some cases a centrifuge,  to remove all traces of water and contaminates. Boats with large storage capacities that don’t get used too often are prime candidates for this sort of service. Larger yachts will often have their own on-board fuel polishing systems. Is fuel-polishing worth it? My answer is that it depends on how much fuel you’re storing. If you can store 1,000 gallons or more and don’t replenish the fuel frequently, I’d say yes, it may be worth it. Less than that, probably not, as long as you run your engine fairly often, stay on top of your filter changes, and bleed any water from your separators religiously.

Diesel treatments are necessary, but which ones? There are dozens of competing products on the market, many of them with the same ingredients. Consult your engine manufacturer or an experienced mechanic for advice on which products are recommended for your engine and locale.

In conclusion, understand that diesel fuel is very different than gasoline in many ways, but it’s the fuel of choice for a large sector of the transportation sector globally. In the recreational boating realm, it’s considered safer than gasoline for inboard engines because of its comparatively low volatility. And diesels are about 30 to 35% more efficient than their gasoline-fueled counterparts. Just remember that with today’s newer diesel engines, it’s imperative to follow the manufacturer’s recommendations to the letter when it comes to things like fuel additives, engine oil, and cooling system contents. Don’t let the resident dock know-it-all give you a bum steer on how to handle the things mentioned here, as the cost could be way more than you ever imagined.