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Marine Fuel Additives

Marine Fuel Additives

An Ounce of Prevention

Do you really need that other stuff you pour into your fuel tanks?

Do you use a fuel additive? If so, why, and how did you choose it? Are you sure you need it? If you haven’t considered these questions you could be damaging your fuel system or wasting your money. Or both.

There are as many opinions about fuel additives as there are about how to catch fish, but if you’re trying to winnow fact from hype, start here: Despite how we love to hate oil companies, domestic gasolines and diesel fuels are of reliably high quality. In fact, if you live in a moderate climate, run your boat regularly, and properly maintain your filters, you may not need any additive. 

But even if you do, you need to know which one. Take octane or cetane enhancers. (Octane is a measure of gasoline performance; cetane is the diesel equivalent.) For anything but extremely high-compression gasoline engines, pump gasoline is fine, and an octane enhancer will have no effect. Similarly, all diesel engines operate at peak performance on #2 Diesel—what’s sold at your local fuel dock. Skip the enhancers.

What about ethanol-related problems in gasoline? Can additives prevent them? Most such problems are caused either by the deterioration of incompatible components, dissolved sludge and other tank contaminates, or the water that ethanol attracts, usually via the fuel-tank vent. Most boats manufactured after 2000 use components compatible with ethanol. As for sludge, if you’ve been using ethanol fuel for some time, as we all have, any sludge has likely already been dissolved and burned in your engine. Water? It can be removed by a water-separating fuel filter, and whatever does manage to reach the tank is usually combusted without causing damage. (Most fuel additives don’t remove water anyway, just disperse it so it can be combusted.) So most gasoline-powered boats don’t need an additive—with one exception: If they sit idle for more than a month or two—especially in cold, damp weather—they’ll need an additive that prevents the oxidation of fuel and fuel-system components. Without one, gum, varnish, and other deposits can form.

Diesel fuel is even more susceptible to storage-related issues, so adding a fuel stabilizer before layup is doubly important. You probably know such contamination is a threat, but you may not appreciate just how unstable diesel fuel is. Writing on the Web site, Gary Morgan cites a University of Idaho study in which diesel fuel was found to degrade 26 percent after just 28 days of storage. He adds, “The shelf life of diesel fuel is approximately 100 days from the day it is refined [emphasis mine], then it begins to break down and form sedimentation. The breakdown process causes diesel to cluster up and fall out of solution, dropping to the bottom of the tank. This forms diesel sludge.” Sludge is also known by its technical term, asphaltene.

Other diesel contaminates include water, microbes, and sediment. Microbes need water to grow, so an effective fuel-water separator with a high-micron filter element can kill three birds with one stone. What microbes do survive this gauntlet can be killed by a biocide additive, after which they’re either removed by the filters or pass harmlessly through the engine. How can you know microbes are present? Realistically you can’t, which is why you should add a biocide once a year as a prophylactic. But follow directions; the active ingredients in most biocides are corrosive in high concentrations.

What about sludge? Since biocides have no effect on it, what does? Some additives claim to contain solvents that dissolve asphaltene. I’m skeptical, but even if they do, you’re left with a bunch of suspended gunk that will wreak havoc on your filters. I never found an additive that worked on my boat. The only thing that did was someone climbing into the tanks and attacking the stuff with a metal scraper. Sludge is tough!

So how do you pick an effective additive? Alas, we’re mostly left to sort out the competing claims ourselves, which is why your engine manufacturer/distributor/service facility is a good place to start. But even with their recommendations in hand, you need to do your homework. Be wary of products that purport to solve multiple problems; choose one targeted to your specific issue. Read and follow the instructions, and don’t over-apply. Even the best additives can damage your fuel system if applied incorrectly. Finally, don’t wait for a problem to announce itself like I did. Regularly examine your filter elements for sludge, look for water in your separator’s settling bowl, and watch for signs of fuel discoloration. To paraphrase, an ounce of prevention is worth a quart of additive.

This article originally appeared in the April 2014 issue of Power & Motoryacht magazine.