Skip to main content

Most folks have never had an engine or engines fail at precisely the wrong time. Here’s how to continue being one of them.

You might prefer a dirty martini, but your engines are happiest when they have clean fuel to drink. Debris, sludge, microbes and water can render even the healthiest- motor hors de combat, often at the least opportune moment. It doesn’t have to be that way: High-end primary filters will keep solid gunk in the tanks from traveling into your engine and strip out any water in the fuel, and all they ask in return is simple maintenance; it’ll save you a lot of heartache down the road. Adding an inexpensive vacuum gauge will let you monitor the state of your filters at a glance, so you’ll know when to service them. What could be easier?

Dirty fuel has always been kryptonite for diesels. Contaminated fuel will silence even the mightiest compression-ignition engines; modern common-rail diesels are even more susceptible than direct-injected engines. Gasoline engines weren’t always so fussy—back in the days of carburetors, a little water or dirt would often pass right through—but today, with sophisticated high-pressure fuel-injection systems, they’re just as temperamental as diesels. So, the bottom line is, gas or diesel, if you want your engines to purr like kittens, supply them with pure fuel and nothing but. The best place to start is at the fuel dock.

Fill up at reputable marinas or boatyards, ones that filter fuel thoroughly.

Fill up at reputable marinas or boatyards, ones that filter fuel thoroughly.

Freshen Up

Clean fuel, both gas and diesel, if left untreated can grow its own contaminants when it stands too long in a boat’s fuel tank. But the same thing can happen ashore, in the fuel dock’s storage tanks. And those contaminants might come aboard with the fuel, especially if you top off at a less-than-savory fuel dock, one without properly maintained filters and water separators. Rather than save a few cents per gallon by buying no-name fuel from a guy with an eye patch, fill up at a reputable marina or boatyard, one that filters the fuel thoroughly before dumping it into your tanks. (I’ve found this to be a much bigger problem outside the U.S., where fuel delivery systems often involve 55-gallon drums on the back of a truck.)

Also avoid places that don’t sell much fuel; what’s in their tanks might have been there for months. Over time, water (typically from condensation) collects under the fuel, nourishing the growth of microbes and fungus. High-sulfur diesel, the kind we used to burn before we worried about exhaust gases and high fuel consumption, tended to kill these beasts before they could multiply, but today’s fuel has only a tiny fraction of the sulfur of yesteryear, too little for assassination purposes. Even if you kill the critters with a biocide, their bodies can still clog your filters.

Ethanol-blend gasoline deteriorates differently, but can cause just as much trouble, and the prime cause is, again, water. When blended gas mixes with sufficient water—and ethanol can draw in water from humidity in the air, skipping condensation—the fuel separates into its component parts of gasoline, ethanol and water. The loss of ethanol lowers the gasoline’s octane rating, and we know what happens when you try to run an engine on water. Water can also cause corrosion inside the engine. Ethanol can scrub gunk off the inside of the tank and carry it into the engine—this was a big problem when ethanol gas first arrived at the fuel dock, exacerbated by many gasoline-powered boats not having primary filters. Ethanol gas that’s been in storage for too long has likely phase-separated, unless the gas has been treated with a stabilizer. That’s why it’s so important to treat the gasoline in your tanks when you store your boat.

What do marinas and boatyards do to keep their fuel fresh? I called a dozen marine fuel retailers in New England, from Maine to Connecticut, and asked how they stabilize the gas and diesel in their storage tanks over the cold, damp winter, when not much fuel is pumped. Exactly no one said they treated their fuel for long-term storage in any way—they rely instead on whatever stabilizers were added at the refinery and on the filtration systems in their shoreside fuel lines. A couple of the marinas sold ValvTect fuel, which is treated at the refinery in a way that keeps it fresh for at least a year and usually longer, according to a ValvTect rep I spoke with. (ValvTect also requires that its dealers have their tanks tested regularly to ensure they’re free of water and contaminants.) Remember all this when you take on your first load of fuel in the spring—you might be getting six-month-old fuel, so make sure your filters are clean, and have spare cartridges on hand.

Good Filters Make Good Shipmates

Whatever happens in a shoreside fuel tank can happen in your boat’s tanks, too. Your best defense is to buy from a reputable fuel dock that gets a lot of business, and then burn the fuel before it gets old—in other words, go boating! You spent all that money on your boat, so use it. Engines run better when they get plenty of exercise (who doesn’t?), and frequent refueling keeps stale-fuel gunk from accumulating in your tanks. You don’t have to keep tanks topped-up, either—pump what you need for a couple of weekends. Just don’t let the fuel level get below a quarter tank. This means more trips to the fuel dock, but you’ll get docking practice and have fresher fuel on board. Add a stabilizer if you’re concerned; it’s cheap insurance. If you have gasoline engines, try to find a dock selling non-ethanol gas. Make sure your fuel fill caps are tight, and won’t let water seep in.

Even if you do all of the above, you must also have a high-quality primary filter mounted between the fuel tank and each engine. You probably already have primary filter(s), but if yours are old, or you’ve changed engines or you just want new and improved, think about replacing them. Racor is the industry standard, Dahl is also good, but there are others. The most important thing is to pick a filter that can handle your engine’s fuel flow at full throttle. Check with a mechanic, ideally one familiar with ABYC standards, when choosing new filters.

Make sure your primary filters are clean. Keep tabs on the presence of water and drain the filters when necessary. Many filters can be fitted with water sensors and alarms that warn you when water is present. Start each season with new cartridges or replace them at least once a year if you boat year-round—more often if necessary. Install the correct cartridges—most filters can take 10- or 30-micron cartridges, and some can take finer and coarser ones, too. (A micron is a millionth of a meter; a coffee filter is about 20 microns.) Ten micron removes more contamination, leaving only microscopic bits for the fine filter on the engine (typically 2 micron)—but your engine manufacturer may recommend a 30-micron primary filter cartridge. Read your manual and choose accordingly.

Primary filters for diesel engines often have clear bowls at the bottom, great for checking for water but not always up to ABYC standards if mounted in the engine room. If things get too hot in there, plastic bowls can melt. Meeting the standard requires a heat shield over the clear bowl, or a full metal bowl. Regardless of how it’s done, the filter has to withstand 2 1/2 minutes of exposure to fire without leakage. Filters for inboard gasoline engines have metal bowls—clear bowls are okay for outboards since they’re not in an engine room. (I’d go with metal bowls all around and make draining the filters part of scheduled maintenance.)

Suck It Up

A vacuum gauge will tell you when to change cartridges. Some primaries come with vacuum gauges, but adding one is simple and inexpensive. (Or you can go complex and install one with a gauge at the helm; that’s overkill, in my opinion.) Vacuum increases as the filter cartridges block up—just like you have to suck harder to drink lemonade through a straw with a crushed end. Run the engine wide-open under load with a clean filter cartridge to determine the baseline vacuum reading, then monitor the reading as you rack up engine hours. When the vacuum reaches the maximum recommended for the engine, as listed in the manual, change the filter. Dent Marine makes a good vacuum gauge, with a resettable pointer that records the highest vacuum, so you don’t have to squat in the engine room and watch the gauge until it maxes out. (For more on installing a vacuum gauge, see “How to Stay on Top of Your Boat’s Fuel System Health” at

Follow the above advice, and that of your mechanic, and you’ll probably never have to polish your fuel. Fuel polishing is a fancy term for circulating fuel through multiple stages of filtration until all the phase-separated contaminants, dead microbes, water and other nasties have been removed. Some boatyards have fuel polishers, or you can find an independent contractor online. If you want to start your own business, FuelTec builds a full line of commercial polishers, made in the U.S., including pneumatic systems which are safer for use with gasoline.

If you’ve got a big boat, with diesel engines and capacious tanks, and you don’t do much boating, you might also consider installing an onboard polishing system. Such systems are relatively inexpensive and will keep diesel fuel clean during extended storage. Plumbed independent of the engine’s fuel system, the polisher can be run on a regular basis when the boat is dockside. Enjoy your dirty martini while your fuel’s being cleaned.

A vacuum gauge will alert you to when you need to change cartridges.

A vacuum gauge will alert you to when you need to change cartridges.

The Baja Filter

When I was a kid, I read every book I could find about cruising to faraway places. Invariably, these cruises took place aboard sailboats, but also invariably, at some point there was mention of keeping the fuel clean in places way out at the edge of the charts. And the method of choice—this was long before fuel polishing became popular on recreational vessels—was the “Baja filter,” essentially a glorified funnel fitted with a fine screen to catch crud and, with luck, most of the water suspended in the fuel, which usually arrived in a 55-gallon drum on a horse-drawn cart. Why it was called a Baja filter, I don’t know—I don’t think the Baja California Peninsula has especially bad fuel. (Today, now that it’s a playground for the rich and famous, Baja fuel is probably among the cleanest on the planet. Moreover, I’m told you can find perfectly fine fuel all up and down the Mexican Pacific coast.)

Just for fun, I ran an internet search and found the modern version of the Baja filter. It’s made of aluminum and flows two gallons per minute through three filters—coarse, fine and water separator. The ‘net said it was made by Attwood—was being the operative word: I called Attwood customer service, and the rep said the Baja filter had gone to product heaven a few years ago. Too bad—I’d have bought one, just in case I ever find myself short of fuel somewhere west of nowhere. But there’s hope: Several companies, including Racor and FloTool, sell plastic filter funnels that cost less and probably work just as well. You might want to keep one on board, just in case.

This article originally appeared in the April 2020 issue of Power & Motoryacht magazine.