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Let’s Get Fuel-ish

Fuel tanks don’t last forever, and replacing them means making some decisions.

Live long enough and one day you might need a hip, maybe both of them, replaced. Keep your boat long enough and eventually you might have to replace one, or all, of your fuel tanks. I’ve been through both ordeals, and I can’t say which is worse. Extracting a leaky fuel tank can mean many hours, maybe days, of expensive boatyard labor. And then you have to put the new one in, and rebuild everything, while the meter keeps right on spinning. Sure, my doc charges about ten grand an hour, but he’s in and out in 90 minutes. Talk about Hobson’s choice.

Workers at Tiara Yachts install a plastic fuel tank in a new 38 LS.

Workers at Tiara Yachts install a plastic fuel tank in a new 38 LS.

Luckily, it’s a choice most boat owners won’t have to make, unless they keep their boat for a long time, or decide to buy and rehab an older vessel. Most fuel tanks last for 15 years, maybe 20, if properly installed and maintained. But there’s always the fly in the fuel: For example, low-carbon mild steel, or “black iron” tanks, once favored by many Far East builders, are susceptible to rusting if the exterior paint coating isn’t maintained, or if water collects in the tank and corrodes it from the inside out. The classic scenario, and one I’ve experienced myself, results from water leaking through the deck or around the fuel fill, and puddling on the tank top. Since the tanks are usually tucked under the side decks and with little clearance between tank top and deck beams, only the most scrupulous owner will discover, and dry up, this damaging puddle. Usually it goes unnoticed for years—the sides of the tank look great—until the tank top rusts through, allowing water to leak into the tank and, eventually, fill the fuel filter, ruining a nice day of boating.

Keep a black-iron tank dry and epoxy-coated on the outside and free of moisture on the inside, and it’ll have a long life. Under the new USCG/EPA regulations effective in 2012, carbon-steel tanks are approved for gasoline only if hot-dipped galvanized both inside and out. (Once steel is galvanized, it’s no longer black iron. That term originated in the plumbing industry to differentiate galvanized from plain steel pipes, which were usually painted black.) Galvanized tanks are okay for gas, but diesel fuel attacks the zinc. Non-galvanized tanks are okay for diesel, which is less likely to accumulate water in the tank than ethanol-blend gasoline, and keeping black-iron tanks from rusting on the outside isn’t rocket science. But there are better options for both fuels: aluminum and plastic.

Foam? It’s for Beer

Corrosion isn’t limited to black iron. Any metal, even aluminum and stainless steel, will crevice-corrode if water gets trapped against it without exposure to fresh air. Aluminum is corrosion-resistant in air (the metal forms a thin skin of aluminum oxide, which protects against further deterioration), but will corrode if water seeps between the tank and whatever it’s sitting on, or under the strapping that holds it in place. Insulating the tank with high-density neoprene, plastic or another non-moisture-absorbing material, both under the tank and under the strapping, will prevent this if the insulating material is completely bonded to the tank. Most installers glue the insulator to the tank, typically with 3M 5200 polyurethane adhesive/sealer or something similar.

Smaller boats—center-consoles, for example—often carry an aluminum fuel tank that’s foamed in place under the deck, frequently in a coffin compartment on the centerline. No matter how well the compartment is sealed, water eventually gets between the foam and the tank, creating, over time, pinholes in the aluminum that leak gas into the foam and create a sort of Molotov cocktail just waiting for a spark. The good news is, accessing these tanks isn’t usually too difficult, although it sometimes means removing the console. Getting the foam away from the tank can be problematic; a longshoreman’s hook does a pretty good job of ripping it out. Don’t be surprised if it smells like gas. Once the tank is out and the compartment cleaned, install a new tank, and forget the foam; the next guy to remove the tank will thank you. Foam floating on a schooner of beer after the job is done is okay. But not around the fuel tank.

Foam is not totally verboten, though. According to Brian Goodwin, technical director of the American Boat and Yacht Council, ABYC standards don’t preclude foaming-in tanks, and some boatbuilders still do it, but other means of securing the tank are required—straps, brackets, etc. Also, the foam has to adhere completely to the tank to keep moisture away, with a bond that’s stronger than the foam itself, and it has to be non-absorbing. Closed-cell flotation foam usually meets the standards, Goodwin says.

And there’s another thing: According to an NMMA position paper, “The Negative Affects [sic] of Ethanol on Recreational Boat Fuel Systems,” ethanol-blend gasoline can cause corrosion in an aluminum tank from the inside out, due to ethanol’s propensity to absorb water, which leads to water accumulating in the fuel tank. Not only might this cause corrosion in itself, but the electrical conductivity of the water could also cause galvanic corrosion, suggests the NMMA. The association’s position in this paper is that E10 ethanol isn’t likely to cause these problems, but increasing the level of ethanol to E15 might do so. Seems to me that water’s water, no matter if it comes from E15 or E10, so I’d hedge my bets with regular use of a fuel treatment to keep the water out of the tank—and when I order my new aluminum tank, I’d have the builder use thicker plate than required, for added corrosion protection.

Fantastic Plastic

What about fiberglass tanks? For starters, fiberglass is strong, light and doesn’t corrode, characteristics that sound just right for a fuel tank. Some first-class boatbuilders (Bertram and Hatteras, for example) once used fiberglass tanks for both gas and diesel boats—lesser builders did the same, and all was well until ethanol was added to the gas. Ethanol is a solvent and can wash uncatalyzed components in fiberglass resin out of a fuel tank’s laminate and carry them into the engine, leaving boat owners with fat repair bills. Goodwin says that fiberglass is ABYC-approved for diesel, if the proper high-spec, flame-retardant resin is used. Early fiberglass tanks were built with ortho resin, but today’s glass tank builders use vinylester or epoxy, which are claimed to be ethanol-resistant. Nevertheless, I’d restrict fiberglass tanks to diesel fuel, just to be safe. And even then, maybe not. (Hightide Marine is a source of fuel tanks and other parts for classic Bertrams. But instead of true-to-the-original fiberglass, High-tide now sells fuel tanks built of aluminum.)

Vetus Fuel Tanks

But why risk installing fiberglass tanks when there’s another choice, one that’s proven to work with both E10 gasoline and diesel? It’s plastic, which many people think is the tank material of the future, although it’s been in use for at least a decade, maybe longer. To be more specific, we’re talking polyethylene, usually cross-linked (XLPE) but sometimes linear, and rotomolded to create a one-piece, seamless tank that’s about as inert as any substance can be. Hey, it’s plastic. It’s tough, it won’t corrode and won’t be affected by gasoline or diesel. Or will it? Scuttlebutt says XLPE allows fumes to permeate the material, not enough to cause an explosion, but enough to smell. (FYI, the human sniffer can detect gas or diesel fumes at a concentration under 1 part per million; gas fumes aren’t flammable until the concentration reaches 1.4 percent, or 14,000 ppm. So, we can smell gas long before it’s set to explode. But let’s face it—a gasoline smell on a boat is still disconcerting.)

But many of today’s first-class boatbuilders have been using XLPE tanks for years, including Tiara Yachts. Indeed, they’re installing a single XLPE plastic tank in the new triple-outboard Tiara Sport 38 LS. The capacity: 331 gallons of gasoline. (The boat also carries 30 gallons of diesel to power a genset.) So, I asked David Glenn, Tiara’s marketing manager, about the odor issue. “No way we’d drop boats with fumes!” he said, sounding a bit peeved that I’d even suggested it. Glenn said they’d had the odor issue more than 10 years ago, but not now; the new polyethylene tanks have a liner to prevent permeation. All Pursuits also have polyethylene tanks, he added.

Moeller Marine rotomolds Tiara’s tanks, and tanks for many other boatbuilders, too. Steven Fulton, a tech-support guy at Moeller, says hydrocarbon molecules can pass through polyethylene, enough to cause odor. It’s easily controlled with ventilation around the tank. But Moeller’s newer tanks have an extra layer of nylon on the inside, to resist molecular permeation and meet the current EPA regulations. Boats with engines built in 2012 or later must use the new tanks if the tanks are mounted belowdecks. (Those boats must also use low-permeation fuel lines and other components. When was the last time you replaced your fuel lines? Too long ago, I bet.)Moeller still builds the old-style tanks, which are okay for pre-2012 engines. And by the way, Moeller agrees: You should never foam a tank in place.

Moeller’s not the only XLPE game in town, and since they don’t sell directly to consumers, you’ll probably buy your plastic tanks from someone else, like maybe SeaStar Solutions, a manufacturer of steering components as well as rotomolded tanks for sale to the general public. Using the company’s RoLoPerm technology, its tanks are molded as a three-layer sandwich, inner and outer XLPE layers around a barrier layer that stops permeation.

And Vetus sells multi-use tanks (for diesel, fresh water and waste), as well as tanks specifically made for diesel. Both are made of linear polyethylene, not cross-linked. Linear polyethylene isn’t as strong as cross-linked, but it’s more flexible and still plenty tough—whitewater kayaks are built with it—and can be repaired in the unlikely event of damage. Vetus lists these as diesel tanks, omitting any mention of gasoline, and says added wall thickness of the tanks solves the permeation problem—which isn’t legally an issue with diesel. The EPA rules cover only gasoline.

Finally, there’s the issue of labeling. Every tank meeting USCG and EPA requirements must carry a label, affixed to the tank so it’s clearly visible, attesting to the tank having passed the appropriate tests mandated by the Code of Federal Regulations 33 CFR 183 Subpart J - Fuel Systems. It’s thrilling reading, of course, but failure to test and label the tank properly makes the tank illegal, even if it’s the world’s best fuel tank in other regards. So, when replacing a fuel tank aboard your vessel, be sure the one you choose is built, tested and labeled in compliance with the law. That means not asking your brother-in-law with a TIG welder to knock one up in aluminum, but finding a professional tank-fabrication shop to build a legal, and approved, tank.

This article originally appeared in the May 2018 issue of Power & Motoryacht magazine.