Tested Tankage - PassageMaker

Cruising powerboats require high-capacity fuel tanks. These tanks generally come in one of three materials: aluminum, composite or steel. With proper maintenance, any of these tanks will provide decades of service.

While composite (fiberglass and resin) tanks require little or no attention, an aluminum or steel tank can fail (start to leak) in less than five years. In most cases, once a fuel tank starts to leak, it must be replaced. That process often requires removal of engines or a generator, cutting apart the old tanks, and fabricating and installing replacements. In some cases, joinery must be removed, and cabin soles cut apart.

The aluminum tank above rested on moisture-absorbing material. As a result, poultice corrosion attacked the contact areas, leading to tank failure. 

The aluminum tank above rested on moisture-absorbing material. As a result, poultice corrosion attacked the contact areas, leading to tank failure. 

Depending on the size of the boat and the work required, tank replacement can cost roughly $30,000 to $60,000. Given the cost, it pays to do everything you can to maximize tank life.

Aluminum tanks are found in roughly 90 percent of cruising boats, and those tanks can fail for any one of the following reasons: fabrication issues, contact with copper, repeated exposure to seawater, or internal contamination. Fabrication or design issues are not something you will likely know about or can do anything about. Fortunately, we don’t see failures due to fabrication problems very often.

I started my boatbuilding career in Maine in the 1970s. At that time, the Paul Luke yard in East Boothbay had shifted from wood to aluminum, and we were building sizable cruising sailboats. We were told not to come into the boat with change in our pockets: A stray penny dropped and lost in the bilge or in a tank could do serious damage later. Aluminum and copper are far apart on the galvanic scale, and copper will damage the aluminum.

On boats, we use aluminum or zinc anodes to protect underwater hardware. This system works well because bronze seacocks contain mostly copper, and aluminum and zinc will readily become sacrificial to protect the seacocks. That relationship, however, works against an aluminum fuel tank: Any copper fittings in contact with the aluminum can cause the tank to become sacrificial. Bronze or brass fittings may be used but should not attach directly to the tank. A stainless fitting attached to the tank can receive the bronze fitting, mitigating the risk of galvanic corrosion.

Seawater can attack the tank exterior. Aluminum boats have no problem with exposure to seawater, so why do tanks have this problem? Aluminum gains corrosion resistance by using oxygen to form a protective surface layer. If you damage or remove that film through abrasion or prolonged exposure to moisture or chemicals, then you open the door to poultice corrosion.

Some manufacturers make sure their owners have access to the tank interior by installing large access ports on the tank face. Even with good inspection ports, interior baffles can restrict access and make cleaning a challenge.

Some manufacturers make sure their owners have access to the tank interior by installing large access ports on the tank face. Even with good inspection ports, interior baffles can restrict access and make cleaning a challenge.

For this reason, the tank should be kept free of damp materials left in long-term contact. Particular attention must be given to the means of securing the tank in place. The tank should rest on a material that does not hold moisture and that allows for airflow under the tank. Placing strips under the tank ends and under the baffles provides good support and raises the bottom of the tank off the tank bed. Applying a marine-grade sealant between those strips and the aluminum will keep dampness from forming between the surfaces. Good airflow around the tank allows any stray seawater to drain off and dry, protecting the aluminum. American Boat and Yacht Council standards call for a -minimum ​-quarter-inch of air space between the tank and the flat mounting surface under it.

Let’s assume you have a well-built tank, properly installed. A mysterious process unfolding inside can undo the best execution. In this scen-ario, the outside of the tank usually looks healthy, and the first sign of trouble arrives in the form of weeping and minor drips from the bottom. In these cases, the tanks fail from corrosion on the inside. During the past 10 years, we have seen a significant increase in the number of tanks that failed this way. Several ingredients combine to create this unfortunate situation.

First, there’s water. Water finds its way into your tank in several ways. Virtually all diesel fuel contains some moisture. In addition, moisture condenses on the inside of metal tanks when the metal cools and the tanks are not full. In some cases, vent lines also allow water into the tank. Water has more density than diesel, and these droplets sink and collect at the bottom of the tank.

This water provides a breeding ground for microbes. The microbes feed off molecules in the fuel, eventually reproducing in enough numbers to form a biomass at the fuel--to--wa-ter interface. As dead cells and waste form, they settle out as sludge that collects on the bottom of your tank. Your tank now contains bacteria and fungus, along with their waste products.

One more bit of science before we move on to solutions: What does all of this growth have to do with corrosion? Beneath that accumulation of sludge, a variety of organic acids form. These chemicals attack the protective oxide film on the surface of the aluminum.

This problem of microbially influenced corrosion, or MIC, plagues the aircraft industry as well as industrial fuel storage plants.

TESTING FOR PROBLEMS

All new fuel tanks must be pressure tested, and the outcome of the test should be indicated on the tank’s identification plate.

The test calls for the application of a minimum of 3 psi, or at least 1.5 times the maximum rated pressure indicated on the manufacturer’s identification label.

Performing a pressure test on a new, empty tank that is resting on the shop floor can be accomplished with relative ease. However, in a pre-purchase inspection, or if you suspect a leak in your own tank, testing becomes a challenge.

In the absence of visible leaks or suspicious pools of fuel, a pressure test will be the only way to verify the tank integrity. Unfortunately, this test requires an empty tank, with all openings, including supply, return and vents, blocked. Gaining access to those connections can be all but impossible on many installations.

PREVENTION

Start with properly built (to ABYC standards) and installed tanks. To protect against external corrosion, make sure you have ample airflow around the tanks. Keep any moisture--absorbing materials, such as sound-damping insulation or wood braces, away from the metal. Look for sources of continued exposure to seawater, such as a leaking deck fill or a spraying shaft seal.

To protect from internal corrosion, minimize the amount of water that enters, and keep your tank clean. The deck fill has on O-ring to keep water out; make sure the ring is in place and in good condition. When storing your boat, keep the tanks mostly full to minimize condensation.

From the exterior, this small pinhole leak looks minimal. 

From the exterior, this small pinhole leak looks minimal. 

This photo shows the same area from the inside of the tank, with extensive corrosion taking place due to the presence of microbes feeding off the water and fuel. 

This photo shows the same area from the inside of the tank, with extensive corrosion taking place due to the presence of microbes feeding off the water and fuel. 

As for keeping your tanks clean, frequent use helps agitate the fuel and keeps it moving through the filters. Modern engines cycle far more fuel than they burn, using the fuel to cool and lubricate different components. While you might only burn 15 gallons per hour, the engine will be pulling closer to 60, returning the unused fuel back to the tank.

Fuel additives can also be helpful, but that is a topic for a separate discussion.

Fuel polishers provide a valuable benefit, provided that they effectively agitate the bottom of the tank. To accomplish that objective requires a robust polishing system that moves a high volume of fuel and returns it in a way that stirs up the bottom.

Regardless of frequency of use or fuel polishing, it pays to inspect the interior bottom of each aluminum tank periodically. How often inspections should be done depends on a number of factors: frequency of use, fuel polishing and climate.

High-humidity environments will create more condensation, leading to the breeding ground discussed earlier. In most cases, having a look inside the tank every three years should be sufficient. Keep in mind that tanks have internal dividers, or baffles, that prevent the fuel from moving with too much force. Your tank might be divided into three chambers, and you must have a way to look inside each one. Remarkably, and unfortunately, many builders provide tanks with no inspection ports, or too few ports to give access to each chamber. A borescope can be helpful for inspecting inaccessible areas.

According to ABYC standards, every tank must have an identification plate. This plate tells us that the builder used the proper alloy (5052), the correct thickness (6 mm) and pressure tested the tank to 4 psi.

According to ABYC standards, every tank must have an identification plate. This plate tells us that the builder used the proper alloy (5052), the correct thickness (6 mm) and pressure tested the tank to 4 psi.

If the tank has an accumulation of sludge at the bottom, then the waste material must be removed. With good access, it should be possible for you to reach into each chamber and wipe and clean the tank bottom. If you cannot reach into a chamber because of inadequate inspection ports, then it might be possible to rig up a wand to a pump, so that you can pump fuel under some pressure in hard-to-reach areas. After that, keep draining or pumping off the diluted sludge. If your tank lacks adequate inspection or access ports, then consider installing an aftermarket product such as the one sold at seabuilt.com.

We have seen aluminum tanks fail within five years of installation, and we have seen them last 20 years and more. By following the precautions described, your tanks should provide many years of problem-free service.

Steve Zimmerman is the president of Zimmerman Marine, which operates five boatyards in Maryland, Virginia, North Carolina and South Carolina. Zimmerman has been building and repairing boats for more than four decades.

This article originally appeared in the November/December 2019 issue.

Related