Inspecting Fuel Tanks

There is an important component in all cruising boats that is mostly ignored. Years go by without a problem, but down the road it happens. The alarming odor of diesel fuel in the engine space or bilge signals a fuel leak. And sometimes, much to the owner’s surprise, the leak is coming from the fuel tank itself.

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Unfortunately attention to detail is required during fuel tank installation. A job well done does a lot to avoid the issues that can surface over time. But this is not generally the case, particularly with older boats, and this can lead to significant problems. Even high quality older boats, such as the well regarded Grand Banks trawlers, are known to have fuel tank issues. In most cases, total replacement of the original fuel tanks is a given for an older GB. The tanks must be cut up and removed, and replacement tanks installed at significant expense. (This has become such a routine project for older GBs that the entire process is well known and a reputable diesel engine shop knows what is involved and what it costs.)

Unless you are building a new boat, where you have some control over the design and installation of the fuel tanks, you have to rely on the builder, which is not always the sure thing you might imagine. Does each tank have inspection ports, large enough to reach in and inspect and clean? If the tank has baffles, and any tank above a certain volume should have baffles, are there inspection ports into each baffled chamber?

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Most all fuel tanks of the last several decades employ pickup tubes to draw fuel out of the tank, rather than fuel fittings down at the bottom of the tank, long banned by the Coast Guard. These pickup tubes should be removable for inspection and maintenance, but many are not, unfortunately, simply because the top of the tank is only a couple of inches from the overhead, usually a side deck. And do you know if your pickup tube has a screen attached to its end, down near the bottom of the tank? If it does have a screen, it should definitely be removed and thrown away, as a clogged pickup screen is an absurd situation in today’s world of superior primary fuel filters (the first filter of several in a fuel delivery system). If there is dirt or gunk in the tank, it is much better to pass the dirt out of the tank and let it get caught by the large primary filter.

 This fuel pickup tube is wrong for several reasons. The lower end should not have a screen, and the tube's end should be cut at an angle, with the lowest part of the tube end just off the bottom of the tank, perhaps 3/4-inch off the bottom. But if it can't be removed to inspect, you'll never know if it was done correctly, or cut too short, leaving gallons of fuel out of reach.

This fuel pickup tube is wrong for several reasons. The lower end should not have a screen, and the tube's end should be cut at an angle, with the lowest part of the tube end just off the bottom of the tank, perhaps 3/4-inch off the bottom. But if it can't be removed to inspect, you'll never know if it was done correctly, or cut too short, leaving gallons of fuel out of reach.

Two installation issues with fuel tanks involve being too close to potential bilge water and located directly under deck fittings. Once the bedding material of a fuel fill fitting dries out, it might allow water to run past the fitting and down the hose to pool on the top of the fuel tank directly underneath. It is only a matter of time before a metal tank will corrode with pinhole leaks coming as a result of standing seawater.

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The same happens with tanks in close proximity to bilge water sloshing around against the bottom and sides of a metal fuel tank. If you own an older boat with insulation surrounding the fuel tanks, you may be in trouble if the material holds water against the tank, and of course you can’t see anything. Out of sight, out of mind...until a diesel smell indicates you have a problem.

 What is behind this insulation? I wonder if this owner has any idea of the condition of this fuel tank.

What is behind this insulation? I wonder if this owner has any idea of the condition of this fuel tank.

Fuel vents are necessary, but was the builder careful when installing these vents to ensure no seawater can get into these vents? Have you looked at your fuel vent recently?

Depending on the tank material, are the tanks properly supported as installed? Plastic tanks especially need to be supported along their entire bottom surface, not straddling structural members which is fine for metal tanks. And, of course, no fuel tank should be allowed to move or creep while the boat is underway. That is unacceptable for many reasons, and even the thought of a shifting fuel tank gives me the creeps, which I once found on Blue Angel. The screw through the strap holding the tank was too short. It pulled out.

Back at the deck fuel fill, is the O-ring gasket inside the cap still pliable? If it is old, or missing, you need to take care of this at once. Even if the O-ring looks okay, a dab of silicone grease is a good measure to ensure it remains watertight when tightened after fueling.

All fittings must be chosen to do their job without causing havoc in some other way, such as brass or bronze plumbing fittings installed directly into aluminum tanks. Dissimilar metals don’t play nicely together.

Fiberglass fuel tanks are probably the best material for fuel tanks today, as they allow unique, custom sizes and shapes, something difficult to achieve with manufactured plastic tanks. While some argue that tanks integral to the hull are best to ensure maximum utilization of space in an engine room, integral tanks might be compromised if the hull bends or flexes. In the event of a grounding or being holed, integral tanks are done for (although they might keep the hull intact if the damage in only within the tank's area). It is generally considered best to build fiberglass tanks similar to metal tanks, with proper installing, structural supports, and tabbing.

Of course there is much more to fuel systems than just the fuel tanks, but if you are considering an older boat, there is a good chance you can do more to keep your tanks dry and healthy. Unfortunately, most builders no longer build with the idea that fuel tanks should be removable. Many sailboats don't carry enough fuel to require tanks that are difficult to remove, but Valiant Yachts comes to mind as a builder who made this an important element in their construction philosophy, with everything in the boat able to come out the companionway. That is an unusual building practice. If you ever get the chance to follow the construction of a new cruising boat, you will see it is the rare builder who gives much thought to the eventual need to get at or remove parts of the boat years after it launches.

And with the nature of boats, they will all need this, sooner or later.