Features May 2007 Issue

Diesel Fuel Tank Replacement

Replacing a failed or corroded diesel fuel tank present a wide range of options. In this special report aimed at cruising sailboats, we examine the pros and cons of aluminum tanks, polyethylene tanks, stainless-steel tanks, fiberglass tanks and iron tanks, and offer a cost-effective solution.

If given a free hand at design, material selection, fabrication and installation, what would Practical Sailor ideal replacement diesel fuel tank look like? The ideal diesel fuel tank installation should possess, above all else, access. A boat fuel tank that is completely and permanently buried beneath or behind fiberglass, timber, insulation or joinerwork is a tank cannot be inspected for corrosion, chafe, structural damage or leaks. Fuel tank baffles, inspection ports, and properly insulated fittings (in the case of a metal tank) are essential. Diesel tank contamination is inevitable, so any tank design for a cruising sailboat must take that into consideration.

                                                ****

Itís sad but true: Boats often last longer, much longer, than their diesel fuel tanks.

If that day should come when you find yourself looking at your own reflection in a spreading slick of diesel fuel in your bilge, what choices will you have? Should the replacement tank be an exact duplicate in size, shape, design features, and material as the original?

The answer to these questions may depend upon how long the original lasted or what difficulties you faced with that tank while it was in service. Was the tank outdated, or was it a tried-and-true design and material? More importantly, did the design, materials, or installation condemn the tank to an early death? Lastly, a more practical question will be how will you get the old tank out and bring a new one in?

Diesel Fuel Tank
Thick plate stock allowed these iron tanks to endure more than 20 years in a less than ideal installation.

Tank Options

While diesel fuel does not possess the explosive characteristics of gasoline&emdash;dieselís flash point is a relatively safe 130 F, which classifies it as "combustible," compared to gasolineís -45 F, making it "flammable"&emdash;it remains a credible fire and environmental hazard. Therefore, the security and integrity of a fuel tankís design, fabrication, and installation are of the highest importance.

Industry guidelines for recreational marine diesel fuel tank installations are limited primarily to those established by the American Boat and Yacht Council (ABYC). An entire chapter within the councilís voluntary "Standards and Recommended Practices for Small Craft," H-33, "Diesel Fuel Systems" is dedicated to the diesel fuel system design and installation, including tanks. This guideline spells out such things as the test pressure of a fuel tank, the grades or series of aluminum alloy from which a tank may be manufactured, and the amount of movement that is acceptable in a fuel tank once it has been installed.

While ABYC guidelines are exceptionally detailed, itís important to remember that some items represent the minimum standard and a voluntary one at that. (While the U.S. Coast Guard sets forth a number of mandatory requirements for gasoline-powered vessels, itís mute on the subject of diesel-fuel systems.) In some cases, Practical Sailorís notion of a durable, safe, and long-lasting diesel tank installation differs from that of the ABYC guideline. For example, ABYC section H33.10.5 states that "fuel tanks shall be installed and restrained so the fuel tank does not move at the mounting surface more than one quarter of an inch." We prefer to see no movement.

For the most part, however, the ABYC standards on diesel-fuel system and tank installation, if followed, will result in a safe, long-lasting installation.

The ideal tank installation should have, above all else, access. A fuel tank that is permanently buried beneath or behind fiberglass, timber, foam, insulation, or joinery work cannot be inspected for corrosion, chafe, structural damage, or leaks. While it may not be practical to install a fuel tank in such a way that all sides are accessible (the bottom of a fuel tank is almost never accessible), the importance of access cannot be overstated. Itís worth noting that ABYC diesel-fuel system guidelines suggest that "consideration should be given to the ease of replacement of the fuel tank." This may be considered a tall order, particularly on modern cruising vessels where a premium is placed on accommodation spaces and installation of accessories.

An additional feature of the ideal tank would include baffles. These are nothing more than vertical, and sometimes horizontal, walls or barriers within the tank. Itís difficult to determine if a tank is baffled by looking at its exterior. Perforations or limber holes within the baffles allow fuel to move slowly from one chamber to the next. Depending upon the tankís shape, itís considered desirable and beneficial to install baffling on most tanks larger than about 30 gallons, or 30 inches long.

Baffles do two things: First, they reduce the movement of the fuel within the tank, which under some circumstances can upset the stability of the vessel as well as aerating the fuel. In the latter case, if this aerated fuel passes to the engine, it could lead to performance problems or an outright shutdown. Second, baffles are a means of internally stiffening and strengthening the tank. For larger or irregularly shaped tanks, this may be a necessity.

Because contamination within diesel fuel tanks is a virtual certainty, inspection or clean-out ports are a prerequisite for Practical Sailorís ideal tank. Although itís preferable for them to be mounted on the top of the tank, ports may be side-mounted if necessary. The addition of these ports during tank construction adds very little to the bottom line. While it can be done, installing inspection ports after the tank has entered service is considerably more complex, time-consuming, and costly. Itís important that an inspection port be installed into every baffled chamber since you wonít be able to clean any chambers that donít have inspection ports. A 6-foot-long, 60-gallon, rectangular fuel tank might have two vertical baffles, creating three chambers.

Even if some or all of the ports will not be easily accessible once the tank is installed, itís still worth installing them. If the tank becomes contaminated, it could be moved or even removed for cleaning. Otherwise you might be faced with the unpleasant and potentially costly task of installing inspection ports later in some far-flung locale.

The pickup (supply) and return tubes on the ideal tank should be removable rather than permanently installed. To reduce aeration of the fuel, the return lines, as well as the fuel pickup lines, should be attached to tubes that extend to the bottom of the tank. The pickup tubes are prone to clogging with debris, so itís important that they be removable as well as accessible. In the past, pickup lines were often fitted with screens, but itís better to have any debris travel to the primary fuel filter where it can be easily serviced rather than accumulating at the end of the pickup tube, which would require more time and effort to remove and clean. To avoid chafe and obstruction of fuel flow, itís important that the pickup tubes not make contact with the bottom of the tank. Ideally, the bottom of the tube should be placed approximately a half-inch from the lowest portion of the tank bottom.

Some thought should be given to securing the tank. How will the tank be installed so that it cannot move, even a fraction of an inch, once it enters service, while avoiding chafe or corrosion damage?

Many original equipment tank installations use bracing, cribbing, or surrounding joinery work to keep the tank in place. The drawback to this, for metallic tanks, is the propensity for this material to hold water, which can lead to corrosion. If you have the option of designing a new tank, it may be worth considering the addition of mounting flanges. These rugged angle sections are welded directly to the tank during its manufacture. The flanges allow you to fasten the tank&emdash;preferably using substantial through-bolts rather than screws&emdash;to a shelf that is integral with the vessel. This approach may take some thought and design modifications to the mounting location, but it will pay dividends. The tank can then be securely installed with minimal contact between the tank and any surrounding structure or material, reducing the potential for corrosion, chafe, and movement.

Finally, consideration must be given to plumbing fittings and their attachments. Plumbing fittings&emdash;including pickup tubes, returns, vents, and fills&emdash;come in two varieties: those that are permanently attached to the tank body and those that are threaded in place. Permanent fittings have the advantages of being cheaper (usually) and being less prone to leakage. However, permanent fittings canít be replaced if the size is wrong or if they are damaged. If they are accidentally stepped on, for instance, they will sometimes deform or even crack at the welded interface with the tank.

Threaded, removable fittings, on the other hand, offer the advantage of being easily modified to accommodate different size hoses and they are less prone to damage. Removable fittings can also be disassembled for inspection and cleaning.

Threaded, removable plumbing fittings, along with any hardware that makes contact with a metallic tank, must be galvanically compatible with the tank material. Thus, copper-alloy plumbing fittings such as brass and bronze should not be allowed to make direct contact with aluminum tanks. An "insulating" bushing, typically 300 series stainless steel, must be installed between the tank and the copper-alloy fitting. Alternatively, the plumbing fittings themselves may be made of 300 series stainless steel. These are probably worth the extra expense, replacements may be difficult to obtain in out-of-the-way places.

In some cases, even if these incompatible alloys are properly isolated, corrosion may still occur. If water leaks onto a copper-alloy plumbing fitting & emdash;or any other hardware for that matter&emdash;and then onto an aluminum tank, it will carry with it salts of copper. These will land on and imbed themselves into the aluminum, where they will begin a slow but almost unstoppable galvanic corrosion process. Therefore, itís important that even properly isolated fuel tank alloys be kept as dry as anything can be kept aboard a sailing vessel.

Tank Material Options

In our quest for the ideal fuel tank, Practical Sailor considered a number of common marine fuel tank materials. The primary materials used for fuel tanks are aluminum, polyethylene, fiberglass, occasionally steel or iron, and now stainless steel, roughly in that order of popularity.

Aluminum

Aluminum is first on the list for several good reasons. It is Practical Sailorís replacement tank material choice for most installations. It is easy to work with, readily available, comparatively inexpensive, light, strong, and corrosion resistant, although far from corrosion-proof.

There are some prerequisites when selecting aluminum for fuel tank fabrication. The alloy used must be 5052, 5083, or 5086 series and a minimum of .09 inches thick. This gauge is ABYC approved, however, 1/8-inch (.125 inches) is preferable, and 1/4-inch (.25 inches) should be considered for "extreme" applications, such as bilge installations or where maximum durability and longevity is sought. Every fraction of an inch of wall thickness may buy a failing installation years of life.

If aluminum possesses so many good attributes, why use anything else? Unfortunately, as many boat owners will attest, aluminum is anything but indestructible. One of its primary foibles is its susceptibility to some corrosion, particularly pitting, galvanic, and poultice.

Pitting is caused by upsetting the corrosion-resistant film formed on the surface of aluminum, sometimes due to variations in available oxygen. Once it takes a foothold, the pit grows deeper, which creates a more powerful cell, accelerating the next form of corrosion, which is galvanic. Galvanic corrosion is the interaction between dissimilar metals in the presence of an electrolyte. In aluminum tanks, this process may take place between a copper-alloy fitting (brass or bronze) and seawater, or between a pitted aluminum surface and seawater. You must ensure that all metals that are in contact with the tank are compatible with aluminum.

Poultice corrosion results when aluminum remains in constant contact with a wet surface, such as wood, carpeting, insulation, or stagnant water. If allowed to make contact, these demons are the harbingers of an early death for any aluminum tank. The result is prodigious amounts of white, gooey aluminum hydroxide. (It looks like freezer-burned vanilla ice cream.) This will quickly compromise the tank surface.

The best defense against this scenario is careful attention to installation details. No hygroscopic material should be allowed to make continuous contact with an aluminum tank, period. A proper aluminum tank installation calls for 1/4-inch by 2-inch strips of non-hydroscopic material, such as neoprene or high-density plastic (Starboard for instance), spaced two inches apart and placed between the tank bottom and the shelf on which it is mounted. This will prevent the tank from resting in water, and enables air to circulate beneath the tank, while allowing condensation to evaporate. Additionally, the installer must be sure to bed or glue the insulating material to the bottom of the tank. If this is not done, water or condensation will find its way between it and the tank, and corrosion will set in. Any other mounting arrangements, such as cribs or beams, must include this insulating material.

Steel, mild or low carbon

Low-carbon or mild steel was once a popular material for fuel tank fabrication. In spite of its rust prone tendencies, a stell tank is quite strong and comparatively inexpensive. If uncoated, it develops light surface rust at first, which usually becomes more pronounced over time. Steel usually rusts from the surface in, retaining its basic shape while getting smaller. In the hopes of extending the lives of steel tanks, many boat builders swathed them in fiberglass. This is a short-lived prevention for rust that is wisely prohibited by ABYC guidelines. Interior corrosion can attack any metal tank, but this can be kept at bay with a well-designed and maintained system, one with a sump that is routinely checked for water.

Stainless Steel

Another ferrous material that is worthy of mention is stainless steel. Until a few years ago, this material was not permitted by ABYC guidelines unless the tank was limited to a relatively small size of 20 gallons and used the unusual and costly shape of a cylinder with domed heads. The concern with stainless steel is its potential for weld embrittlement. The area adjacent to welds, because of the heat to which theyíve been exposed, can become hardened, and thus brittle. If the tank flexes, as many tanks do throughout their service lives, then there is the chance that this less-flexible material may develop cracks, particularly in a damp, salty environment. The attitude toward stainless steel diesel tanks, however, has changed somewhat, and todayís ABYC guidelines do provide reasonable standards for stainless steel fuel tank fabrication. While weld embrittlement may still occur, itís less common with todayís modern welding techniques and thicker plate stock. Stainless is also susceptible to crevice corrosion (akin to aluminumís poultice corrosion), which occurs when stainless steel is exposed to oxygen depleted water for extended periods, so precautions are needed to prevent its constant contact with stagnant, oxygen-depleted water.

In order to meet ABYC guidelines, stainless steel tanks must be a minimum of .075 inches thick and utilize 316L or 317L low-carbon alloy. (The "L" suffix denotes low carbon and is a prerequisite for any stainless steel that is to be welded.) If low-carbon stainless steel is not used, carbide precipitation, a scenario where the area adjacent to the weld becomes "chromium impoverished," may occur, causing stainless to corrode much like ordinary steel.

ABYCís general approval of stainless steel for larger fuel tank installations is relatively recent, so there arenít many compliant stainless steel fuel tanks in service. It is, however, an acceptable material, albeit exceptionally expensive, particularly with the recent increases in the cost of stainless steel (See "Keep a Close Watch on Marine Metal," February 2007).

Polyethylene

One material, which has been gaining steadily in popularity for fuel tank fabrication, is cross-linked polyethylene. Hunter and Beneteau use polyethylene fuel tanks in their boats, and it is commonly used on gasoline-powered powerboats. It is virtually indestructible, corrosion-proof, strong, light, and comparatively inexpensive. Its drawbacks include that it is impractical for custom fabrication&emdash;most manufacturers require multiple tanks be made, anywhere from five to 100 units&emdash;although hundreds of shapes and sizes are available. Additionally, because the tanks are roto-molded, full internal baffles are not possible. (Partial baffles in the form of "speed bump-like" shapes along the tank sides or bottom as well as tubes between tank walls can be included in the tank mold.)

Chafe is another issue poly tanks must contend with, but so must all tanks. Careful attention must be paid to poly tank installations due to their propensity for hydrocarbon expansion. Poly tanks will expand or grow in all directions by approximately 2 percent when first exposed to fuel. (It is possible for a 60-inch-long tank to grow as much as 1.2 inches.) Once the tank has expanded, it stabilizes for the remainder of its life. Additionally, all poly tanks must be fully supported across the entire bottom of the tank (good advice for any tank installation), meaning they must rest entirely on a shelf. Plumbing fitting installation may also become a weak link in these tanks. As the tank bulges&emdash;and larger tanks will bulge&emdash;the threads can begin to deform, which could lead to leakage around plumbing fittings. Perhaps the greatest drawback of poly tanks is their inability to support inspection ports. Because of the hydrocarbon expansion issue as well as their flexibility, leak-free inspection ports are difficult, if not impossible, to install. Although these tanks are well-suited to small, and thus often very wet, gasoline-powered runabouts and center consoles, the inability to clean out these tanks thoroughly makes them questionable for diesel applications. One note of caution: If used for this purpose, plastic fuel tanks must be fabricated from cross-linked polyethylene, or XPE, not linear polyethylene, or LPE.

Fiberglass

The final tank material, and probably the most unsung, is fiberglass. Fiberglass was, before the advent of polyethylene, the only non-metallic alternative. It is strong and corrosion-proof. Care must be taken to use the proper resin (vinylester or epoxy), and meticulous lamination technique is a prerequisite. These tanks lend themselves well to the do-it-yourself project, provided the person has some experience with fiber-reinforced plastic (FRP) lamination, because odd shapes and sizes can be custom fabricated. They should not be made integral to the hull unless the vessel was originally designed and constructed for this type of installation, otherwise you will risk delamination during severe wracking, as in the case of a grounding or collision.

The preferred installation methods are the same as other tanks, flanges, shelves, and cribs. Certain shapes can be tabbed in place, provided these secondary bonds are done properly. They are immune to corrosion, so isolation from moisture, bilge water, and hygroscopic materials isnít important. A number of production boatbuilders (Cabo Rico has used non-integral FRP tanks for many years with good results) have switched from steel or aluminum to fiberglass for fuel and other tank construction.

The only physical shortcoming of fiberglass is its susceptibility to damage as a result of exposure to certain fuel additives such as ethanol. The boating press has been rife with stories in the last two years about the effects of gasoline containing ethanol (sometimes referred to as gasohol), usually in a 10-percent concentration. When used in fiberglass tanks, this has caused serious damage to the tanks as well as engines. The good news is that diesel isnít yet spiked with alcohol, although we canít entirely rule out the possibility. (E-diesel, a mixture of diesel fuel and ethanol, is still in the experimental stage.)

Conclusion

Given our criteria for the best tank at a reasonable price, Practical Sailorís ideal tank material for most applications is aluminum. Custom aluminum tank fabrication is reasonably priced, and it has a very long track record, so the behavior and weaknesses are well understood. If cost is no object, a well-made fiberglass tank will serve well in areas that will inevitably see water, but finding a qualified builder can be difficult. The higher expense of a stainless-steel tank might also be a good choice for certain applications, but its track record with diesel is limited. If you do opt to have a tank custom fabricated (this would include aluminum, stainless steel or fiberglass), itís critical that the fabrication be undertaken by a professional, dedicated tank builder and not just a machine or fiberglass repair shop. Fiberglass shops will often build what you specify, even if it is flawed or violates ABYC guidelines for fuel tanks. A dedicated fuel tank fabrication shop will often advise you if your design goes against convention. For off-the-shelf aluminum, stainless, or poly tanks, simply purchasing from a reputable manufacturer of these products will often, but not always, produce good results. Shop carefully and let features and quality be the watchwords that are weighed against economy.

Comments (0)

Be the first to comment on this post using the section below.

New to Practical Sailor?
Register for Free!

Already Registered?
Log In