Skip to main content

Don't overlook your fuel system

There are few systems aboard boats with the capability to completely disable us at a most inopportune time, that have the potential to create large environmental issues, and can provide the boat owner with both significant repair bills and down time. Your boat’s fuel system, whether gasoline or diesel, has that potential.

There are few systems aboard boats with the capability to completely disable us at a most inopportune time, that have the potential to create large environmental issues, and can provide the boat owner with both significant repair bills and down time. Your boat’s fuel system, whether gasoline or diesel, has that potential.

Although most boatbuilders try to work within industry-accepted practices in addition to governmental regulations, there are practical considerations and cost factors involved in the building process. And I have frequently surveyed boats where the owner has implemented changes in the system that may not have been designed or executed to the appropriate standards, nor meticulously maintained. The fact is, regardless of your engine’s condition, without good, consistent fuel management you could, at the very least, be facing reliability and performance issues.

Good fuel management begins before you fill your tanks. Although we can’t control the fuel prior to pumping it into our boats, we can inspect its condition and deal with it as necessary.

Taking on fuel

Of primary importance to your fuel system is keeping out water and particulate matter. When taking on diesel fuel, especially from an unfamiliar vendor, consider pumping it into a clean glass jar and letting it sit for a few minutes.

If the quality of the fuel is suspect and you have no other alternatives, you may consider filtering the fuel if it’s practical as it is being pumped on board. For initial filtering, I recommend a Racor Fuel Filter Funnel, a fast-flowing combination funnel/filter that separates free water and contaminants as small as .005 inch from gasoline and diesel fuel. It is available with flow rates from 2.7 to 15 gallons per minute. This process slows fueling considerably, so I suggest you make sure the fuel dock is OK with it. I use the 3.9 gpm flow rate for my outboards, which will capture more than 8 ounces of water.

A word of caution: Petroleum products flowing over a plastic surface generate static electricity. Although the funnel is manufactured from electroconductive polypropylene, it still needs to be electrically bonded with the fuel source. I keep a flexible 12-inch jumper wire with small alligator clips attached to the funnel. When fueling, I attach one of the clips to the fuel nozzle while the other remains on the filter.

I have been aboard boats, both new and old, that are considered to be well-designed and -built. Unfortunately, many all-too-familiar issues surface over time — particularly with systems that have been designed with ease of vessel manufacture in mind and with little or no attention to the subsequent inspection, maintenance and repair issues that undoubtedly will occur.

An often overlooked path for water entry into the fuel system is through the deck fill, since the primary consideration for its location unfortunately is often building convenience. Fuel fills can be bombarded with sea water while under way, most notably with sailboats, and rainwater and wash water take their toll at the dock. The only means of preventing water ingress at the deck fill is typically with a small section O-ring or gasket.

When performing surveys, I sometimes don’t find any O-ring or seal at all. If there is a seal, it might be in poor condition and, therefore, ineffective. I keep several spare seals and a replacement fill cap where the deck key is stowed. I replace the seals on my trawler several times per season.

Water also can find its way into your fuel through a tank vent fitting that has been poorly located or installed with the incorrect orientation. Tank vents should be above the boat’s maximum heeled waterline, with the openings facing aft and slightly down. It should go without saying, but make certain all deck fills are clearly marked for the type of fuel required, water or waste. Take nothing for granted at the fuel dock.

Although pumping fuel into the bilge or onto your deck ranks very high on the list of what not to do, almost half of the used boats I inspect have degraded fuel fill hoses that will allow just that to occur. (This is the hose that connects the deck fill to the fuel tank.) USCG Type B1 or B2 fuel hose is permitted in certain applications, as per 33CFR 183.558, but the use of USCG type A1 or A2 hose is highly recommended for all fuel fill applications, and the hose must be double-clamped on both ends.


Most current fuel tanks are constructed of aluminum or fiberglass, although many steel and iron tanks are still in use. Tanks should be accessible for complete inspection without requiring major structural disassembly and, based on 33CFR 183.550, may not be integral with any boat structure nor move at the mounting surface more than 1/4 inch in any direction. To help guard against corrosion, metal fuel tanks should not be in contact with any hygroscopic materials, either in the mounting system or covering the tank.

Fuel tanks constructed of mild steel or black iron were popular years ago when many builders wrapped the tanks in fiberglass. Unfortunately, the wrapping only served to mask rust. These tanks need to be carefully inspected for water accumulation between the fiberglass covering and the tank wall. Fiberglass tanks, when fabricated with the proper resin (vinylester or epoxy) have provided years of trouble-free service and can be easily fabricated into complex shapes. Unfortunately, many fiberglass tanks were built using polyester resins, which appear to suffer both permeability and structural problems with today’s ethanol-based fuels.

Aluminum fuel tanks continue to be popular in today’s market. Ensure that all metals in contact with an aluminum tank are compatible with aluminum. Copper alloy (brass or bronze) fittings should not be used with an aluminum tank unless a stainless steel bushing is used between the dissimilar metals.

Each tank should have an access port for cleaning and inspection. I have found that a 6-inch-diameter opening is the minimum practical size. Most well-constructed tanks have internal baffles, often requiring several access ports for thorough inspection and cleaning. All fittings should be removable from the tank by unscrewing them from the female threaded “welding bosses.”

In addition to the fuel pickup tube, each tank should have a stripper tube that accesses the lowest portion of the tank (deeper than the fuel pickup tube) to pump out the water and sludge that eventually accumulates on the bottom of the tank. All fittings should be at the top or on the highest horizontal surface of the tank. Tank drains, fuel feeds and crossover tubes are not permitted from any other surface.


An essential part of fuel management is the filtration process. A first-rate fuel filtration system will allow your engine to efficiently achieve its rated performance without interruption.

Most gasoline engines are equipped with some form of fuel filters, although they typically can be improved upon with little effort. I recommend the installation of a high-quality aftermarket fuel/water separator in addition to the standard engine-mounted units installed by many builders. The basic inboard engine OEM spin-on fuel filter appears similar to an oil filter and, like most oil filters, does not provide a drain or means to monitor its condition. The filter can become clogged and water-soaked without you knowing there is a problem until the engine begins to run poorly.

Since the condition of the filters should be monitored frequently, I prefer to install in an easily accessible location a Racor 10-micron remote spin-on filter that includes a separate metallic bowl with drain plug. The drain plug allows the removal of accumulated water and debris without a complete filter change. Most factory filter heads can be upgraded with a similar spin-on filter that can serve as the secondary filter. With an outboard engine installation, I use a similar Racor remote filter, but one with a see-through bowl and self-venting drain.

Diesel fuel systems are best approached with multistage filtration. The primary filter is your first line of defense. Its task is to capture, separate and store contaminants, and it should be located nearest to the fuel tank. A good primary filter will be in an accessible location, contain filter elements that are easily changed, and will include a large-volume water separator with a check valve, sediment drain bowl and have the ability to be monitored remotely by a vacuum gauge and water probe. What I’ve described is manufactured by Racor and is commonly found as standard equipment on many high-end voyaging boats and commercial vessels. The double manifold MAX models allow you to isolate one filter/separator for service while the other keeps operating, eliminating the need to shut down the engine if a filter change is required while under way.

Engine and filter manufacturers agree that your primary filter element should be rated at either 30 or 10 microns, depending on other system particulars. On my diesel trawler, I have installed the MAX model as my primary filter, using 30-micron elements, and another single-element unit with a 10-micron filter element closer to the engine. I retained the factory engine-mounted 2-micron filters as the final line of defense.

When modifying your fuel system configuration, be certain to take the fuel flow rates and pressures into consideration. I have found both the engine and filter manufacturers to be very informative and cooperative when consulted.

In addition, there are fuel polishing systems available — both for on board installation and transportable dockside units — that can help to bring your existing fuel up to a maintainable standard.

Current fuel concerns

As the use of ethanol becomes more widespread, we likely will face more fuel contamination issues created by the water absorbed through fuel tank vents in addition to the existing condensation issues. Ethanol attracts and absorbs moisture and also is a solvent that can dissolve varnish and other material in fuel systems. In some cases, it can dissolve components of the fuel system itself, such as seals, gaskets and some early fiberglass fuel tanks. The resins used in some of the fiberglass tanks appear to be leaching from tank walls, weakening the tanks.

The resin released by the ethanol also can make its way through the fuel system, where it sticks to valves and other internal engine parts. The black, sticky substance has been the cause of bent pushrods and stuck intake valves, and can contribute to the destruction of the engine. (Please refer to the July and October 2006 issues for a detailed look at ethanol problems and concerns, or search the story archives at Soundings Keyword: ethanol)

Diesel fuel is changing as well. Although there are staggered compliance dates throughout North America, as of Oct. 15, 2006, S15 (ultra low sulfur diesel, or ULSD) was available at most fuel retailers in the United States for highway use. S15 contains only 15 ppm of sulfur. Fortunately, with the exception of California, the EPA regulations for marine diesel fuel transition to ULSD are several years down the calendar. We are, however, currently required to be using low sulfur diesel (500 ppm sulfur) for non-highway and marine use. I find it unlikely that fuel retailers will be dispensing more than one formulation of diesel fuel, so in reality we may be dealing with ULSD now.

There are several diesel fuel properties other than sulfur content that will change as a result of using S15. The process of reducing the sulfur — known as hydrotreating — also removes naturally occurring lubricity agents critical to engine life, alters the aromatics content, and changes the density of diesel fuel resulting in the reduction of energy content (BTU/gallon). The change in aromatics can cause aged seals to shrink, and some of the new S15 fuels are expected to be more susceptible to oxidation, which could attack seal material and cause it to prematurely age, according to fuel industry sources.

As of Jan. 1, 2005, all diesel fuels have been required to meet the ASTM D975 standard for lubricity. The Clean Diesel Fuel Alliance states, “as necessary, additives to increase lubricity and to inhibit corrosion will be added to ULSD fuel prior to its retail sale.”

If you’ve been around boats for any length of time, you’re aware of the numerous fuel system issues that have to be dealt with. Fortunately, most can be easily overcome through careful service and maintenance.



Diesel fuel filters: Which should you choose?

If you want your diesel engine to run properly, you need the right fuel filters.