It is an axiom of boating that the fuel filter will always clog at the worst possible time. Most diesel fuel tanks have quite a bit of crud floating around in the bottom and a bio-film of slime on the sides. In rough water, when a running engine may be vital and changing a fuel filter most nightmarish, this stuff gets stirred up and sucked into the fuel system. Maximum RPM dropping off is the first sign and the engine can be barely running in minutes.

Most commercial vessels and many yachts have dual filters so that the operator can switch to a clean filter while changing the clogged one but this common system is far from a perfect answer for a small vessel. First of all, the filters are likely to be right next to the engine and working in a heavily rolling boat with alternator belts that can snag hair, clothing, and fingers less than an arms length away is not for the faint of heart or stomach. True duplex filters are not available for engines of auxiliary sailboat size and changing dual single filters will often introduce enough air into the fuel system that the engine will stop and the fuel lines will need to be bled, another nightmarish job, seasick in the dark, as the boat drifts towards danger. Much better not to have filters clog at all.

Fuel contamination is an ongoing process. Spores of, Cladosporium Resin, a fungus, pulled in through the air vent are a primary culprit. These and other organisms grow and clump together along with products produced in the fuel itself as it breaks down in the presence of air. The crud settles to the tank bottom and coats the tank walls with the bio film that can be dislodged by the washing action of a boat subjected to extreme motion.

It's also possible to pick up a big load of filter clogging sludge from a poorly maintained marina pump. Running fuel through a filter as it's loaded is a good idea but not always practical due to the extra time at busy fuel docks. Fuel with fungal spores may not be visibly contaminated but sludge will blossom if conditions are right aboard your vessel.

In a dual filter system, one of the filters is just sitting there, doing nothing useful, until you need it. Why not put it to work cleaning out the low levels of contamination that are slowly building up in the tank? Many vessels also rely on a second, electrically driven, fuel pump to provide fuel to the engine driven pump. Why have a spare sitting in a locker? Better to hook it up to that second filter and have your fuel being cleaned whenever the engine is running.

Here is the system I advocate in designs for both new construction and retrofit. If it is possible to add new connections to the tank, the new construction schematic should be followed but this is not always possible. I have used a variation of the first arrangement with no problems.

The typical marine fuel filter, such as the common Racor's, is a surface filter. This has a thin paper element that functions much like a fine screen. For maximum effectiveness at removing water, another important function, it should be sized to the fuel flow although this won't be possible for most small engines since small enough filters aren't sold. This is not the filter you want for cleaning your fuel in multiple passes. The second filter should be an industrial depth filter with a thick fiber element that can trap and hold 3 - 4 times as much of the crud it filters out. A good choice for a mid size sailboat diesel is a the Shelco FOC-394-V. Since it is not a marine product, it costs less than a Racor or similar brand. Best of all, elements are only a couple dollars apiece, about 1/10 the cost of marine filter elements. You do have to buy a case of 60 though but that should be a lifetime supply. This filter uses half length filter cartridges. Engines over 25 h.p. should use the next size larger filter with full length cartridges. Shelco can advise on the best size for the fuel flow. The larger filter is a better choice for any engine if space and weight are not an issue.

A better and higher capacity filter element is the Hytrex blown plastic type. They cost about six bucks apiece but one will last me through a season with nearly 1000 miles under power unless I encounter a load of bad fuel. They are available in small quantities at the link.

The electric pump that pulls fuel through the Shelco polishing filter has a three position switch, ON - OFF - AUTO. It is normally left in the AUTO position so that fuel starts flowing through the polishing filter as soon as the engine is started. The electric pump is of sufficient capacity to circulate the entire contents of the tank in about an hour. Note the Racor pressure / vac guages on the schematics. Noting the readings on these gauges when the system is first running will provide early warning of filters beginning to load up. Good practice before heading into a dicy situation under power is to bring the engine up to full RPM and take a peek at the gauges.

The flow of the polishing system is 20 - 30 times that of the engine draw alone. This makes for much greater suction at the fuel system inlet in the tank which promotes getting the crud drifting around the bottom up into the fuel system and into the filter. The whole idea is to clean the fuel as much as possible before it even flows through the filter that supplies the engine. Much of what you need to remove from the fuel is not in the form of hard particles but soft goo, some of which will extrude through the filter. The recirculation of fuel through the polishing filter causes the goo and smaller particles to clump together so that they become large enough to get permanently trapped by the filter on the next pass. The single pass standard fuel filter can't do this and the stuff passes through to the engine mounted fuel filter. When that filter starts to load up so that fuel pressure rises, the gunk is pushed through the final filter into the injectors.

Consider the situation where fuel is found to be heavily contaminated. While the typical duel filter system may let you keep the engine running with repeated filter changes, all filtered fuel must go directly to the engine. The rate of filtering can be no greater than engine fuel flow so dealing with the contamination may be a lengthy process unless an outside fuel polishing contractor is brought in. You may not find one of those in remote cruising grounds where fuel supplies are most likely to be questionable. I've heard of cases where it took over half a dozen filter element changes to clean a bad load of fuel. At the price of Racor filter elements, that would pay for my Shelco filter and polishing pump right there.

With the system shown above, the Racor filter is likely to load up first due to its lower retention capacity. The engine can then be run from the polishing pump and filter via the bypass while the Racor element is changed. The Racor should be changed as soon as practical in case the Shelco does load up. Another possible scenario is for the Shelco to load up first due to it's higher fuel flow. High vacuum readings on it's gauge are the early warning that a contaminated filter situation is developing.

With a typical dual filter system, it will be necessary to keep running the engine to filter fuel once the vessel is secure. The cleaning process will not be completed until the entire load of fuel has been consumed because there is nowhere for it to go after it has passed through a filter. The polishing filter system, on the other hand, can keep re-circulating fuel with battery or shore power until it has made several passes through the filter. Two dollar Shelco elements can be changed as necessary.

The polishing system can also be run at the dock with the engine off for a few hours after picking up a new load of fuel or at commissioning in the spring to be sure that the fuel is clean.

I have used this system for a full season burning about 100 gallons of fuel. About half of the fuel was purchased from a high volume gas station on shore where fuel is usually cleaner than from marinas. At the end of the season, I found this stuff in the sediment bowl of the Shelco polishing filter. This is only a portion of it because wind blew the plate over while I was cleaning out the filter housing.

There was not a speck of debris in the sediment bowl of the Racor filter which looked just as clean as when I filled the system in the spring.

For pictures and details of the system on my boat, which is a bit more complicated due to dual fuel tanks, click here.

Here is great and idea for additional redundancy for which credit is due to a fellow named Rich H who is a filtration expert by profession and was invaluable in developing my system and the ideas on this page:

This is a small tank inserted in the line between the polishing system output and the main fuel oil tank. It should be located as high as possible in the boat to provide gravity feed to the engine. It should be large enough to provide about an hour of running time but can be as large as space allows. This tank is always receiving a fresh supply of just filtered fuel from the polishing system and so this fuel is ready to go into the engine with no further filtering. If the system has been switched to run from the polishing filter and it's element loads up or the polishing pump fails, fuel will simply drain back by gravity to keep the engine running long enough to get the vessel into a safer situation or change the Racor element. No further action will be required unless the isolation valve has been fully closed which would only have been done in the case of problems with the engine mounted pump and filter. The anti-siphon valve should be mounted as high above the tank and the highest point in the line from the tank to main tank as possible so that fuel will generally not reach it. In the case of a new installation, where the polishing line runs to the bottom of the main tank, the anti-siphon valve could be dispensed with as fuel would siphon back through the whole system making the full contents of the main tank available. The engine would start running on unfiltered fuel as the emergency tank emptied but would still be protected by the final engine mounted filter.

I am presently installing a tank like this on my boat that will double as the gravity feed tank for the Newport diesel cabin heater that I am installing. It will automatically be refilled whenever the engine or polishing system is running. The schematic for that set up can be seen here.

Everything presented on this page are just ideas and I can take no responsibility for any actual installation due to the many variables in individual boats. If you put a system like this in, you should obtain professional advice on its details and installation.

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