Deck-Fill Fuel Filters

The expensive Baja filter, oft-mentioned in these pages, did very well in these tests—but West Marine's new Teflon-based filter did better for less, by means of a simple water-separating scheme.


Any analysis of the simple act of pouring an oily liquid called”Number Two Diesel” in your fuel tank can get very complicated. What’s coming aboard is not only a burnable fuel, but a lubricant. It’s also a coolant. Along with the fact that oil burns much slower than gasoline, it’s the self-lubricating nature of a diesel engine that is largely responsible for its long, useful life.

Deck-Fill Fuel Filters

There are, however, two attendant bits of unpleasantness—water, and contaminates like dirt, dust, rust, scale, bugs, and bones.

A diesel fuel pump, which is the heart and soul of the engine, operates at extremely high pressures and therefore can’t tolerate even a speck of dust in its super-polished interior. The fuel pressurized by the pump must be squirted into the cylinder past the very simple, spring-loaded injector ball (whose tolerances are as little as .0002″), against an upper engine cylinder that already contains awesome pressure. This superheats the air (to about 2,000°), which ignites the oil spray.

Diesel fuel does not mix with water; it sits on top. Therefore, any water that makes its way into your fuel tank will accumulate on the bottom of the tank. Water can be present in the fuel you buy, and it can come from condensation. A damnable variety of nasty creatures can thrive in this water, but, dead or alive, they eventually sludge up the fuel.

Squirting even a tiny bit of water through that diesel injector orifice instantly makes rapidly expanding steam that wants to destroy something. As mentioned above, the oil is a coolant; steam is not.

The bugs (we’ll call them HUMbugs—see the sidebar) are bad, too, whether bacteria or fungi (yeast and mold). Many spores are airborne. You frequently get some in the fuel you buy. They live in the water and dine on the oil. And they are said, under optimum conditions (meaning warm), to reproduce in six hours, multiplying many times. With a really bad case of HUMbugs, contaminated fuel can start clogging your secondary filters in a month. Even if you kill them repeatedly with biocides, their skeletons will be a problem.

If you stay at the dock or operate in nothing but very calm, flat water, the crud and water will remain at the bottom of the tank and those miniscule creatures can live in peace, proliferate to the extent of their food supply, and live like Ozzy Osbourne. No obvious harm done in that case; in fact you might get by for many a year while the little skeletons pile up at the bottom of the tank.

However, shake up the stuff and pump it into the system, and you’ve got trouble, right now. The primary fuel filter, the first one in line coming out of the tank, will clog, and the engine will lose power and slowly stutter to a halt. This will happen at the absolutely worst time.

To preclude drifting into the shipping lanes, taking the rocks, or wiping out your neighbor’s slip, you need a few lines of defense.

Every diesel engine we know of comes with an engine fuel filter. Most boats also have (or should have) between the tank and that engine filter, a good filter, sometimes two (Racors in parallel have been the rage for years), with a see-through bowl that makes it easy to check what’s collecting therein.

The First Defensive Position
It’s often said that a diesel engine will rarely fail you—if you keep the cooling system in shape, don’t neglect the lubricating oil, give it lots of fresh air to breathe, and feed it nothing but clean fuel.

On the clean-fuel issue, this report deals with the very first filter in the defensive array. It’s the rudimentary fuel oil filler funnel. Most have a seemingly crude screen or two to catch the bad stuff; some are considerably more involved. The question is: Are they worth the effort?

Of course, they slow down the refueling, and the guy on the dock will be pecking away: “Hey, you don’t need that filter. We’ve got the cleanest fuel in town.” And your answer can be: “Well, we’re about to find out, aren’t we?”

A half-dozen specks in the filter aren’t of much concern; but as you pay the bill, the dirt is good stuff to show to that loudmouth. If the funnel starts showing sludge (and there’s that telltale odor of sulfur), stop immediately, cast off your lines, utter as many epithets as you can remember from your days as a muleskinner, and seek another fuel dock.

As Nick Nicholson, our editor-at-large, reported recently after completing a sailing trip around the world: “You can’t really tell much about diesel fuel unless you decan’t some in a clean jar and let it settle. We’ve bought water-clear fuel in New Zealand, ugly brown in Tonga, cloudy yellow in the Galapagos, and blue in Trinidad. Before any of it goes in Calypso’s tank, it goes through a Baja fuel filter. It’s a bit of a pain to disassemble and clean every time, but it beats purging a fuel system underway.”

Deck-Fill Fuel Filters

Practical Sailor collected 10 filter funnels for a test to see which removed what. How much better is Nick’s $200 Baja filter than a $6 red plastic funnel with a simple bronze screen about the size of a quarter?

Let’s try them all— two Attwoods; a Baja; two from a fancy automotive catalog called Griot’s Garage (including a beautiful chromed model from Germany); two from the RC Company; several Seafits (made for and sold by West Marine), and a new, somewhat “techie” filter from West Marine. This last one —with a stainless steel, Teflon-coated screen—seems very attractive because the West Marine catalog described it as “self-cleaning.”

All it took to qualify for this joust was a filter screen of some kind in the funnel. There are plenty of others out there, but at the funnel-mit-screen level they’re all very much alike. This is a good representative sample.

The Diesel Fuel Samples
Two fuel samples were prepared. Sample “A” was new diesel fuel, pink in color, obtained from the busiest fuel dock in Newport, Rhode Island. Goat Island Marina personnel said the fuel was brand new, having been delivered several days earlier by two 4,000-gallon trucks. To a 2.5-gallon jug of this fresh fuel was added a quantity of tap water equal to 10% of its volume, plus about a cup of household dust. The fluffy dust, obtained from a small vacuum cleaner used solely for household dusting, was difficult to measure.

Sample “B” was very dirty fuel pumped from a tank aboard a sailboat, in Bristol, Rhode Island. The brown-black sample, pumped from the sailboat’s tank until the tank was empty, was supplied to Practical Sailor by Dan Kerr’s Kiwi Marine Services, Ltd., in Portsmouth, Rhode Island. The contaminated fuel had several times clogged both a Racor and the engine”s primary filter. Even when both filters were changed and the fuel lines were bled, the Perkins 4/107 still refused to start. It’s unlikely that you’d ever encounter such dirty fuel when refueling, but it’s not unheard-of.

Two and a half gallons each of Batch “A” (the new fuel with additives) and Batch “B” (the very dirty fuel pumped from the sailboat tank) were readied in containers calibrated in both metric liters and English quarts and gallons. That was enough to supply one quart each to run through each of the 10 filters being tested.

Giving Gravity a Chance
As a crude test to see visually what might settle out by simple gravity, each batch was thoroughly shaken (as was done over and over and over at every juncture of the testing) and a quart of each was transferred to a transparent container.

At the bottom should be any solids (dirt, scale, rust, slime, microorganisms, their skeletons, etc.) with water next and oil on top. Left to sit for 24 hours, the contents quickly settled enough to show a water/oil line.

In Batch A, the cruder bits of debris showed well in the clear water, as other finer specks settled in the oil and dropped into the water.

In the charcoal gray liquid in Batch B, the waterline showed but no solids could be seen because of the dark color.

After being left undisturbed for five days, the specks of dirt in Batch A must have been growing HUMbugs—small whitish clods in the water.

What was happening in Batch B is not known; no visible change in either the very dirty oil or what appeared to be a bottom layer of dirty water could be detected.

On the sixth day, one quart of each batch was poured through each of the 10 funnels, which were thoroughly washed and dried between batches, into clear containers. For the Batch A tests in this case we carefully withdrew water from the bottom of the jug with a turkey baster, put it into containers, then added dirty fuel on top to equal the original 9:1 ratio of fuel and water in the big jug. (There was no way to shake and pour the mixture; the oil and water separate too quickly.)

Deck-Fill Fuel Filters

The quantities of dirt and water observed in the funnels was noted and the filtered liquid was left to develop a steady water/oil line.

As noted in two columns on the chart, the Visual Test indicated that only two of these funnels appeared to remove all of the water and a great deal of dirt. They were the Baja filter and the new West Marine unit with stainless steel/Teflon tubular filter.

Of the rest, most appeared to remove some of the coarser dirt. A few removed some water. For less than $10, not much should be expected.

Because there were no notable differences in what the filter funnels extracted from Batch A and Batch B, the two “Visual Test” columns on the chart (one for water, one for dirt) are for both batches.

Second-Phase Testing
It was time for some lab work. To get more refined results about which filter performed best, several ounces of each of the 20 samples of filtered diesel fuel (10 each of Batch A and Batch B) were drawn up in clean syringes and placed in small, thoroughly cleaned bottles. Included were samples of the original (unfiltered) Batch A, Batch B, and the clean fuel.

The bottles then were delivered to Strandtech, the lab owned and run by contributing editor Rick Strand.

The water remaining in each filtered sample was measured in a process utilizing water-absorbing polymer beads—the same kind used in bulkier form to remove water from oil and diesel fuel. The laborious process involves placing a carefully measured amount of beads in the solution, letting them soak, removing the beads by filtration, making corrections for retained mineral spirits, and weighing everything very precisely.

The percentages of water remaining in each batch are shown on the chart (see sidebar). The lower the number, the better.

Dirt filtration was measured in two ways. With the deliberately doctored Batch A fuel, microscopic particle counts (done with 250 times magnification in 0.1 square centimeters on slides) were made, and samples were assigned places in our standard rating system of poor, fair, good, and excellent.

The Batch B fuel was so filthy, its cloudiness precluded particle counts. After the water was removed, the remaining dirty fluid was force dried. The residue was placed on a white background and rated visually on the same scale as Batch A.

The new $29 West Marine filter emerged triumphant, the best by far and a good buy in any of the three sizes. The expensive Baja filter we’ve long touted also did very well, but it was beaten by a very simple move by the designers of the West filter: Keep the water away from the screen in the first place. While the Baja’s finest filter is probably just as good, it inevitably lets more water through.

Also surprisingly effective were the fine-screened filters from Griot (model 44797 at $25), and Attwood (model 8888-1, at $7.25).

The awful truth is that none of these filters yield pristine clean fuel. Some water remains, even if but a tiny bit. And that leads inevitably to HUMbugs. Nor can they clean up really dirty fuel, like Batch B. Luckily, you’ll probably never encounter such fuel at a fuel dock.

The best of these filters extract a lot of dirt that normally might wind up in your primary or secondary fuel filters. As an example, the worst filter showed a particle count of 12; for the best filter, the particle count was 1, a 12-fold difference.

So, is it worth the hassle of filtering fuel at deck level? The tests strongly indicate that as a routine safeguard, the answer is a resounding “Yes,” and the filter of choice is the West Marine Model WM-F8C.


Also With This Article
Value Guide: Deck-Fill Fuel Filters

Have We Got A Dust For You

Wanted: The Perfect Fuel

Contacts— Attwood Marine, 616/897-9241, Baja & Seafit, West Marine, 800/262-8464, Griot, 800/345-5789, RC, 800/356-7699,

Darrell Nicholson
Practical Sailor has been independently testing and reporting on sailboats and sailing gear for more than 50 years. Supported entirely by subscribers, Practical Sailor accepts no advertising. Its independent tests are carried out by experienced sailors and marine industry professionals dedicated to providing objective evaluation and reporting about boats, gear, and the skills required to cross oceans. Practical Sailor is edited by Darrell Nicholson, a long-time liveaboard sailor and trans-Pacific cruiser who has been director of Belvoir Media Group's marine division since 2005. He holds a U.S. Coast Guard 100-ton Master license, has logged tens of thousands of miles in three oceans, and has skippered everything from pilot boats to day charter cats. His weekly blog Inside Practical Sailor offers an inside look at current research and gear tests at Practical Sailor, while his award-winning column,"Rhumb Lines," tracks boating trends and reflects upon the sailing life. He sails a Sparkman & Stephens-designed Yankee 30 out of St. Petersburg, Florida. You can reach him at


  1. I never took into account the fact that the tainted gasoline had blocked both a Racor and the engine’s main filter on many occasions. I remember when I was a college student I was proudly answering the question but now I realize it was different. Thank you for helping me understand the right information about marine fuel filters.

  2. As near as I can tell, none of this article is pertinent any more. The aluminum Baja filter is gone, a collector’s item. None of the other items are available. West Marine sells a plastic deck fill filter.