Capt'n Wil's Diesel Polishing System

By Wil Andrews

Webmaster's Note: This article was originally written for Trawler World Online, now known as Trawlers & Trawlering. It is reprinted with permission at the suggestion of Al Casanova who used the information contained in this article to design the fuel polishing system aboard Almar Too, his Nordic Tug 32.

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Diesel Fuel Polishing System Components

1 Gulf Coast Filter F-1
2 Gulf Coast Filter F-1 element
3 Gulf Coast Filter O-1 JR
4 Racor 500 2-micron element cut away to show filter depth
5 Walbro 6802 continuous-duty diesel fuel pump.

A Work In Progress

The most common reason for marine diesel engines in general, and recreational trawler diesel engines in particular, to fail to run is associated with "bad diesel fuel." The things that make the diesel fuel bad generally have nothing to do with the original quality of the fuel. The problems come about because of the condition of the fuel we buy and what happens to the fuel when it gets stored, sometimes at the suppliers' facilities, but most often in our own vessels.

To get anything from this epistle, you must accept the fact that this whole thing is a work in progress. The work is to find out how to keep the fuel we burn Just Like New. The progress in this work comes in fits and starts, and next year I'm sure I'll know more about the solution to the problem than I know now. Getting the whole answer is difficult because while it is of vital concern to us recreational trawler captains, it is a minor problem when considered in light of the events of the nation and world. There is no money dedicated to finding the solution to this problem by either government or industry. There are companies selling products proclaiming that their product will solve the problem, but I am not aware of any formal scientific studies that speak to the causes and solution of this problem. That leaves it to amateurs like CaptnWil, and others, to see this work in progress to a conclusion for the recreational boating world. I encourage all of you to make tests and keep records so the whole body of information can be shared by all of us. We make judgments based on our observations, experiences, and faith in advice given by others. It is important that we strive to be able to change those judgments when the evidence suggests a change is called for.

I feel confident that many individual organizations that have experienced this problem have come to a successful solution for their particular situation, but they have no reason to share their experience with us. That leaves us pretty much on our own.
What follows is my final solution to my problem. I hope it may be of help to you

In The Beginning

Like most who have experienced bad fuel problems, I first became aware that such a problem was possible in 1989 when my "new to-me," four year old trawler engine quit while I was taking her home from a thousand miles away.

The boat had spent two winters in Chesapeake Bay mostly at the dock, made the trip down the ICW to the Keys, and I took possession in Miami. The engine had less than one thousand hours on it and the fuel tanks were less than half full. With the joy that is only possible that first day you own a boat, we set out with eight guests toward North Carolina. Guests came and went along the way, and thankfully, there were no guests when the engine stopped underway the first time. Before I finally figured out that the problem was a clogged fuel filter, and then found the proper element, the engine stopped eight times. The last time it stopped underway was in a passage that warns not to anchor because of unexploded ordnance. For a lot of reasons, that whole voyage is an experience I'll never forget, but that's another story.

I did not know that this work in progress had really begun until the engine stopped while underway again in Chesapeake Bay that summer. It was an uncomfortable feeling to be unable to move in the middle of the bay with night rising on and the wind coming. By this time, I understood that the symptom of the problem was a clogged fuel filter. I recognized that the problem was most likely to occur when the boat was in a seaway and the fuel got stirred up. That realization brought terror to my mind when we got caught in a July storm just off Cape Lookout with 60-knot winds and mountainous waves. If the engine had stopped then, those I love the most would be cast into the breakers of Cape Lookout.

Keeping the engine running is always preventing an inconvenience, but sometimes it is a matter of survival. I continue this work because of both reasons, but most importantly because of the latter.
Early Efforts

After going through a Racor element every day for several days, I decided to get to the bottom of the problem and clean the tanks. The bottom of each tank had enough black sand-like stuff to fill two five-gallon buckets each.

What was that stuff? It was the stuff at the bottom of the tank. I have never had a satisfactory answer about what it was. I am sure there were ways to find out, but I have only seen speculation and theory about that stuff. That speculation and theory led to putting an additive in the fuel to prevent it from forming again which additive is unimportant.

The Pour-in Solution

It is very popular today to think that something you can pour into a fuel tank will take care of your fuel problems. There is a large body of experience, including my own, to indicate that any pour-in solution only treats some of the symptoms, but that's not the real problem with this procedure The problem is that while the immediate symptoms may vanish, this cure begins to create its own set of problems. If you kill the bugs, their dead bodies will cause problems sometime in the future they just become part of the jelly that wants to collect at the bottom of the tank, and we know where that leads. If you emulsify the water in the tank, it still goes through your injection pump and injectors, and will lead to a whole new set of problems.

The Absolute Solution

We'll talk in a little while about the Polishing System Solution, but if your system qualifies, you can get along just fine without any extra system.

The Maintenance Supervisor of the North Carolina Ferry System has been most kind and helpful to me in my investigation of oil bypass filters and diesel fuel storage problems. His observations and experiences are from many engines on many automobile ferryboats that run long and hard. They put more hours on most engines in a week than most recreational boaters do in a season. They burn more fuel in a month than we burn in a lifetime. When he tells me his conclusions of his observations, I listen and believe.

The older ferryboats have Detroit engines and the newer boats have Caterpillar engines. There are some facts about the operation that have special meaning for us in seeing why this operation doesn't have a fuel problem, but we do:

1. The Ferry System buys diesel fuel on State Contract and it is delivered directly from the pipeline to his storage tanks.

2. There is never a time when the entire fuel supply remains in the tanks more than a very few weeks. In the summer, the cycle is not more than one week.

3. The engines on the ferryboats circulate a much larger quantity of diesel fuel than they burn. They circulate so much that all the Detroit engines and many of the Caterpillar engines are equipped with coolers in the return line to the tank to keep from overheating the fuel in the tank. Keep this in your mind. It will come up in a different light a little later.

4. The fuel in a ferryboat does not stay in the ferryboat fuel tank more than a day when it operates and never more than a few days in most other cases.

5. The engines are fitted with Racor diesel filters with water separators and the normal fuel filters that are supplied by the engine manufacturer just like most recreational trawlers. Water separators are tended everyday.

6. At one time, a pour-in additive was used in the fuel. During that time, high injector maintenance was experienced. After a time, use of the pour-in additive was discontinued, and after that cycle of injector maintenance was complete, the injector problems decreased materially.

7. Fuel storage tanks on land are entered and cleaned regularly.The most important thing to understand is that the very high volume of fuel used and long hours of operation in this installation, magnify everything about the operation.

If you can guarantee you get clean fuel and guarantee it will be stored in clean tanks and guarantee it will remain in those tanks no longer than a week or two, you won't need a Polishing System you already have the Absolute Solution.

The Circulation Myth

The amount of diesel fuel circulated by the system is a very important factor in connection with our storage problems. When I first started researching this fuel storage problem, I heard stories that the Lehman-Ford 135 circulated 60 gph well, at least 30 gph. Wrong! No one knew the real answer because manufacturers do not commonly provide that information, and no one I ever talked to had ever measured it. I was shocked to observe that the Lehman-Ford 135 returns 1.8 gph to the fuel tank at 2200 RPM.

I expect the high circulation rate story came from the fact that Detroit two-cycle engines use circulated diesel fuel to cool the injectors. Most four-cycle diesels in the range of sizes used in recreational trawlers don't cool the injectors with circulated diesel fuel so they only need to circulate enough fuel to make sure the injection pump always has a positive head. That is, the lift pump must just pump enough fuel to guarantee that fuel is always available to the injection pump at positive pressure.

This low flow rate has a direct bearing on our general problem, but it also has a specific effect of the action of Racor filter units that have what is called turbine action. In most installations, the flow rate through the filter is far too low to allow the desired turbine action.
It is possible that your engine has a greater pumping rate than indicated above. It is a very simple matter to measure the quantity of fuel returned to the tank. Just break into the return line to the tank and catch a known quantity of fuel in a known time and compute the flow rate. You can do this at the dock with the engine in neutral. Place a bucket under the return line and have a large zip-lock ready to catch the measured fuel. Have the engine started and increased the RPM to cruising speed. Then catch fuel in the zip-lock for a timed interval. The longer the timing interval the more accurate the results will be.

Magnetic Fuel Units

Long-time CaptnWil readers will remember that one manufacturer of these units threatened CaptnWil with legal action because of dissatisfaction at not being able to manipulate the way a West Marine Trawler Fest seminar was conducted. For that reason, I will offer no opinions about these devices. I will just say that a very effective Diesel Polishing System can be fashioned without using these units and the rest of this report will not mention them again.

They are only mentioned here to let long-time readers know the reason why.

Where Does The Crud Come From: Water

It is generally agreed that water in the fuel tank causes all sorts of hell. The experts tell us that algae is born, lives, multiplies, and dies in the surface between the water and diesel fuel. These critters and their residue are a major source of trouble. The moral is: no water, no algae.

Most of the water gets into our tanks by water condensing on the inside surface of the fuel tank and less often on the surface of the diesel fuel itself. This occurs every time the temperature of a surface is below the dew point temperature of the air to which it is exposed.

Without getting too technical, more moist air will have a higher dew point temperature than less moist air and be more likely to cause condensation. It is also helpful to note that the condensation always occurs on the warm side of the surface -- the beer glass sweats on the outside, and your port lights sweat on the inside in the winter. Thank God for the first, but lament the latter.

This all means that condensation can only occur in your diesel tank when the temperature of the surfaces of the tank and/or the diesel fuel itself is below the dew point of the air INSIDE THE TANK. If you see condensation on the outside of your tank or on your fuel lines, there is no condensation going on inside your tank. The problem can only occur when you don't see such condensation. Condensation on the surface of the diesel fuel itself is seldom considered, but is a fact. It works similar to fog at sea.

There are two ways to prevent condensation on a surface. (1) Raise the temperature of the surface above the dew point of the air or (2) Reduce the dew point of the air below the temperature of the surface. Your automobile defroster works like the first case and your air conditioner works like the second case. Desiccant filters can also be made to work like the second case.

The most common means to prevent condensation in fuel tanks is to keep the tanks full. If they are full, there is no air in them so no condensation can occur. It is especially important to keep the tanks full when the boat is idle for a period of time.

Desiccant filters in the vent lines have tremendous possibilities. If they can be fitted properly, they will keep most of the moisture in the air from entering the fuel tank, which will lower the dew point of the air to very low levels. Their installation and care in common recreational trawlers offer some problems that I have not been able to solve. The limiting factors include their physical size and the requirement that they do not get contaminated with diesel fuel. I hope someone will come up with a means of using them with our diesel tanks because they could almost eliminate the condensation problem in diesel fuel tanks.

Where Does The Crud Come From: Dirt

There is more junk in fresh diesel fuel pumped from a high volume supplier than I would ever imagine. That is one of the reasons that engine manufacturers always put diesel fuel filters on their engines. We seldom see this crud because most modern fuel filters are spin-on types with the element hidden from view.

I have a Gulf Coast fuel filter mounted on my pickup truck. One of its features is that the top of the filter container can be removed at any time for inspection of the element. On a recent 10,000-mile land trip, I replaced the element before leaving and had to replace it at 5,000 miles because the crud in the element caused fuel flow problems on steep grades at full load. During that trip, all fuel was purchased at high volume truck stops.

It is one thing to get such dirt into a 20-gallon fuel tank and quite another to get it into a 300 or 400-gallon fuel tank on a trawler. The complete fuel turnover rate in the 20-gallon tank guarantees that the entire contents of the tank will be kept stirred up and the crud will quickly end up in our fuel filter. In the 400-gallon, on the other hand, the dirt will just settle to the bottom of the tank like the silt in the delta of a river. It will build there over time so that it can cause its worst evil at our most critical moment.

Where Does The Crud Come From: Other Stuff

Then there's that black stuff that begins by discoloring the fuel filter, then discolors the fuel, then makes the fuel black and puts jelly-like stuff on the fuel filter, and then just shuts down the whole fuel system.

I can't find anything about this condition in the literature, but I suspect it is the result of the solids from the original crude oil settling out and returning to their natural state. No matter what the cause, it is an ever-present, ever-continuing condition that adds to that awful mix at the bottom of our tanks. Let diesel fuel stand long enough and it will turn black.

The Real Nature Of The Beast

Except in rare cases, the factors that make our fuel storage problems proceed at a very slow pace. It may take months or years for the problem to become evident, but when it shows its ugly head, it is "full growed up." It is mean and ugly and spoiling for a fight. Remember that the first time it happened to me there were ten-gallons of crud at the bottom of each fuel tank.

The important thing to remember about my first experience with this problem is that just removing the symptoms did not remove the problem. The problem came back three years later.The second time it happened to me I vowed to solve the problem so I would not get the symptoms.

Design Considerations

While it now seems obvious, the real objective escaped me for some time in this venture. The objective is to return and/or keep the fuel just like new. That's just what the NC Ferry System does by their high turnover rate. But that statement needs to be refined a little. We need to determine just where we want to make the fuel just like new. The answer to that question will determine where our design goes.

The common system with one secondary and one primary fuel filter tries to keep the fuel that enters the injection pump just like new and the only effect on the remaining fuel is the returned clean fuel. Once we understand that most of our engines return very little fuel to the tank it becomes obvious that this system has little or no effect on the fuel in our tank. The fuel in the tank keeps getting less and less just like new, and filter replacement intervals decrease.

Depth Type Filters

Depth type filters provide better filtration than edge type filters and are available with much larger capacities than edge type filters. Like most on this list, I was introduced to depth type filters very late in the game. I did not even know of their existence until after I initially solved my fuel problem with a polishing system using a Racor edge type filter.

Gulf Coast Filters (GCF) manufactures the depth type filters I have experience with, but there are many other manufacturers of such filters. I have every reason to expect that those filters perform well.

The basic concept of the depth type filter is that the fuel passes through a lot of filter media. For example, the fuel travels from one end of a roll of paper towels to the other as it passes through a GCF F-1 fuel filter thus the depth. It is that eleven plus-inches of contact with the filter media that makes this filter so efficient. It is its huge volume, when compared to the volume of a Racor-500 or 900 that gives it such an impressive capacity.

Depth type filters are not required for the diesel polishing system, but they will increase the efficiency of the system and reduce the maintenance. I will describe the application of these filters as we go along, but you can just substitute any other quality filter you desire.

The System

I do not have a separate polishing system and a normal fuel supply system. I modified the existing fuel system to incorporate the polishing capacity into it.

I also recommend that a single fuel/polishing system supply all the fuel to every engine on the boat. If your total diesel circulation requirements are not more than 60 GPH, a single Racor-500 or GCF F-1 will handle your needs quite well. Systems that require greater circulation rates are beyond the scope of this report and will not be covered here. CaptnWil will be happy to discuss designs for such systems if desired.

The system consists of the appropriate filters connected to a continuous duty electric fuel pump with the piping arranged to circulate the fuel from the tank through the filters and back to the tank.

The system has evolved over the almost two-years that I have been investigating this problem. That evolution has come about as I have learned more about the subject from my own observations and the experiences of others.

The single most important discovery is that a great amount of circulation is necessary to keep a substantial quantity of fuel just like new. My pump circulated about 50 gph and it ran at least 48 hours every week. With a fuel capacity of 600 gallons, I turned the fuel over a little more than three times per week. Continuous circulation is not too much.


The major components of the system are:

1. Walbro fuel pump. The factory phone number is (517) 872-2131. I bought mine from Peterson Co whose phone number is (800) 537-6212. This little pump is protected by a five-amp circuit breaker so it draws hardly any current. It is marketed to replace the original diaphragm operated fuel pump on diesel engines and is rated for continuous duty.

If you choose another pump, make sure it is rated for continuous duty.

2. Gulf Coast Filters F-1 fuel filter and water separator. The phone number is (800) 398-8114. This filter is recommended to be the main fuel filter if it can be fitted in the space. It uses a roll of Bounty paper towels. The element replacement cost of this filter is always attractive, but if you have a very dirty system to clean up, you will appreciate it even more. In addition, it filters in the sub-micron range and will add life to your injection pump and injectors.

This filter has the best water separator I have seen. It is efficient and very large.

The only drawback about this filter is the vertical clearance it requires. It must be mounted vertically for the water separator to work. The whole thing with clearance above the filter requires about 33 inches of total vertical clear space. The vertical clearance can be reduced to about 24 inches if a tilting mount is fashioned to facilitate removing the element.

If you can make it work, your troubles will be rewarded with a marvelous filter.

This filter is rated at 250 HP and can be paralleled if the engine capacity is above that.

3. Gulf Coast Filter O-1 Jr. If you can't make the F-1 fit, you can use a Jr. It was originally designed as a small bypass oil filter, but can serve very well as a fuel filter.

It comes with a molded-in orifice to restrict the flow in oil bypass work. If you use the filter for fuel, drill out the orifice. I use one on a 180 HP engine.

This unit uses a roll of toilet paper as the element and can be fitted most anywhere in any position. Its drawbacks are that it has less capacity than the F-1 and does not have a water separator. If you use this filter, you must make sure a water separator is in the system.

4. Racor Filter and Water Separator. If you choose not to use either of the GCF units above, you will need one like the Racor. This unit is well known to all and is available from many sources. The larger the size the longer the element will last, but its capacity will be very small, and its element replacement cost will be large when compared with the GCF filters above.

The Racor water separator works fine and the element won't pass water. Be sure to use the 2-micron element in this and any other work.

There are other similar units on the market and I expect they work fine, but I don't have experience with them. Of course, Racor elements can be obtained almost anywhere there is water.

5. Tubing, Valves, and Fittings. Type L soft copper tubing or USCG-approved hose can be used. The copper is better, but the hose is easier. I went with the hose. I suggest flare fittings for either material. Use the same size lines as is on your fuel system now.
Several valves are in the system. The West Marine catalog lists _-inch ball fuel valves that I like and also conventional Tempo fuel valves. Either will work well. Use Loctite 242 on the screw fittings. I've never had a leak with this stuff.

Two vacuum gauges are specified. Every vacuum gauge must be fitted with a shut-off valve. If the gauge doesn't have a shut-off valve, and it breaks, you can't run the engine and if you have a bottom outlet fuel tank, you run the risk of emptying the fuel tank into the bilge. Do Not! I say, Do Not use the Racor gauge that replaces the T-handle. It cannot be fitted with a shutoff valve.
A check valve will be included in the piping diagrams. It can be obtained from most local shops that do diesel injector pump and similar work. Its use will be discussed in the circuit description section.

6. Timer. While a timer is not required, providing one costs little. I used an Intermatic Spring Wound 12-hour Timer Switch, model FF12HH. It provides a disconnect switch as well as a 12-hour timer. I bought mine from Grainger.

Fuel Piping Basics

A basic fundamental in designing a diesel fuel system for any boat is to design the system so that the boat will not have to stop if a fuel filter gets clogged or an instrument breaks. There are many situations where not having engine power is extremely dangerous. It is so dangerous that the Rules Of The Road have a special category and rules for it, "Vessel Not Under Command." It is necessary to design the piping so that just adjusting a few valves will allow fuel to flow so the vessel can remain under command.

This basic requirement is at the heart of the diagrams shown. It is the reason that valves are shown for each vacuum gauge and the reason for the two-filter arrangement in both diagrams.

You will notice that the GCF F-1 and O-1 JR. are piped in series with a Racor in "Polishing System With Gulf Coast Filter." This is possible because the F-1 and JR will filter particles much smaller than the Racor and the Racor will remain clean almost forever. If the F-1 gets clogged up, you may safely bypass it with its bypass valves and just use the Racor until you are in a place to replace the GCF element. In like manner, if the Racor gets clogged up you can just bypass it and operate on the GCF alone until the Racor element is changed

In "Polishing System With Racor Filters," the Racor filters are piped in parallel because they have the same filtering efficiency.


In both cases, the vessel can continue underway by adjusting the valves.

Everything possible should be done to keep from having to make filter changes underway. That is the purpose of the vacuum gauges. If they are inspected often they will, in most cases, give ample warning so panic actions underway are not necessary. The bypass piping allows for the unexpected situation when it occurs.

System Operation

It is recommended that the Walbro fuel pump be operated all the time the engine is operated in addition to any weekly pumping schedule as required. This will mean that you are polishing the fuel any time you are underway. If you are on a cruise that means you may not need to operate the polishing system on its weekly schedule. You can just operate on the weekly schedule when the boat is idle.

In any event, you should pump enough to turn the fuel over at least three times per week. More is better than less.

Check Valve

I try to avoid using check valves in any piping system where possible because they are prone to fail because of crud accumulating on the working parts. This system uses a check valve that lives in the very clean diesel fuel that we have just polished. It should stay clean and have a very long life.

The system is designed so that the Walbro just keeps a positive head on the engine lift pump while underway and bypass most of the fuel around the engine. If the Walbro were to fail while underway, the engine lift pump would try to draw fuel from the engine return line and the engine would stop running because of air in that line. The check valve will prevent that problem. The two-way valve in the same line can be used to close the return as well, but the check valve just makes it more convenient.

Three-Way Valve

The three-way valve is provided for those who do not want to run the Walbro underway. In that situation, turn the three-way valve to the proper position and adjust the two-way valve in the line that has the check valve.

Fuel Circuit

This is a CaptnWil Law when using the polishing system:

Return the fuel to the same tank it came from.Any other procedure may lead to a fuel spill. That means making sure the valves at the fuel tanks are always set properly.

Other pumping arrangements may be more efficient at cleaning up the fuel, but the risk of a fuel spill removes them from consideration.

Day Tanks

Day tanks have become a popular topic of late, and a few words are necessary about them in connection with a fuel polishing system.
The object of a day tank is to provide a "day's" quantity of fuel that is guaranteed to be clean and dry for the engine. Sometimes it is used to overcome the problem of excessive suction heads on the engine lift pump.

It is important to remember that the clean and dry fuel in the day tank will be exposed to the same conditions that cause the fuel in the main storage tank to become bad. Given enough time, the fuel in the day tank will get to the same condition as the fuel in the main tank. A filter and a water separator are still required on the outlet of the day tank.

To provide the desired results, this tank must, in fact, be a "Day Tank." That is, the fuel in this tank must remain there for only a short time. All the problems of long-time fuel storage in the main tank will be present in the day tank if fuel remains in it for long periods of time. Once it ceases to be a "Day Tank" it must be treated just like any other tank.


All of the usual safety wiring practices should be followed and the pump should be protected with a 5-amp fuse or circuit breaker. It will be convenient to locate the switch or timer close to the helm.

Used Gulf Coast Filter F-1 element and Racor 500 2-micron element:
F-1 element shown after 500 hours with clean fuel.
Racor element cutaway showing pleating of element.

Introduction to Captn Wil Andrews

Wil Andrews has a passion for testing equipment and developing systems.

For years, Captn Wil, as he is widely known, has applied his engineering background in the search of the best in diesel polishing, anchors and other systems for trawlers. Recently, family health considerations led to the sale of AfterSail, a Krogen 42, but Captn Wil continues to be an active and respected participant on Trawler World List.

When you see the signature Captn Wil, you better be ready to toss preconceived notions and advertising claims out the porthole.

Much of the equipment testing and system development undertaken by Wil Andrews was accomplished aboard his beloved AfterSail, a Krogen 42 Pilothouse.



Copyright © 2008 SENTOA • Last Update October 2, 2008 • Questions? Contact the SENTOA Officers.