In a perfect world the fuel that you put into your diesel tank aboard the boat would be perfect, free of contaminants, water and dirt. But sadly the world is not ideal and fuel is often full of all sorts of stuff. One would think that fuel in a tank would be safe from further degradation but this is not so. As a tank heats up and cools down moisture laden air is drawn into the tank through the breather pipe where it condenses on the surface of the fuel then sinks to the bottom of the tank. This happens many time during the course of the year and often goes unnoticed by the average boat owner, the amounts of water may be small and either lie in the bottom of the tank below the level of the pick up tube or are mixed in the with the fuel through the natural movement of the boat where they are carried through to the engine and burnt off as a natural part of the combustion process. Water is not the only thing that can prevent the engine from running properly, general dirt and poor quality fuel are all out to do the nasty on the engine.
A natural question to ask is why do cars and trucks have fuel polishing system. Basically it all boils down to the fact that cars are used frequently and the fuel rarely stays in the tank for more than a few days or a couple of weeks at the most. Fresh fuel is constantly added and there is little opportunity for it to degrade. Leave a a car or truck with a half empty tank of fuel out in cold for a year and the chances are it will be very hard to start being afflicted by the very things that affect our boats.
How a fuel polisher works
Basically a fuel polisher is no more than an extra set of filters through which the fuel is circulated thus removing most if not all of the contaminants. Obviously it is essential to have a good filtration system on the engine and this most often consists of a large primary filter and secondary filter before the fuel reaches the injectors. The problem arises in that on most sailboats the engines are quite small and as we want to sail rather than motor the engine is used infrequently. In a sail boat with a modest 20 gallon tank it is not unusual for the majority of that fuel to remain in the tank for many years where it continues to degrade. All tanks have to be vented and during the warmth of the day the air in the tank expands, add to this the fact that the fuel returning from the engine via the injector return pipe is warm. At night when the temperature drops air is sucked in through the tank vent and condenses on the inside of the tank and the surface of the fuel where it drops to the bottom of the tank where it can be picked up by the pickup tube. Small amounts of dirt and water will be filtered out by the engine filtration system but that only works when the engine is running. What is better is if there is system that filters the fuel on a regular basis when the engine is not running. Fuel polishers have been available on many large power yachts for many years but it is only that sailors have started to wake up to the fact that they may be a good idea on their boats too.
Filters are measured in microns, this has nothing whatever to do with the physical size of the filter but is the size of particulates that the filter will capture. Parker who make the very popular Racor range of filters offer filter sizes of 30, 10 and 2 micron. For most sailors the 10 micron size offers a good balance between adequate filtration and not becoming clogged readily. It is also worth mentioning that the average Racor filter is capable of filtering upwards of 50 gallons per hour which is far more than the total amount of diesel that we have in our tank.
In it's most basic form a fuel polisher is simply a pump and a filter through which the fuel is drawn from the tank passed through a filter then back to the tank. Even a quite modest fuel polishing system is capable of processing 50 gallons per hour and if run a couple of times per week will keep the fuel in tip top condition. Many fuel polishers incorporate some sort of automatic time switch which can be programmed to turn on and run the fuel through the filter even when the boat is left unattended. This can be a great option especially for boats they are laid up during the winter months it will mean that a battery will have to be left on board to supply power for the pump but power consumption for even a fairly large system is going to be less than 2 amps so it might be worth considering taking off the large boat batteries for winter storage and replacing it with a smaller cheaper battery from the local Auto Zone or Napa for the sole purpose of
running the polishing system pump, remember that you will need to periodically charge this battery though. There are two basic variations of installing the system components and these are shown in the accompanying diagrams; either a dedicated fuel polishing loop or incorporating the fuel polisher into the feed to the engine. I think that with bigger engines it may be better to have the dedicated loop especially when the fuel tank supplies fuel to more than one source such as the propulsion engine and a generator. As I only have a small auxiliary engine I opted to install it into the fuel feed line. This has the added advantage of adding an additional filter into the line before the fuel feeds to the engine and allowed me to do away with my existing rather small primary filter which I incorporated into the fuel delivery line when I built the boat. Much more fuel is sucked up than can be used by the engine and this excess fuel passes through the polishing module and returns to the the tank by the return line. My tank is only 6 gallons when full and experience has shown that more than 10 gallons an hour passes through the filter with the engine running, thus a couple of hours under motor will pass all the fuel through the filter 3 times, more than ample to keep it in tip top condition.
I bought the Racor polishing kit which contains the filer, module, pipe and fittings for around $550 but if you want to add the timer then expect to add another $150 to that price.
Step by step to installation
Sit down and with a pencil and paper draw a plan of where you want to install the polishing filter, pump and any other components that you want to install in your system. This will help you decide pipe runs so that before cutting into your existing system you will have all the required parts, tools and accessories to hand. My system is simple and there are only two main bits, the filter and the pump. I installed these on the bulkhead inside a locker fairly close to the fuel tank. There is no reason why the fuel polisher has to be installed in the engine space and in fact there are lots of reasons for not doing so. Engine room space is often severely limited on many boats and installing another piece of hardware will make changing filters that much harder. Also the filter needs to be installed vertically.
If teeing into existing cut the fuel lines with a set of cutters or a knife avoid using a saw as the swarf will get into the fuel and could find it's way to the injector pump which will not like it at all. You will need to cut into both the flow and return lines from the tank.
Install some nylon tee fittings and secure the pipes with stainless pipe clamps.
Ensure that the pipe is suitable for fuel use.
Bolt the filter in a convenient position remembering that you will have to get the top off later to change the filter element, also you will need access to the bottom of the glass bowl to drain off any water that separates out.
Note that the filter is marked in and out for flow coming in and clean fuel going out.
Next install the polishing module which can be close to the filter. Leave the cover tapes in place to prevent contaminates getting inside until you are ready to connect up the pipes.
You will need brass NPT fittings to to screw into the module and filter. Use Teflon tape to seal the threads but keep it back from the ends so bits don't find their way into the fuel.
Nip up the fittings up but don't over tighten.
Everything connected up. The pipe coming out of the top of the module goes back to the tank as the return. The pipe on the right is the suction pipe the pipe to the left goes to the engine.
Additional notes
Some installations use copper. Don't cut these with a hack saw instead usea pipe cutter for a neater job which is less likely to distort the pipe.
Slide on the clamping nut
Then slip the brass ferule inside the pipe
Push the pipe into the fitting and then tighten with the correct size wrench.
The neat completed joint.
Finally double check all hose connections to ensure that there are no air leaks. Although this is not directly part of the fuel delivery system to the engine leaks on the vacuum side of the pump will mean that air can find it's way into the fuel system when the polisher is not running.
Connect up the electrical connections which merely consists of connecting up to a spare breaker. If installing a timer the wiring will be a little more complicated but I just had to run a couple of wires to a spare breaker at the chart table so I can run the polisher at a time of my choosing.
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