Upgrading the fuel system and adding a polishing circuit

Like most boat owners during their first season of ownership, we had the odd issue to sort out when we sailed Ariel out of St. Joseph and toward Traverse City that first summer of 2003. Most of the issues were fairly minor, but one issue slowed our progress northward considerably and, at one point, left us without an engine during an approach through a narrow channel in unsettled seas and a good blow: algal bloom and sediment in the fuel tank. After changing filters and doing what we could to clean the diesel tank, we resolved to find a more permanent fix.

I spent some time browsing the trawler forums and decided Ariel needed an updated filtration system with a fuel polishing circuit. I purchased two Racor 500 FG filters off of Ebay and gathered the necessary bits to plumb the two filters in series. I mounted them on a piece of Starboard and routed the valves and lines such that I could switch between a primary filtration circuit, a secondary filtration circuit (the old Racor R24s), and a fuel polishing circuit. The fuel polishing circuit required a pump, so I bought a Walbro 6802 and mounted it in the port cockpit locker, above the fuel tank.

This system has been in service now for over 10 years, and it has worked flawlessly. It’s more than adequate for our fuel flow and capacity, but that’s what I wanted: a system that would end our worries.

The idea behind a fuel polishing circuit is to cycle the entire contents of the diesel tank through the filters, thereby removing any sediment or water that – unchecked – could cause more problems. I generally run the polishing circuit back at dock after a lively sail, when the contents of the diesel tank are likely to be stirred up.

This is the original concept as I drew it:




And the layout before it was mounted in the engine compartment:


The original fuel setup with the R24s and an old Fram, the orange top just visible above the oil fill.


The new fuel filtration setup in place.


New setup plumbed and ready for service.


On/Off switch for the fuel polishing circuit.


Walbro fuel polishing pump and mounting location in port cockpit locker.

walbro walbromountedbig


And here’s some of the original write up providing background and info about the installation from the old website, should you care to know a few more details:

When we finally tackled the fuel issue a day later, we discovered that Ariel was suffering from a major buildup of crud in her diesel tank. After removing the Racor spin-on element, catching it and the spilled fuel in a ziploc baggy, I gave the bag a good shake and held it up against the sunlight coming in through the companionway. It was virtually opaque; I could just make out thousands of tiny black specks held in suspension.

During Ariel’s winter layup, I will be draining the tank and doing another thorough cleaning to ensure a fresh start for next season. The most important part of our approach, however, is the addition of two Racor 500FG turbine filters in series, with our old Racor R24S plumbed on a manifold as backup so that we can switch to a fresh filter if the 500s begin to show signs of restricted flow. This setup will drastically improve the engine’s reliability factor, and it is something called fuel polishing that will make our crud woes a thing of the past.

Unlike tractor-trailers, trawlers, and other vessels that have a high fuel turnover rate, sailboats – especially here on Lake Michigan where tides and remote, inland slips are non-existent – often have fuel that is months (if not years) old. Invariably, condensation forms inside the tank, dripping into the fuel, creating a wonderful environment for growth. It is this growth – unchecked for 24 years – that caused all of our woes. Without a higher turnover rate, there is little that can be done to prevent this problem. Even biocide will not solve the problem once it presents itself; once the creatures are dead, their carcasses will still clog filters. But how to get a higher turnover rate? Fuel polishing.

By plumbing an electric fuel pump into the fuel system that circulates fuel through the filters and back into the tank, it is possible to keep old fuel clean and free from devastating blooms of crud. Completely turning over the fuel at least three times a week is the recommended practice. This is what we hope to accomplish this winter with our dual Racor 500 fuel filter installation, and a basic fuel polishing system.

After last summer’s nonsense and the pain of spending over $100.00 on Racor spin-on elements, I spent some time searching Ebay for a cheaper (in the long run) alternative to our current R24S filter. I was able to find two new 500FG turbine filters for 75.00 a piece with a much cheaper element–8.00 each vs. R24S at 25.00 each. Now that I’ve got the filters, I need to determine the best way to set up the filtration system so that it’s possible to change filters while underway, and maintain a fuel polishing circuit. After a bit of musing, I came up with the sketch above. This system would utilize all but one of our current filters, so we wouldn’t be wasting any pieces, plus it allows for an easy way to prime the fuel lines and filters–not to mention ensuring the likelihood of having an engine when we need it most.

The Walbro pump, located where it is, and the priming pump on the R24 allow for easy priming on both sides of the manifold (valve). After changing the filters in the 500FGs, all that is necessary to bleed the system up to the lift pump is to close the valve located just after the two 500FGs, flip on the Walbro and let it run for a minute or so. Once fuel has been drawn through the 500FGs, it’s a simple process of opening the valve to direct fuel to the engine, then operating the lift pump until there’s fresh fuel at the tertiary filter’s bleed points–although I doubt that would even be necessary since there shouldn’t be any air between the second valve and the tertiary filter. Similarly, switching the first valve to the R24, I can easily use it’s built-in pump to prime it. Here’s a picture of the pump I’m after, it’s a Walbro 6802, 7 psi shut-off, max. capacity of 60 gph, continuous-duty pump. The pump is often available new through Ebay or through Go2Marine.com for about 100.00.

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