Managing Fuel Quality

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When we left Seattle on our trip around the world, we were expecting fuel quality issues to be common and carried 48 Racor 900 fuel filters. Surprisingly, we used less than ten on the entire trip. Most of the fuel we got world-wide was surprisingly good. We did encounter some minor fuel quality issues, but each incident never consumed more than two or three fuel filters. Regardless of the fuel quality at the source, how do you ensure that only great-quality fuel reaches the engine?

One tactic is to filter the fuel on the way into the tanks using a Baja Filter. This is a common tactic with world-cruising boats. The downside is that pre-filtering slows fueling dramatically. It takes 1.5 to 3 hours to fuel us as it is and might take more than a day with an external filter. It just doesn’t feel practical when taking on more than 1,500 gallons.

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The second approach is avoidance: only go to locations that sell in large volumes to commercial boats so you know the fuel is good. This is a good tactic, doesn’t take any longer, and sometimes results in better pricing. However in some locations, such as pictured below in Port Vila, Vanuatu, we only found a single place to fuel. And that single fueling option didn’t normally stock as much fuel as we needed. To deliver the fuel we required, they needed to make special arrangements to get our fuel delivered by truck. Unfortunately, the truck would deliver only to the bulk tanks in the fueling station, who would then deliver it to us. The consequence was that the fuel station’s mostly-empty tanks got a large delivery of fuel, stirring up water and sediment, right before we fueled. This was pretty much the worst possible option, but since it was the only one, we took it. So, as much as we like the second approach of being selective about where we get our fuel, it also isn’t a general solution for Dirona.

Fueling in Port Vila, Vanuatu. Click image for a larger view

Since we don’t pre-filter and we can’t always be selective, that leaves us with the third tactic: expect some poor quality fuel and manage it. On this model, we worry less because instead of worry that we might get bad fuel, we more or less expect we’ll get some bad fuel once in a while and design the system to deal with it.

Here’s what we do. All fuel being delivered to Dirona goes into the two bulk side tanks of 835 gallons each, or into the on-deck fuel bladders when we are using them. Any fuel leaving our side tanks destined for the wing day tank or the supply tanks (where all engines but the wing get their fuel) is explicitly pumped through a 25-micron fuel-transfer filter. (We don’t “gravity-feed” from the side tanks into the supply tank). And fuel transfered below decks from the bladders first passes through the 25-micron fuel transfer filter before entering a side tank. (See Fuel for the Crossing and Managing Fuel Economy for details on our fuel bladder design.)

The transfer filter is a large Racor FBO-10, pictured below, with a truly massive filter area. The FBO-10 commonly is used in bulk-transfer, commercial fuel-management applications. It has the advantage of supporting large transfer rates, but it also has a large filtration area so fewer filter changes are needed. It takes a lot of block that filter. In fact, we’ve never blocked it, although we have purchased large amounts of rust, small amounts of water, and even some large cockroach-like bugs. The FBO-10 just catches it all.

In the picture at the top of this post, James has drained off the bottom of the FBO-10 and is checking to see if there’s a water problem. We only do this once every six months. In this particular case there is no water and only a little bit of sediment.

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By the time the fuel gets to the supply tank, it has passed through the FBO-10 25-micron bulk filter. From there the fuel passes through a Racor 900 2-micron filter, then a 10-micron on-engine filter, followed by a 2-micron on-engine filter. So all the fuel gets filtered at least four times on the way to the Deere injectors.

Our fuel filtration model is a little different than most and, like so many things in boating, this is an area of hot debate. Generally, the industry recommendation is to have stepped filtration on the way to the injectors, where each level of filtration is finer than the last. This distributes the filtration job over all filters and allows early filters to last well and do bulk filtration of the big stuff. Putting very fine filtration on the primary filter is loved by many, but frowned upon by most of the professionals. The concerns are legitimate and focus around two issues: 1) fine first-level filters won’t last long (they plug up fast), and 2) if not watched incredibly carefully, this can lead to fuel pressure problems on-engine due to unnoticed blockage.

Our original design followed the industry-recommended stepped-filtration approach. Between the supply tank and the engine we had a 30-micron filter and, on-engine, we had a 10-micron filter followed by a 2-micron filter. Because our engine is a high-pressure common rail system with fuel pressures frequently near 20,000 PSI, fuel quality is very important. That is why the manufacturer installs a 10-micron followed by a 2-micron filter pair on the main engine. The downside of this approach is that 2-micron filters can plug up quickly and might need replacing frequently. That on-engine filter pair, pictured below, costs US$140 and, more of a concern, can only be changed with the engine shut down.

Since most fuel delivered at the dock has never been filtered at anywhere close to 2 microns, the fuel often carries a lot of small particles. This isn’t an issue in that these particles will be filtered out, but it means 2-micron filters need frequent changing, often in the 200-500-hour range. The cost is a minor concern, but we really, really don’t like to have to shut down the main engine when at sea.

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With all these factors in mind, we instead went to a 2-micron primary off-engine filter instead of the original 30-micron. This filter costs US$13 rather than US$140 and, more importantly, can be changed without stopping the engine. Pictured below are twin Racor 900 filter housings, where one filter is in use and the other side is ready to go. When the reading on the pressure gauge between them approaches 7 in Hg (inches of mercury) of vacuum, we switch over to the backup filter using the lever James is holding. With all the systems still running, we can then change the old filter at our leisure.

We have fuel pressure readings on the dash, and we check the fuel pressure gauges in the engine room on each shift change when underway. That generally mitigates the risk of not noticing the filters are plugged, the reason manufacturers dislike this non-progressively-stepped filtration design.

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The final concern is that you can go through a lot of those 2-micron filters if you take on bad fuel. We solve that problem in two ways: 1) the very large 25-micron Racor FBO-10 in front of everything to catch the big stuff, and 2) we have more than 40 of the 2-micron primary filters on the boat, so we can afford to go through multiple a day and still not run out.

The end design has four levels of filtration: 1) Racor FB0-10 with a 25-micron element, 2) Racor 900 with a 2-micron element, 3) John Deere on-engine first-level filter at 10 microns, and 4) a John Deere second-level on-engine filter at 2 microns. We mitigate the faults with this non-stepped design by having many filters and lots of monitoring and alarms. Dirona Fuel Manifold has more detail on other aspects of the fuel system design.

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If, while operating, we learn that we have a bad load of fuel, we can configure our fuel system to be able to run the engine off the supply tank while at the same time “polishing the fuel” by running all the fuel from each side tank through the transfer filter and back to the same side tank. This should allow us to recover fairly quickly if we do have a problem. Thus far we’ve never had enough of a fuel quality issue to require this. The worse we’ve seen is needing to change fuel filters in 180 hours, which actually is pretty good.

Fueling in Rodrigues, Mauritius

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17 comments on “Managing Fuel Quality
  1. Mike Cero says:

    I’m concerned about the additional strain and premature failure of the fuel pump when using a 2 instead of 30 micron filter for the primary?

    • MVDirona says:

      Most marine professionals recommend staged filtration where the first stage is 30 micron and the final stages is whatever is install on the engine. On our boat John Deere has 10 micron and 2 micron installed on engine. Off engine we use 25 micron on our transfer filter, then 2 micron, then the 10 and 2 micron filters on engine. The reason we use 2 micron off engine is 1) we can change these without shutting down the engine, 2) they are $13 vs $140 for the on engine pair, and 3) we have lots of them.

      The only downside of finer primary filtration is they can plug up faster. But, whether you use 2, 30, or anything in between, you have to change filtration when it starts to resist flow. We recommend putting a vacuum gauge on the primary and we change at 5 to 7 inches of water vacuum. To be extra safe, we also have Maretron monitoring of the primary fuel vacuum but this isn’t required.

      If you regularly check the pressure and change below 7″ of water vacuum, there will be no negative impact on fuel pump or other fuel system parts.

  2. Tom Felt says:

    James do you use any fuel additive in addition to your six month treatment?

    Thank you for all your posts. We love reading about your experience

    • Hey Tom.Your comment reminds me that it’s been a while since our collective paths have crossed. For sure, we’re all closer to being in the same part of the world at this point.

      On additives, I don’t use anything other than the periodic biocide. I’m not against using additives but note that many/most of the fleet operators don’t use additives and the manufactures are neutral to negative on many of possible additive choices. When I worked for GM (admittedly many years back), they were largely negative on the use of additives for either gas or diesel engines. I’e heard nothing but good things about Stanadyne and it’s both highly recommended and seems to perform well. The only reason we don’t use it is it’s a hassle to enough enough on board to keep up with our fuel burn. We move the boat a lot and, some years, we would have used a ridiculous amount of Stanadyne where we using it. We are now at 9,400 hours and need the injectors replaced. Arguably an additive might have delayed this upcoming replacement but nearly 10,000 hours is a lot and I find myself suspecting I might change injectors at this point with or without additive use.

      • Tom Felt says:

        James. Thanks for the detailed response. Deana and I hope to cross paths with you and your Beautiful wife.

        Currently our N52 is safely on the Hard ground for winter in Preveza Greece.

        As always your insight is so greatly appreciated.

  3. Alex Goodwin says:


    Thanks for being very understanding as you correct my mistaken assumption that all the fuel removed from the tank gets burned up on the first pass.

    So, given that your fuel system averages 14 laps (frinstance) between supply tank and main engine, combined with a rule of thumb I read somewhere that filtering a tank 7 times when recirculating is equivalent to 1 filter pass when transferring tank-to-tank, that 2-pass-equivalent (14 laps when recirculating) would mean actually >75% of at-filter-rating particles get removed.

    No wonder you’re not too worried.

    • I agree we are fairly well filtered. The only point from your note I didn’t understand is why circulating through the engine passing through 2 micron, 10 micron, and 2 micron is better than a single pass through the 25 micron transfer filter. I would think that a pass through a filter whether driven by the transfer pump or the engine fuel pump would be equivalent.

      • Alex Goodwin says:


        Not sure where or how I gave that impression – I certainly didn’t mean to. From what I understand, I agree with you – a filter pass is a filter pass, no matter the pump.

        • Yes, makes sense. Filtering the heck out of all the fuel is a great way to get past small fuel quality problems and to ensure that delicate high pressure fuel components live a long life.

  4. Paul Wood says:

    Hi, James & Jennifer,
    This is probably a daft question but that looks like red diesel. I know it’s legal to run a boat on that fuel for propulsion, but what about the cooking and heating, do you ever run into issues regarding fuel duty? Is it like a 60/40 split at the dockside pump or does it not matter.

    The UK Excise are absolutely mustard on the correct use of fuel and it’s payable duty.

    • Different juristictions dye the fuel differently or don’t at all. In the US, we usually have red dyed fuel. In many parts of the world, it’s sold without dye (clear/yellow). The current fuel load was purchased in Ireland and it’s an unusual green color. We have not yet fueled in the UK but only have 450 gallons left so will want to fuel soon. We’ll probably get a load in Dublin but haven’t firmly decided yet.

      The percentage split between heating and propulsion with different taxation on each is not something we have seen outside of the UK and, depending upon where we next fuel, it’s possible we won’t take on fuel at all in the UK. I’ve read about some boats that have fueled in the UK have run into problems in other EU countries because they were operating with red dyed fuel.

      • Rob Heath says:

        Hi James,
        If you are coming down the English channel you might want to consider stopping off in Guernsey or Jersey for fuel, It is significantly cheaper ( less than 1/2 normal UK price )! Rockland, an N47 that I follow has an annual pilgrimage to take on a load of fuel in Guernsey.

        • It doesn’t look like we will e close to Guernsey or Jersey on this trip and we only have 400 gallons left so we’ll need to get fuel prior to our stop for boat work in Southampton. We’re currently thinking of fueling in Dublin.

  5. Alex Goodwin says:


    Not sure if you answered this question in the fuel manifold post comments (so it may be a dumb one), but are your off-engine filters (the 25-micron prefilter and the 2-micron one in the lever-changeover setup) absolute (>98% removal of particles at or bigger than rating) or nominal (>50%)?

    • Hi Alex. Racor uses a nominal rating on their 900 series of filters. These filters are sufficiently fine that the 10 micron and the 2 micron on engine only ever get changed on a time interval which is what I want.

      • Alex Goodwin says:


        Thanks for the quick reply. Good to hear you’ve got a system that works for you, Jennifer, and your fur-bearing employer, Spitfire.

        For cases such as the Port Vila top off you mentioned in the post, do you just polish the fuel you’ve taken onboard longer?

        • Even Port Villa wasn’t really that bad from a fuel quality perspective so we didn’t polish at all and just ran business as usual. Business as usual means all fuel gets a pass through a 25 micron filter to the supply tank. All fuel heading to the engine comes from the supply tank and first passes through 2 micron, 10 micron, and 2 micron. Most of the fuel that gets pumped to the engine is actually returned to the supply tank. The engine only burns a tiny portion of the total fuel moved. As a consequence, all fuel does many “laps” between the supply tank and the main engine. Effectively, we are always polishing the fuel in the supply tank.

          If we ever got unusually poor fuel, we can polish in parallel where the large transfer pump and transfer filter just circulate all the fuel from a given tank repeatedly. The main always runs and returns to the supply tank. In parallel, we can polish either side tank or the supply tank. But, thus far, we have never had enough of a fuel problem to require this explicit polishing step. Perhaps the biggest fuel risk we face right now, is the build up of asphaltenes or other fuel fallout in the main tanks or perhaps some growth fed by water build up. I treat the tanks with a biocide every 6 months to keep the later under control but eventually we’ll want to drain the tanks, climb in and clean out all the built up debris. When I was touring the Holland America cruise ship Westerdam (//, they were cleaning their heavy oil settling tanks. Yuck! Even dressed in a full disposable hooded suit with a face mask and filtered air, that didn’t look like fun. I’ve got pictures of me standing up in our tanks but that was before they had fuel in them. The good news is they are accessible but the bad news is the job won’t be a fun one.

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