High Humidity = Drain Racor Frequently

One of the things that has really surprised us in the high humidity of Florida is that we have to drain the Racor after every single time we run the engine. The fuel — and gunk — in the photo above was what we drained after just 3.3 hours of run time.

If we run the engine all day (as we did when we left the boatyard on the Okeechobee and motored to Ft. Myers), we’ve found we have to drain the Racor after about four hours or the engine will stall. Of course, this usually happens just when we’re in a tricky pass or narrow channel . . .

Basically, what happens is that during the day, the air in the fuel tank heats up and expands. Then at night, when temperatures cool down, the air contracts and draws in moist air from the outside and all that moist air condenses and puts water droplets into the tank. The water drops to the bottom of the tank and where the diesel meets the water, microbes thrive and become sludge.

Keeping your tanks full helps considerably as there is less air being exchanged every day. But there’s no way to totally avoid it. You can’t close off the diesel vent or the daily expansion and contraction will harm the tank itself. And while there are some sophisticated air dryers to go on the vent, they are expensive and complex to install.

When we cruised the Sea of Cortez – a desert area – we never had problems with moisture and sludge in the fuel tanks.

Here, with 80 to 90% humidity (and a fair number of days over 95%), it’s a different story.

The good news is that we have a Racor filter to catch the water and sludge before it hits the engine. Actually, we have two Racors so that if one gets stopped up, we can switch to the other while we drain or change the first (as may be necessary). IMPORTANT NOTE: While a Racor will trap water, it does not have a check valve to stop fuel flow when the Racor is full. In other words, if the Racor is full of water and sludge and you don’t drain it, water in the fuel can pass to the engine. The engine may stall and  you can have significant damage to your engine.

Bottom line: you can’t just assume that because you have a Racor filter, your engine is protected. You have to drain the Racor frequently. Our rule is every four hours of engine time or at the end of a day, whichever comes first.

Dave has to use a small cup to catch the fuel and water that drains out, and it’s hard to see the condition of the fuel in it. So he pours the drained fuel into a plastic bag and we tip it towards one corner to see how much “crud” falls to the bottom in a few minutes.

If the boat has sat for a while or if we see that we’re getting more water and gunk than normal when we drain the Racor, we polish the fuel (read more about DIY Fuel Polishing).

You can prevent a lot of VERY expensive diesel repair by doing this every time!

A quick note: diesel that’s yellowish in color (called “clear”) is no different than the pink (called “red”) as far as your engine goes. It is merely a dye to show what taxes have been paid. The difference is whether it’s designated for road use (clear – you pay road taxes when you buy it) or off-road (red and no road tax but often fuel docks charge more to begin with due to their convenient location). There’s also blue diesel that’s only for US Government vehicles. Boats can use either clear or red (or blue if it’s owned by the federal government; there are serious fines if non-US Government vehicles are found with blue diesel).

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  • Tony
    Posted at 04 January 2016 Reply

    Hi Carolyn!
    Doesn’t the company that you mentioned using the air dryer’s in the food packages have an “in fuel line” dryer?

  • Jim Shell
    Posted at 04 January 2016 Reply

    Sweet Louise! That much water and crud from condensation in a partially full tank! We boat in Galveston, Texas with high humidity and might drain our Racor once in two or three years. We always filter every drop going into our tank through an aluminum Baja filter, which is probably no longer available.

    Maybe something else is going on to get that much contamination?

    • Carolyn Shearlock
      Posted at 04 January 2016 Reply

      Sorry if I gave the impression that all the fluid in the bag was water and gunk — just the teaspoon or so in the corner. We also run all our fuel through a Baja filter. We’ve talked to other boaters here in Marathon who have a bigger problem with water than we do. You’re lucky if you’re not getting much — it may be a difference in the tank and vent configuration.

      • Jim Shell
        Posted at 04 January 2016 Reply

        Still, you are collecting a very significant amount of contamination more that we are. For the Racor to catch the teaspoon or more think about the actual amount in the tank…maybe a quart or more. Wouldn’t it be nice if we could have a low tank drain (pump out) to periodically remove the contamination that settles there?

        • Carolyn Shearlock
          Posted at 04 January 2016 Reply

          I’d love it if our tank had a sump (our previous boat did). It’s a pre-made plastic one and of course, they don’t. We thought it was something in our setup until hearing numerous other people here in Marathon having fuel contamination problems. Long-timers say that it’s common when there is unseasonably hot/humid weather followed by cool spells.

  • Raquel Hernandez
    Posted at 04 January 2016 Reply

    Brian Liddy

  • Wes Westerop
    Posted at 05 January 2016 Reply

    Well, there are fuel vent line dryers that work real well, such as H2Out http://www.h2out.com/#!air-vent-dryers/c1mor (no direct sales; buy on Amazon: http://amzn.to/1PdrqJj). There are also polymer absorbing pads that you can dunk in your tank to absorb water on the bottom, http://www.strickequipment.com/cgi/commerce.cgi?preadd=action&key=03-60038
    I have an H2Out filter on my 1995 Hunter Legend and it works wonderful. I have indeed only had a teaspoon of water in my racor in 2 years. I also use a filter when putting diesel in. You do have to take care when putting diesel in, maybe slow down, so that the tank can vent properly. I only noticed one, when fueling in LA at a commercial fuel station.

  • Bil Wakefield
    Posted at 05 January 2016 Reply


    I second the H2Out filters Wes described.

    They work great. We went through ~260 gallons of diesel the last 2 years and have yet to drain the Racor filters (but I do anyway just to remain proactive… We also have sump drains and so far the tanks have no water… knock on wood…)

    Be aware they have aluminum casings so to inspect the desiccant color you must pull off the top rubber cap. [i.e., they must be installed in an accessible location for maintenance…]

    This may be a worthwhile project for you given what you describe… sorry…



    • Carolyn Shearlock
      Posted at 05 January 2016 Reply

      The accessability is one of the problems that we encountered in trying to figure out how to install them on our boat. We keep looking and trying to figure out how to make it work . . .

      • Bill Wakefield
        Posted at 05 January 2016 Reply

        Carolyn, I absolutely understand.

        Accessibility stops a lot of our projects in their idea phase.

        Even though our fuel tank vents were reasonably accessible enough for installing the H2Out driers, I don’t consider them ‘conveniently’ accessible for maintenance.

        Therefore I may re-route [lengthen] the fuel tank vent lines to accommodate a more convenient location in the future…

        This is one of those painful decisions for us: Should we purchase [to have on hand- or take up more storage space…] the product we want so we can install it when we gain access to the area because of one or more other projects? I prefer to leave the inventory on the vendor’s shelves, but all too often we find the opportunity to install presents itself on an unplanned basis, and if we don’t have the product on hand…

        Best wishes keeping up with fuel moisture control!


  • JB
    Posted at 05 January 2016 Reply

    I didn’t read all the comments yet but you have a bigger problem then humidity. Check the o-rings on the deck filler. That is waaaay to much water. Did you have the tanks polished back when you had original fuel problems? Blasted not just drained and separated

    • Carolyn Shearlock
      Posted at 02 July 2016 Reply

      You were right, but it took another six months before the o-rings were cracked enough that we could see it (and that was with a magnifying glass). We looked when you mentioned it, but didn’t see a problem. Then we did: http://theboatgalley.com/the-cost-of-an-o-ring/

  • Andreas
    Posted at 07 January 2016 Reply

    Hi Carolyn,
    in Europe we have for Boats only the clear but in different grades, the normal Diesel contains minimum 10% FAME or called BioDiesel which brings for Engines not so frequent in use some Problems as it is not long term storage stable. The other is called C.A.R.E. Diesel which is made out of vegetable Oils and is long term storeable, and gives due to a higher cetan no. a bit smoother Engine run. May you should try to get the latter mentioned Diesel as it do not need Grotamar.

  • Suz Grosby
    Posted at 07 January 2016 Reply

    Do you use a biocide every time you fill the tank? Sailing from RI to humid FL we found this essential

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