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).
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).