+xI'm new to sailing, but 'old' from the oil industry. It occurs to me the advice given in the the sailing community is nonsense.
- Polishing wont take the blocking crud out of fuel as the deposites form on and adhere to the tank. Anything in suspensions is small and must be removed on a continuous basis -- which the running engine does anyway.
- The crud is more likely to be degraded hydrocarbon which biocides wont touch and will form in even the best quality diesel.
- It's unlikely condensation is forming in sufficient quantity to cause a problem.
Diesel is hygroscopic. It attracts, but does not bond with water. The water takes two forms, emulsified (always present) and freestanding (at the bottom of the tank). I'm not sure how your average marine separator works, but it may not remove emulsified water. A FuelGuard or Diesel Dipper will remove both. A Mr Funnel works on the same principle and is cheaper.
The water leads to two types of contamination...
1. Microbial growth (Diesel Bug) thrives on the (freestanding) water/diesel boundary and asphaltene (more about this to follow). Once you have this growth in your tank adding new fuel to old accelerates the growth.
2. 'Diesel Sludge' is caused by the natural ageing process whereby molecules lengthen and bond to produce varnishes and gums. These drop to the bottom of the tank to form diesel sludge, containing asphaltene.... which in turn promotes the growth of diesel bug!! Anti-bug additives will not stop the formation of diesel slug.
Both 1 and 2 never meet a filter membrane in day to day use. They only cause a problem when they break free from the surface of the tank as a gummy mass that will block it very quickly. The water that's causing the problem lies at the bottom of the tank. Polishing won't remove it unless the polishing device draws from the very bottom of the tank and on a frequent or continuous basis.
So leaving your tanks full or part full of ageing fuel over winter does not sound like a good idea. Adding new full to old aged fuel is not good. It also suggests that during the active season running tanks low before refuelling might be the best approach.... however this might cause deposits to be picked up more readily.
Hi Andy,
Welcome to the sailing world.
Some confusion may be loose-ness in the definition of “polishing”, in the industry as well as casual talk in the boating community. I may be complicite in this in my writing. My “polishing” consists of drawing fuel off the very bottom of the tank, running it through a Racor filter (10 micron) which also separates out standing water.
I have no creds in the oil industry, but I will attest that when I take fuel off the bottom of my tank and run it through a Racor filter and put it back into the tank, the fuel is cleaner and there is less water in it. This is clear from the state of the filter (it gets dirty) which I can see when I swap filters and, luckily only rarely, by water in the bowl of the filter: probably free-standing water. This “polishing”, done when I choose, makes it less likely that the filters will load up and interfere with the running of the engine at a time not of my choosing, like running into a sheltered anchorage after a boisterous passage (I also have a dual-filter set-up).
I believe the Racor water separator works thru some sort of centrifugal process that needs to have a certain level of flow. It does work as attested by the very occasional noticing of water in the bottom of the Racor bowl (it separates from the fuel in a noticeable way).
Agree that getting water out is best done through access to the bottom of the tank, not always easy, in fact, generally a problem on most installs.
Agree that most of the problematic crud adheres to the side of the tank. And, you are also correct that, if the crud stays on the side of the tank, it will never see a filter. But it can get dis-lodged. This is why some vessels have a problem, after a boisterous passage, when they fire up their engine to go into an anchorage and find the engine sputtering just as they have rocks on either side of them. The fuel was agitated in the tank and the crud broke off, went to the bottom of the tank and accumulated until it reached the bottom of the pick-up tube and then, got to the filters.
I make a point, after a rough passage or day sail, to “polish” the fuel as described above and take advantage of the “shaking-up” that has occurred.
And, I believe, the jury is still out as to full or empty tanks over a winter. I have done both and not had a problem either way and research and research seems to find more arm-chair pontificators than those who have actually tested and written about the results such as RC Collins.
I am also clear that, properly dosed for stability (and for bugs etc. which some products do together and easy to do at fill-up), good diesel fuel is good for a very long time (recently attested by the number of boats, including mine, who fired up their engines and used fuel that had been sitting for 2 years because of covid restrictions to boat access). Sure, some had problems, but my guess is that the problems were more dirty fuel and water from improperly cared for fuel than the fuel had gone bad from aging.
I am also pretty sure, that for most usage profiles for the recreational sailor, that frequent or continuous drawing off the bottom of the tank is not necessary. At least, it has not been for Alchemy. Tanks usually have a few inches on the bottom below the pick-up tube where water can accumulate before it gets picked up by the pull of the engine pumps. Even then, most boats have a combo filter/water separator to catch some water. If you draw off the bottom of the tank a few times a year, the small amount of accumulated water will be disposed of prior to becoming a problem. (The biggest danger to water in the tanks is a cracked or missing O-ring in the fill deck-plate. (This can an does let in a surprising amount of water and should be inspected/lubricated as regular maintenance.)
My best, Dick Stevenson, s/v Alchemy