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Old 07-05-2020, 20:05   #31
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Re: Leave a Diesel tank empty or Fill it

Mainsails experiment was in ideal conditions, not in a boat, but with a completely dry tank in a barn. A couple problems I have with that test, one, you cant get your diesel tank completely bone dry realistically. 2nd, the temp difference from being stored in the bilge partly below the waterline was not part of the experiment. Ive had condensation on the walls of my hull at certain times of year. The temp differences of the surface as compared to the air temp is what I think would attract condensation. I keep an eye on our racor and I periodically inspect our diesel tank. It’s always clean, no water. No algae. It’s not always 100% full, but I top it off fairly often.

I would be interested to see mainsails experiment modified to have the tank sit half way in a bath of 65 degree water, on a cold damp day, with a half inch of diesel in the bottom. Then see how if any moisture gets in the tank.
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Old 07-05-2020, 22:50   #32
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Re: Leave a Diesel tank empty or Fill it

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Originally Posted by slug View Post
On big yachts the main fuel. vent line has a valve and can be shut off to avoid humidity and fuel contamination

Also possible to use this system on small craft
Hmmm, interesting. Just thinking about the design concept of vents.

Being relatively large volumes it doesnt take much pressure differential to put a lot of force on the structure. Ie talking in the old PSIs of pressure. Large volume is lots Square Inches for the Pounds to act on= a lot of force with relatively small pressures.

Obviously this depends on factors like, temperature differences between daily high and low, the strength of the tank sructure etc. But the point of vents is to equalise internal and external pressures, so the tank doesnt blow out or collapse, or the engine has to try and suck against a large vacuum.

Or a high internal pressure could induce a leak.

Just sayin.
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Old 07-05-2020, 22:58   #33
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Re: Leave a Diesel tank empty or Fill it

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Originally Posted by kmacdonald View Post
Read this: https://pbase.com/mainecruising/fuel_tankt_condensation

Leave it empty. Less weight.
The conventional aviation fuel tank 'wisdom' is leave tanks full. This reduces the airspace at the top of the tank which is what breathes in the condensation.

But I dont know how much actual benifit this is? I doubt much practcally.

I also agree leaving empty is good if wanting to do any fuel system maintenance like tank cleaning etc.

Also fuel is known to slowly deteriorate over time. How much time is debatable.
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Old 07-05-2020, 23:29   #34
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Re: Leave a Diesel tank empty or Fill it

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Personally I try to keep it full. I use a biocide treatment. I don’t like the idea of condensation in the tank. General practice on anything with a motor was to always store with a full tank. Probably not as big of a deal on a diesel, but with gasoline empty tanks corrode and cause all kinds of issues. diesel is stable in a tank for a long time.
We have a diesel tank. Not metal. Our worry is moisture and fungi etc. We always try to keep our tank full and use a biocide. Never had a problem in 12 years of ownership, with considerable periods of no use. Of course we have the proper fuel filters and change them annually.
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Old 09-05-2020, 11:56   #35
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Re: Leave a Diesel tank empty or Fill it

For several years in Europe, diesel has been containing more and more vegetable oil. Bacteria are growing much faster than before. It is now risky to fill the tank in winter if you are not sure that the tank is perfectly clean. The only way to find out is to unbolt the tank inspection hatch. If you fill the already dirty tank, there is a risk that bacteria will proliferate even with treatment. In spring, the tank must be completely emptied to clean it. It is not simple and diesel is sometimes unusable. I therefore prefer to keep only 40/50 liters in the tank in winter. Before refueling in the spring, I open the hatch again to check that it is still clean. If necessary, it is easy to remove bacteria from the empty tank. The anti-swell plates inside have been modified so that they can be given access to the entire bottom of the tank.
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Old 10-05-2020, 01:25   #36
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Re: Leave a Diesel tank empty or Fill it

maybe I am missing something here - as I understand the basics - if you can eradicate or reduce substantially the moisture content in the tank , then you proportionately reduce the risk of algae growing in the diesel . To achieve that , I fill the tank ( roto moulded plastic ) ,which is under the port quarter berth ,to completely full i.e. to the filler cap ,at DECK level , so that there is no air in the tank at all ! .This remains like that for 9 to 10 months every year , during the peak heat of summer and sometimes snow in winter . The only occasion that I did not fill the tank , I came back to a slimy sludge of algae . I removed the tank to clean it - not a job I want do again , so I now use a measured amount of a biocide everytime I fill and maintain the regime of a full tank over the winter season . I don't believe that temperature or being half above or below the water line etc can have any real effect , this is surely over thinking the problem .
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Old 10-05-2020, 01:42   #37
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Re: Leave a Diesel tank empty or Fill it

Apologies ,I meant to add that I had my tank out 5 years ago to clean and opened it last year to inspect and found it absolutely clean - spotless . If you are filling to deck level , be VERY sure your filling hose and your vent hose and connections are not leaking at all ! ! .
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Old 11-05-2020, 09:46   #38
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Re: Leave a Diesel tank empty or Fill it

Always full - no condensation
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Old 12-05-2020, 06:08   #39
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Re: Leave a Diesel tank empty or Fill it

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Always full - no condensation


Ours spends the majority of the time at or below a 1/4 tank. No condensation either.
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Old 15-05-2020, 07:09   #40
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Re: Leave a Diesel tank empty or Fill it

So I did my own condensation test several years ago, but not for a boat tank. A sealed 5 gallon plastic pail on the ground under cover but exposed to sun and temperature changes. I drilled a 1/4" hole through the side near the top. Eventually it was 1/4 full of water. It all depends upon temperature swings, air flow, humidity, etc. Letting the sun hit it made for very large temperature swings that I doubt would occur on a boat although the liquid being cooled by the ground would be similar to the cooling affect of water.

By the way, put gasoline in the container and the effect will be much greater. Not so true with diesel.
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Old 15-05-2020, 07:40   #41
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Re: Leave a Diesel tank empty or Fill it

Conventional wisdom is leave a tank full with an added Biocide. I used Biobor for almost 20 years in the tropics. Had the tanks cleaned once but it was really unnecessary. That is perhaps because I rarely filled without using a Baha filter.
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Old 15-05-2020, 07:46   #42
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Re: Leave a Diesel tank empty or Fill it

That depends substantially on where in the world you are, and what material the tank is made of. In humid climates, a near empty tank with breathe. That will result in condensation, may promote diesel-bug growth, and may corrode the tank. IM (not so experienced) Opinion, an air bladder might be useful so the bladder breathes rather than the tank. This won't work so well if there are baffles in the tank. As someone already suggested, a dessicant (silica gel?) pack in the breather pipe sounds good too, just check it frequently in case it saturates.
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Old 15-05-2020, 09:03   #43
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Re: Leave a Diesel tank empty or Fill it

I think it might depend on what part of the world you are in. I understand large parts of the US have very low humidity, even on the coast so probably not a problem. Here in Blighty, the humidity is often very high, even on warm sunny days. As soon as the temperature drops the moisture has to precipitate out, which is why we have lots of evening mists and fogs at certain times of the year, as well as dewy grass even if it has been dry through the night without mist or fog. Diesel bug is a real PITA in UK, especially with the newer diesel containing ca 5% biodiesel and soon to go up to 10% bio diesel which is hydroscopic. I run a fuel polishing system on my boat for that reason, and have not had an issue since installing it, but prior to that, my tank was full of 'gunk' at the bottom, which fortunately did not do any damage to my stainless steel tank. I helped clean out a friend of mines fuel system on his much newer boat than mine, after his engine choked and died after a particularly boisterous trip across the Solent, I had to tow him into Gosport and onto his mooring because of it. We cleaned his tank out the best we could (cannot remove it because it is built in) and hit what was left with Marine 16 Shock Treatment. He now makes sure to dose his fuel with Biocide on every fill, something I have not had to do since installing the fuel polishing system. After the Winter lay up I probably collect about 10~20ml of water out of the Racor filter but the fuel stays clean.
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Old 15-05-2020, 10:14   #44
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Re: Leave a Diesel tank empty or Fill it

Nowadays we have cheap Endoscopes. No more guessing just LOOK at your fuel and tank walls, bottom, etc. USB and 30ft, illuminated and water/fuel resistant about 30 bucks. Take a sample with a tube for or water contamination check. EZ. So mine are mostly empty for easy cleaning when/if needed.
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Old 22-06-2020, 09:36   #45
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Re: Leave a Diesel tank empty or Fill it

Certainly the biodiesel which is now added to the diesel has been a -negative- turning point.


I have a 250 liters (66 gallons) built in fiberglass tank, with the interior epoxy painted. For almost 30 years we had no problem, just added the biocide dose when filling-in (an not always) and that was all. Fuel could spend the winter in the tank that being full, half full, or nearly empty, no problems ever.


Since about 5 years ago, it is becoming a mess, with algae and mud growing throughout the winter no matter what you do. I am now switching to enzyme treatment to see if it improves, but am not fully confident either.

Probably I will end up going for the ´tank as empty as possible during the winter´ so that in spring it is easier to check and clean the tank if needed, and also be sure that new fuel added for the summer is fresh and clean.
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