Diesel Tank Full or Not?

[Timccarpenter]

We have a 28' sailboat with a 20 gallon fuel tank (so says the specs..seems less than that by looking at it). Regardless, we had the tank emptied and clean this spring by our marina. The marina filled it with only 5 gallons. We barely use any fuel at all. We motor about an hour each time we go out sailing and are still setting at above a 1/4 tank.

Is it best to keep a diesel tank full to prevent moisture build up? Even if it could literally take years to ever use any significant amount of the diesel? It is better to keep the tank fuller than a 1/4 due to any issues with the level when healed (if under power)?

Starting to think about what to do as we near fall :-/

[Statistical]

Fuller tanks mean less space of humid air which means less condensation of water into the fuel tank. Is it meaningful enough to matter? That I don't know. Personally I consider the 1/4 tank to the be emergency reserve and refill when we get that low but your sailing distance and conditions might make that excessive.

Is this a diesel tank? Diesel doesn't ever really go bad at least not on the timeframes we are talking about here. It can get water in it, it can get gunk in it, it can gets bugs which produce gunk but the diesel itself doesn't go back not even over a year or two or probably more. Yes it does oxidize over time but only the surface layer of the tank (another small advantage of fuller tanks) and the process is slow compared to gasoline. Gasoline on the other hand unless you are adding some stabilizing product will go bad over time. Months possibly certainly in a year or more.

So if it is a diesel tank I would fill it up or at least fill it half full and refill when it gets to quarter tank. Gasoline yeah that is a bit trickier I would only put enough in that it matches expectd annual usage and regularly add a stabilizer.

[Timccarpenter]

Originally Posted by Statistical

Is this a diesel tank? ....

So if it is a diesel tank I would fill it up or at least fill it half full and refill when it gets to quarter tank. Gasoline yeah that is a bit trickier I would only put enough in that it matches expectd annual usage and regularly add a stabilizer.

Yep, Diesel. Thanks for the input!

[rognvald]

Hi, Tim,
Modern fuel conditioners eliminate many contaminants and moisture. For a short season, it might behoove you to keep your tank half full so that the washing machine effect helps mix the fuel and any contaminants/dirt/algae is removed through your fuel filters. We kept our diesel boat on Lake Michigan for 13 years with a 6 plus month Winter overlay and never had any fuel problems. Before we brought it back South via the St. Lawrence, I emptied the tank and cleaned it and then filtered the old fuel back into the clean tank. I've never had a problem with fuel in 23K miles. Good luck.
Rognvald

[old pirate]

I will second the post above and add an additive. Something like “fuel doctor”. It seriously reduces contamination. Possibly double the dose, say 2ml per litre.

[CaptTom]

I've had good luck with BioBor to prevent bio-fouling. That seems to be the most common cause of gunked-up filters, in my experience. Personally I'm a "tank full" guy. I know there's been a lot of heated debate about the amount of moisture which can realistically form, but 20 gallons of fuel is pretty cheap insurance.

[rognvald]

I have used FPPF Diesel fuel conditioner since 2005. This was highly recommended by a renowned diesel mechanic/specialist for it effectiveness. However, there are many choices on the market.
Rognvald

[S/V Illusion]

Originally Posted by Timccarpenter

Yep, Diesel. Thanks for the input!

Not all input is created equal(ly).

First, both the US military and the ASTM have studied the effect of age on diesel. Both concurred that diesel ages and deteriorates in many characteristics after 6 months. Quicker in the presence of moisture.

Also, numerous studies have demonstrated that tanks not open with ventilation cannot accumulate moisture due to condensation. There is no path for moisture to enter or to exchange ambient air. Close the tank vent when in storage. And don’t fill a tank simply because of a myth.

[Dockhead]

Originally Posted by S/V Illusion

Not all input is created equal(ly).

First, both the US military and the ASTM have studied the effect of age on diesel. Both concurred that diesel ages and deteriorates in many characteristics after 6 months. Quicker in the presence of moisture.

Also, numerous studies have demonstrated that tanks not open with ventilation cannot accumulate moisture due to condensation. There is no path for moisture to enter or to exchange ambient air. Close the tank vent when in storage. And don’t fill a tank simply because of a myth.

100% agree with this.

Don't leave extra fuel in the tank just to let it age and deteriorate. Fresh fuel is always better. Fill up before you need to use it, not the year before.

Don't worry about "condensation". Partially filled fuel tanks "breathing in" moist air to condense out inside has been conclusively busted as a myth, by MaineSail among others.

Fuel tank problems are mostly caused by leaky deck fillers, which are the work of the devil. Frequently check the o-ring and make sure you keep it screwed down tight.

[rognvald]

Fuel maintenance is very important. Period. However, I remember many farmers I lived near in Northern Illinois left their older tractors outside throughout the entire Winter. They always started in Spring and were plowing once the ground thawed.
Rognvald

P.S. This is not an advertisement for diesel fuel abuse. R

[CarlF]

Keeping the tank full is mostly urban legend. Internal tanks don't have a condensation problem since the temperature in the tank hardly varies during the day. So it doesn't really matter how full the tank is. It would be different if the tank was outside exposed to the sun.

On the other hand, since diesel fuel was reformulated about 10 years ago for EPA reasons it doesn't last as well in a tank. It's reallly not great for the engine to use diesel that is more than 2 years old. The cetane level and other things are likely to be off the Yanmar fuel spec. While you can try putting in more additives, this is not nearly as good as using fresh diesel.

[grantmc]

Originally Posted by S/V Illusion

Not all input is created equal(ly).
First, both the US military and the ASTM have studied the effect of age on diesel. Both concurred that diesel ages and deteriorates in many characteristics after 6 months. Quicker in the presence of moisture.

Fun fact. Came across a village in remote Greenland that was relatively close to an old US military base from WWII. When the base was abandoned much was left. That included hundreds and hundreds of 40 gallon drums of diesel and oil. The locals have been using it ever since.

[CaptTom]

Originally Posted by CarlF

...Internal tanks don't have a condensation problem since the temperature in the tank hardly varies during the day. So it doesn't really matter how full the tank is...

I suppose that depends on your climate. Anything metal in my boat definitely accumulates condensation, especially in spring and fall when ambient temperatures change most dramatically over a 24-hour period. If I can see condensation on the outside of the tank, I consider it quite possible that there's condensation on any exposed metal inside, too.

Keeping topped off, summer or winter, isn't difficult and has served me well for many decades, myth or no myth. So far this season, the filters and Racors are clean and the engines running well. It's hard to justify changing my strategy.

[NorthCoastJoe]

If you really want to keep moisture out from the vent put in one of these:
https://www.amazon.com/dp/B0986VCVV3...hlbWF0aWM&th=1
As said earlier deck fill "O" rings are the worst offender.

[NYSail]

I run my tank down low before winter……. Below 1/4 tank. In my climate moisture in the tank is not an issue at least for me. I have a racor 500 as a primary filter/water separator and after 8 seasons with the boat have never had water in the bowl. Also I dont add anything additional. Just what has worked for me. I just make sure I add and use fuel and don’t let it sit. Only time I actually fill her is when we leave for our months summer cruise.
Good luck!