To fill or not to fill....(the diesel tanks)

[Dockhead]

Originally Posted by S/V Illusion

Condensation in a partially full tank can occur but is generally more a myth than a real problem. What is a real problem is degradation of diesel fuel. Diesel can get “old” and it doesn’t age well.

If you want to avoid fuel problems, the goal should always be to burn fresh fuel whenever practicable.

This.

I think it's been proven (by our MaineSail, empirically) that condensation in partially filled tanks is a myth.

Diesel fuel -- as long as there is no bio in it -- does last a long time, but fresh is always better. So I would burn it up and don't worry about keeping the tanks full -- provided you always have an adequate reserve in case of need.

Note that keeping the tanks clean can be more of a challenge with low-turnover fuel tanks (whether or not they're kept full). Pay attention to maintenance of the tanks -- regular inspections, cleaning as needed, etc.


Now if you can't get bio-free fuel, all bets are off. The bio stuff is really nasty, really unstable, and highly susceptible to diesel bug. In this case you need to be sure not to let the fuel get old. There are different types of bio additives, and not all behave the same, but some bio diesel only lasts a couple of months before it starts to go off. Beware!

[Pete7]

Agree with Dockhead.

How about emptying one tank, clean it out and leave it empty then just use the other tank. However, do replace the filler o ring seals regularly as this is probably the way water gets into the tanks.

Pete

[Dockhead]

Originally Posted by Pete7

. . . However, do replace the filler o ring seals regularly as this is probably the way water gets into the tanks. . .

Pro tip.



Flush deck-mounted fuel fillers are from the devil. The source of probably 90% of fuel problems.


Worth maintaining fanatically and watching carefully. Just failure to tighten the cap properly can ruin a load of fuel in one minute if you are taking water over the bow sailing upwind in a chop!

[Rucksta]

Originally Posted by stormalong

NEVER EVER close the breather of a fuel tank full or empty. Thermal expansion and contraction will cause partial pressurization/vacuum of the tank stressing the tank and every fitting and hose. Fuel tanks are not made to be pressurized. An extreme temperature change and you may end up with a bilge full of diesel.

Yet when I pull up to the bowser and open the gas cap the tank exhales a sigh of relief.
Why don't cars (not EVs) spontaneously burst into flames everywhere?

Why don't sealed 50 gal fuel drums as deck cargo split seams?

Why do we see sealable vents on portable outboard fuel tanks if a vent is never to be sealed?

The tank in a boat is usually low and partially below the waterline where the temperature does not change much or rapidly compared to a road going vehicle or deck cargo or a tank lying in the sun in the bottom of the dinghy

Lines and clamps are fuel rated with working pressures well above any that would be generated by thermal expansion.

Before taking delivery and installing a new metal tank I want to see it pressure tested.

Tanks are built to pass a presurisation test.

[Bobby Lex]

I'm really not taking sides here on the expansion/contraction venting debate. I don't know which theory has actually been empirically proven.

But FWIW my boat builder specifically advises to open the tank inspection cap when fueling up to prevent damage to the tank from filling with fuel faster than the vent can expel displaced air. He recommends the same when filling the water tank as well.

That recommendation may be coming from the lawyers not the engineers, but when I asked him "is that really necessary?" he answered in the affirmative.

Bob

[stormalong]

Originally Posted by Rucksta

Yet when I pull up to the bowser and open the gas cap the tank exhales a sigh of relief.
Why don't cars (not EVs) spontaneously burst into flames everywhere?

Why don't sealed 50 gal fuel drums as deck cargo split seams?

Why do we see sealable vents on portable outboard fuel tanks if a vent is never to be sealed?

The tank in a boat is usually low and partially below the waterline where the temperature does not change much or rapidly compared to a road going vehicle or deck cargo or a tank lying in the sun in the bottom of the dinghy

Lines and clamps are fuel rated with working pressures well above any that would be generated by thermal expansion.

Before taking delivery and installing a new metal tank I want to see it pressure tested.

Tanks are built to pass a presurisation test.

Boat tanks are pressure tested when new to determine if there is a hidden leak.

Car fuel tanks are made to be pressurized.

I stand by my opinion.

NEVER EVER seal a boat fuel tank.

[mwardmoeller]

"Three things that are of no use to the aviator
The runway behind you
The altitude above you
The fuel on the ground

Love it. Thanks for the advice. Definitely confirmed through the thread that there are pros/cons to both. Leaning towards using fuel return to burn/use/empty one tank and run from that side to use up the fuel. Maybe live off one tank for the summer then fill all (after inspecting/testing) for next sailing season.

Fair Winds, all, and thanks.

[Dockhead]

Originally Posted by Bobby Lex

I'm really not taking sides here on the expansion/contraction venting debate. I don't know which theory has actually been empirically proven.

But FWIW my boat builder specifically advises to open the tank inspection cap when fueling up to prevent damage to the tank from filling with fuel faster than the vent can expel displaced air. He recommends the same when filling the water tank as well.

That recommendation may be coming from the lawyers not the engineers, but when I asked him "is that really necessary?" he answered in the affirmative.

Bob

Open the inspection port before filling fuel and water tanks?!

If that is really necessary, then the vents are not properly designed, is all I can say. Certainly not necessary on any boats I've owned or operated.

What concerns condensation in empty or partially filled fuel tanks, here is MaineSail's article: https://marinehowto.com/does-an-empt...nk-condensate/

He proved empirically that it doesn't make any difference whether a fuel tank is empty or full.

And if you think about it, it's fairly obvious. The myth holds that your fuel tank "breathes" with changing temperature day and night, pumping moist air through which then condenses water out into the tank. This can't be, for a few different reasons. First of all, although the outside air temperature varies day and night, the temperature of your fuel tank likely does not (depending on where it is, how big it is, etc.). Mine does not at all - I measured it. Second, even if it did "breathe", it would be breathing out when it warms up, and in when it cools off. To get condensation from "breathing", you need to be bringing warm moist air into the tank, and then cool it off inside the tank. But any breathing which takes place will be in the opposite direction of that.

So -- complete old wives' tale. Totally busted.

My father believed in this nonsense and so after every single time we went out cruising on his boat, we always wasted an hour or two going into a fuel dock to top the fuel tanks off to the very brim. Despite this he had regular diesel bug problems.

I've never done this on my boat, leaving half or even quarter filled tanks over the winter on a regular basis, and never had any fuel problems (knock on wood). The difference could be better quality fuel in Europe compared to Florida and Caribbean. Or it might be that my Dad had deck fillers for the fuel tanks, and my boat does not.

[Dockhead]

Originally Posted by stormalong

Boat tanks are pressure tested when new to determine if there is a hidden leak.

Car fuel tanks are made to be pressurized.

I stand by my opinion.

NEVER EVER seal a boat fuel tank.

Car fuel tanks also don't just vent to the atmosphere like boat tanks. They are vented through a complicated emission control device. It's a different system completely.

[s/v Jedi]

I have had 350 gallons of dirty Venezuelan fuel and used it happily for 12 years. I filtered it at 30 microns with my fuel polishing system, taking out all the water (had to drain the bowl every 30 minutes of polishing), then used Startron once every year to keep it looking good. Never any problems with it.

Diesel fuel is an oil that doesn’t gum up, that is gasoline. It will last forever as I experienced and I agree with the condensation being a myth.

When you have two tanks and all fuel can be moved into one tank, then fill the empty tank with fresh fuel and just use the other tank up first.

[5BTM]

Originally Posted by Dockhead

Pro tip.



Flush deck-mounted fuel fillers are from the devil. The source of probably 90% of fuel problems.


Worth maintaining fanatically and watching carefully. Just failure to tighten the cap properly can ruin a load of fuel in one minute if you are taking water over the bow sailing upwind in a chop!

My fuel after a summer at the dock.
Rain and a fuel fill that was about 1/16 of a turn loose.

[sailingharry]

I will disagree with the upthread opinions that want you to keep the tanks filled at every opportunity. Indeed, fuel left at the dock does no good. But carrying around years' worth of fuel in an area where you don't go far and don't need much fuel and can get it easily is silly. If you were to take that boat and set sail for England, I would thoroughly agree that you should be topped to the brim. But where you sail, one empty tank is no problem, and I would even say only half of one tank is no problem. Use one tank till dry, shift to the other tank, and when the opportunity presents itself fill the other tank.

We also have two tanks. Our engine, a Yanmar 4JH2E, self primes. Accordingly, I routinely run my tanks until the engine dies, and then shift to the other tank. If your Beta is equally accommodating, you might consider that. This gets as much of the old fuel, and as much of the crud and water as possible out of the tank each time (that's what filters are for, right?)

Just to be clear, I run the tank dry in a seaman like fashion. When I am nearly dry, I shift to the full tank when in tight quarters. I ensure that when the engine does finally run out of fuel that I am in a safe position for that.

[Kd9truck]

So I have a 57 gal fuel tank, I went on the hard with half a tank this past winter. I pumped out the bottom 5 gallons of fuel, because I saw “stuff” in the bottom of my glass bottomed Racor filter, so instead of filling with fuel and additives before putting on hard I got the idea to remove “the stuff”.
Turns out I had about 2 gallons of impurities (water, bio mass) and now I have a clean tank. That may have been the first time my fuel had been “polished”, I have had this vessel 5 years so at 12 years 2 gal in a 57 gal tank might be average not sure. I sail my vessel back and forth off shore 200 nm from the summer mooring to winter on the hard every year so this years rough passage churning the tank prompted the polishing and I felt it was worth it.
I did notice in the photo above provided by another post, it did not look like the fuel was fresh, For me “use it, or lose it” applies they both looked cloudy imho. In this case I would be compelled to polish thought a fine micron filter, old clear fuel is ok imho, old cloudy or discolored fuel is just not worth the maybe 100$-200$ to me.

Some here are from part of the globe that do not have stringent regulations on fuel, I can not speak to that, I only have my own experience to comment on.

Cheers

[5BTM]

The fuel in the photo was at least 4 years old, probably older.

[Lee Jerry]

Originally Posted by Dockhead

Open the inspection port before filling fuel and water tanks?!

If that is really necessary, then the vents are not properly designed, is all I can say. Certainly not necessary on any boats I've owned or operated.

What concerns condensation in empty or partially filled fuel tanks, here is MaineSail's article: https://marinehowto.com/does-an-empt...nk-condensate/

He proved empirically that it doesn't make any difference whether a fuel tank is empty or full.

And if you think about it, it's fairly obvious. The myth holds that your fuel tank "breathes" with changing temperature day and night, pumping moist air through which then condenses water out into the tank. This can't be, for a few different reasons. First of all, although the outside air temperature varies day and night, the temperature of your fuel tank likely does not (depending on where it is, how big it is, etc.). Mine does not at all - I measured it. Second, even if it did "breathe", it would be breathing out when it warms up, and in when it cools off. To get condensation from "breathing", you need to be bringing warm moist air into the tank, and then cool it off inside the tank. But any breathing which takes place will be in the opposite direction of that.

So -- complete old wives' tale. Totally busted.

My father believed in this nonsense and so after every single time we went out cruising on his boat, we always wasted an hour or two going into a fuel dock to top the fuel tanks off to the very brim. Despite this he had regular diesel bug problems.

I've never done this on my boat, leaving half or even quarter filled tanks over the winter on a regular basis, and never had any fuel problems (knock on wood). The difference could be better quality fuel in Europe compared to Florida and Caribbean. Or it might be that my Dad had deck fillers for the fuel tanks, and my boat does not.

MaineSail only looked at a completely empty tank, NOT a partially filled one. There is a difference. I will posit that the condensation does form on the empty tank (inside and out, at times, when conditions are right), but then evaporates during the day, just like morning dew on your lawn or car. In a partially filled tank, some of this "dew" (a small amount, not nearly all, maybe only a drop or two) may drip into the fuel before it has a chance to evaporate. Hence the issue with partial fill. Why does he go through the trouble of draining his tank(s) for the winter?

Note I do not think this is a problem "in season" with use, so tanks do not need to be continuously topped off. But is (or can be) for longer term storage, and therefore shouldn't be left in a partial state and should be either empty of full.