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OrangeCrush · 09-11-2016, 08:34

Hello,

My newly acquired boat is now on the hard in Brooklyn for the winter. I couldn't top off the fuel tanks before haul-out because of a total absence of a diesel fuel dock in Brooklyn. I have two tanks, one 30 gallon and one 45 gallon. I can transfer between the tanks and polish the fuel through either a 10 or 30 micron filter.

I've got about about 15 gallons left. I transferred it all to the rear tank so now the front tank is empty and the rear tank is about a third full. I've got some Kill 'Em biocide that I will add to both tanks. I will be burning some fuel (from the rear tank) with the Espar heater occasionally when I am down on the boat doing work over the winter. The boat will be shrink wrapped but well vented.

My current plan is to make about 5-6 trips to the nearest gas station this weekend to use my only jerry-can to fill up the rear tank. But before I do this rather tedious task, I wanted to see if perhaps the biocide, the ability to polish fuel between tanks, or my plan to use the heater occasionally made this less (or more) important. It will not be marine diesel if I go to the gas station, but I guess addition of a lubricant before next season would make that less of an issue anyway, if there is a difference at all.

I don't mind the chore if it is really the best thing for the motor, but since it's my first inboard I wanted to double check with you experts first...

I appreciate your opinions.

valhalla360 · 09-11-2016, 08:56

The theory is by staying full, you keep water from condensing out of the air by eliminating the air space.

But if you run the calculations on how much water can condense out of the available air space, the amount that could condense over the course of a winter is on the order of a few drops.

If you are getting noticeable amounts of water in the fuel tank, it's likely the result of a leak allowing rain water in. Often the fuel fill is on a level section of deck and if it doesn't seal well, rain water can find it's way in.

**a64pilot** · 09-11-2016, 08:58

If easy to do, I keep my tank topped off, habit more than anything.
I wouldn't bother if I were in your shoes myself, too much work and likely no need.

jkindredpdx · 09-11-2016, 11:33

I top off my tank each November, but wouldn't bother Schlepping cans.

SVTwilight · 10-11-2016, 07:51

Do it and use biocide. You'll be happy that you did!

doublewide · 10-11-2016, 07:54

I went to a diesel engine maintenance conference done by Mack Boring. Several attendees tried unsuccessfully to get the expert to recommend a fuel treatment or additive. Every time someone tried a different tack to solicit advice about additives, the expert said the same thing, "always keep your tanks full to prevent condensation." Nobody could get him to recommend any additive. He repeatedly said, "always keep your tanks full to prevent condensation" or words to that effect. I think he was trying to send us a message.

Sailmonkey · 10-11-2016, 08:09

I don't buy into the condensation thing. There's just not enough air in there, and not enough exchange of air in the tank to keep adding water.

If it makes you feel good, schlep Jerry jugs to the boat, but I personally wouldn't bother.

As far as marine diesel. Other than being pink, there shouldt be a difference (unless you've found something different). We haven't had dyed fuel in the boat in four years (I buy diesel 12 gallons at a time from the corner store and siphon it into the boat)

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Teknishn · 10-11-2016, 08:12

I work on larger boats and yachts and I always try to get the owner to keep the tanks topped off for a couple of reasons. First is condensation, the second is the corrosive effects of oxygen on partially filled tanks.

As an example, was working on a 40-meter yacht and the owner and his wife were enjoying cocktails with guests on the 01-level and complained of a diesel smell. Pulled the owner aside and told him that the smell was oxidation occurring inside his fuel tanks which were only about half full and had been for the past month. (Obviously these were not properly lined tanks). The next day we topped off by onloading 12 tons of fuel and voila! Diesel odor gone!

Oxygen in older tanks will cause corrosion which will cause that diesel odor when partially filled. Worse, that corrosion flakes off at some point and then becomes sediment in the bottom of the tank, which then one finds out at the most inopportune moment that the fuel filter is clogged.

For me, water is less of an issue, that's what seperators are for. Just my 2-cents worth.

**Cotemar** · 10-11-2016, 08:14

My Vote would be to TOP OFF THE DIESEL TANKS.
Condensation is worse in the winter months

OrangeCrush · 10-11-2016, 08:18

Ok... I will probably go ahead with the plan out of over-cautiousness. But wouldn't transferring the fuel back and forth through the filters a few times in the spring get the water out anyway?

It sounds like biocide is better as preventative maintenance than remedy for a bacterial "infection". Kill 'Em is the only one endorsed by Yanmar, it seems, so I feel ok about using it, and my tanks are less than 3 years old. (Although if I had read the safety warning before I ordered it I might have thought twice about bringing that stuff on board my boat. Death from skin contact? Jeez)

Just a question: why does an empty tank not suffer from condensation like a half full one does? If it because of the time lag it takes to bring a volume of liquid to ambient temperature?

What about covering up the fuel vents with a small piece of electrical tape? It would pop off if there were real internal pressure but might prevent the entry of humid air on a daily basis...

**a64pilot** · 10-11-2016, 08:26

Read this
Does An Empty Tank Condensate? Photo Gallery by Compass Marine How To at pbase.com

On the fuel vent question there are desiccant filter made for fuel vents, as the tank breathes the air is pulled into the vent and the desiccant will absorb any water vapor.

Having gone through fuel "bug" problems, I will always add a Biocide, you will not talk me out of it, though many try. Where I buy fuel, every time I'm told that is a waste of money, our fuel already has it in it.

**Cotemar** · 10-11-2016, 08:30

Quote:

Originally Posted by OrangeCrush

Just a question: why does an empty tank not suffer from condensation like a half full one does? If it because of the time lag it takes to bring a volume of liquid to ambient temperature?

What about covering up the fuel vents with a small piece of electrical tape? It would pop off if there were real internal pressure but might prevent the entry of humid air on a daily basis...

Wild temp changes and condensation in the winter months make a half empty fuel tank become a rain forest.

If there is no fuel in the tank then there is more of an even temp change and far less to no condensation.

My boat has a 125 gal diesel tank, so its next to impossible to drain it at the end of the season. Next best thing is to TOP IT OFF

Full fuel tank is good
Empty fuel tank is good
Anything in-between full and empty is NOT good

valhalla360 · 10-11-2016, 08:41

Quote:

Originally Posted by Cotemar

My Vote would be to TOP OFF THE DIESEL TANKS.
Condensation is worse in the winter months

But how much? 3 drops over the course of a winter instead of 2 drops?

If you look at how much water a cubic foot of air can hold, it's just not enough to get noticeable quantities condensing.

I wouldn't even trust the engine reps as this old wives tale has been around for decades and they probably were told it by the old reps and never actually ran the numbers.

valhalla360 · 10-11-2016, 08:49

Quote:

Originally Posted by OrangeCrush

Just a question: why does an empty tank not suffer from condensation like a half full one does? If it because of the time lag it takes to bring a volume of liquid to ambient temperature?

What about covering up the fuel vents with a small piece of electrical tape? It would pop off if there were real internal pressure but might prevent the entry of humid air on a daily basis...

Empty will work if it's truly empty. Water sitting on the bottom of an empty tank will evaporate.

Once you have even an inch of fuel on the bottom, any water (such as from a leak) will sit under the fuel and thus cannot evaporate.

The problem is most boats don't have an easy way to completely empty the tanks.

If you buy the old wives tale, ALMOST empty is your worst case scenario.

**Cotemar** · 10-11-2016, 08:57

Quote:

Originally Posted by valhalla360

But how much? 3 drops over the course of a winter instead of 2 drops?

If you look at how much water a cubic foot of air can hold, it's just not enough to get noticeable quantities condensing.

I wouldn't even trust the engine reps as this old wives tale has been around for decades and they probably were told it by the old reps and never actually ran the numbers.

I have 3 diesel engines, so I try and do the best maintenance I can, so they take care of my family and get them safely home.
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