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Old 07-08-2017, 06:05 PM   #1
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Another new guy question - fuel tanks full or not?

I am wondering is it better to keep your diesel fuel tanks full to avoid condensation, or low to avoid algae? This is in California, so it is a mild climate, if that matters. Also, it is an older boat (1982).

Thanks,

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Old 07-08-2017, 06:41 PM   #2
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Theres a lot of opinions on this. Some places and some tanks develope condensation, other boats and tanks dont.

So depending, keeping tanks full or partial might depend on who you are.
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Old 07-08-2017, 08:26 PM   #3
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Personally, I don't think that condensation is really an issue. I would keep the tanks at a level that would give reasonable fuel turnover. I have no idea what is reasonable. On my saiboat, I would fill up the tank once a year whether I needed fuel or not. Only a 50 gallon tank. I used a biocide in fuel when I did.

Currently, I am thinking that I would like to put as much fuel in the tank as I expect to use in 3 months. I'm not worried about condensation but fuel freshness.
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Old 07-08-2017, 08:27 PM   #4
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I am asking because I have a pre-purchase survey coming up week after next, and I am wondering how they will check the tank integrity. I mentioned filling the tanks up to the owner, and he said it was bad to do that.

Algae, and lugging around a bunch of extra weight were the reasons. He seems like a straight up guy, so I don't think he is trying to put anything over on me. But I feel like I would like some kind of clear assurance on the state of the tanks because the boat is a 1982 model.

It seems like at a minimum topping them up would provide some sort of check. Haven't asked the surveyor yet, maybe he'll have this issue covered.
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Old 07-08-2017, 08:41 PM   #5
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I don't know enough to really help you on this but filling up the tanks could be an expensive proposition for the owner when you may or may not buy the boat. Second, I don't understand the algae reason as I thought algae grew on the layer between the diesel and any water that exists in the tank? If so that would be at the bottom of the tank. Last, the PO of my boat filled up when he got fuel. Used it down and then filled to the top. Was almost full when I purchased it and I filled it up the day before delivery. It gave me some comfort knowing that they weren't leaking. Seems to me that letting the whole tank get soaked in fuel regularly would help prevent some corrosion, no?

Read all of this with great skepticism...others can help much more than I!!
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Old 07-08-2017, 08:48 PM   #6
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Greetings,
Mr. 508. Algae will NOT grow in fuel tanks. Microbes DO grow in fuel tanks and live in any water (not necessarily due to condensation) in the bottom of the tank and feed off the fuel. When these little buggers die off their dead bodies is the stuff that clogs filters. As has been mentioned, use of a biocide eliminates this possibility.
Extra weight? Of course but if you're that concerned about decreased mileage because of the weight of extra fuel, buy a motor home. Meaning, fuel is one of the smaller costs in boating.
So, filling your tanks is NOT "bad to do" and yes indeed, topping up the tanks would be a test. Maybe this "straight up guy" is hiding something or maybe not.
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Old 07-08-2017, 08:52 PM   #7
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I don't know enough to really help you on this but filling up the tanks could be an expensive proposition for the owner ...
I offered to pay for it.
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Old 07-08-2017, 09:12 PM   #8
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Greetings,
Mr. 508. YOU offered to pay for it and the guy still thought it was a bad idea? Take the price of tank replacement off of your offer unless you see those tanks filled.
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Old 07-08-2017, 09:44 PM   #9
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I agree with RTF. I'm kind of a skeptical guy anyway. However, if an owner turns down free fuel, I would assume there is another reason he wouldn't want the tanks full. Have your surveyor spend extra time with the tanks. Check the tops for rust or corrosion, depending on their material. And pay attention to the return line connections, etc. Pressure test???
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Old 07-08-2017, 11:28 PM   #10
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Just talked to him again. He believes it is not good to do that because of the algae, but he's fine with me filling it up if I want to.

Is that the best way to check the tanks? This might sound really dumb, but do they pressure test them? Of course I'll find out when I talk to the surveyor, but I'm just wondering.

And thanks again you guys, especially RT, I really appreciate the help!

Thank you!
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Old 07-09-2017, 12:32 AM   #11
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Quote:
Originally Posted by f508 View Post
Just talked to him again. He believes it is not good to do that because of the algae, but he's fine with me filling it up if I want to.

Is that the best way to check the tanks? This might sound really dumb, but do they pressure test them? Of course I'll find out when I talk to the surveyor, but I'm just wondering.

And thanks again you guys, especially RT, I really appreciate the help!

Thank you!
If his only objection was a misplaced fear of algae, (as RTF pointed out), then go for it if you want to really test the tank integrity, fillerup..!

However, for what its worth. my boat is near Brisbane, Queensland, Australia, a climate not too different from Florida I understand - warm and often humid summers, and sometimes quite cool, as like now, in winter. Yet I never have more than half-filled tanks, or less, and I only ever have a teaspoonful of condensate in the primary filter bowl, my filters last so long I just change them about every 5 years anyway, and all I do is add the recommended amount per volume of Diesel Power additive when I add fuel, which is not often, and have never had a moment's trouble with fuel issues. Mind you, my tanks drain from the bottom, so sludge build-up is minimised anyway, but there can't ever be much, the filters last so well.
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Old 07-09-2017, 02:40 PM   #12
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Quote:
Originally Posted by f508 View Post
Just talked to him again. He believes it is not good to do that because of the algae, but he's fine with me filling it up if I want to.

Is that the best way to check the tanks? This might sound really dumb, but do they pressure test them? Of course I'll find out when I talk to the surveyor, but I'm just wondering.

And thanks again you guys, especially RT, I really appreciate the help!

Thank you!


If there is already "algae" present, you are much more likely to encounter problems when the tank levels are low, and the crud can be stirred up by the motion of the boat.
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Old 07-09-2017, 04:24 PM   #13
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I used to be in the "keep them full" camp, but some resent test data I saw indicated, so far as moisture in the tank, it really didn't seem to make much difference. Most moisture was already present in the fuel when it was delivered or leaked in through bad fuel cap seals, etc. Since then I've been keeping them about half full, 200 - 300 gallons, solely for the purpose of turning the fuel over more often.
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Old 07-09-2017, 04:39 PM   #14
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If considering buying the boat, certainly fill the tanks to check for leaks.

I am in the camp that considers keeping tanks full a non-issue regarding condensation. I have two 160gal tanks on my personal ride and mostly keep them around 1/4 full. In nine years and 20,000nm not a drop of water in the Racor ever. But my fill and vents are not exposed to rain, so I think that is the key.
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Old 07-10-2017, 12:59 PM   #15
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Perhaps the "condensation" mythology was started by all those places in the past that had water in their fuel supply?

I m inclined to call BS, and leave the fuel tanks at whatever level is convenient. Sure makes it better for winter storage and fuel degradation in your favorite old ride.

The extensive regulations that have been put in place over the last 40 years have put a stop to water in fuel at the pumps, where geographically these regs are applicable.

And surprisingly... we aren't having water in fuel trouble like our parents and grandparents had to put up with?

Some myth busting rebel did a test, put empty fuel tank in the garage and open piped it to the outside... no wet at all inside after many months.

Who has the link?

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Old 07-10-2017, 01:20 PM   #16
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I like to keep mine more towards the full side because I'd rather the boat be heavy/more stabile in sloppy conditions.
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Old 07-10-2017, 01:30 PM   #17
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Quote:
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Personally, I don't think that condensation is really an issue. I would keep the tanks at a level that would give reasonable fuel turnover. I have no idea what is reasonable. On my saiboat, I would fill up the tank once a year whether I needed fuel or not. Only a 50 gallon tank. I used a biocide in fuel when I did.

Currently, I am thinking that I would like to put as much fuel in the tank as I expect to use in 3 months. I'm not worried about condensation but fuel freshness.
Dhays,

Regarding freshness... is there a real issue? Does diesel degrade over time? I've heard that is does not... to any significant amount. I've got a friend that has been using the same diesel over the past 10 years when I last topped off and no issue. He just doesn't travel far.

As for a biocide, suspect that might keep the filter cleaner?

I'm not in any camp on topping off or not... I just don't ever want to run out at the wrong time, so will fill accordingly, and see little reason to make a partial fill.
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Old 07-10-2017, 02:17 PM   #18
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Whether the tanks are full or empty there will always be organisms if you don't use a biocide. I do, every time I fuel. Empty allows more condensation. Air in tanks expands as the day warms and contracts as the day cools, drawing in moist night air.
If you use a good biocide, it also has fuel stabilizers and additives that help a good primary filter extract water.
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Old 07-10-2017, 02:26 PM   #19
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Perhaps the "condensation" mythology...

RB

Hi,

The condensation issue is fairly well known and demonstrated in aviation, but I am new to the boating thing, so I have no idea in this realm.

Maybe the boat tanks have a more stable temperature because the hull is usually in the water, and water doesn't change temperature as rapidly as air, so condensation is not much of an issue.

Anyway, seems like one less thing to worry about, so that is good.

Thanks
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Old 07-10-2017, 03:13 PM   #20
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Some random notions--

Condensation is a major consideration in the design and operation of fuel farms. Now, how similar are our boat tanks to a fuel farm? For most, not very. But once a vessel gets into bunkering large volumes the reasons for condensation mitigation creep up.

Along with what micron rating, adding biocides or not, stale fuel or biodiesel - condensation concerns may or may not be an issue in your case. But, we do wear our safety belts, don't smoke, keep our weight down and wear motorcycle helmets (some anyway) don't we?

Me? Fuller tanks during winter. No downside unless your vessel is not used much and the diesel fuel becomes + one year or so.
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