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Old 04-03-2014, 03:39 PM   #1
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Confused about day tanks

What is the purpose of a day tank?

To ensure no air gets into the fuel system? Is the day tank mounted above the engine and so uses gravity feed?

To accurately calibrate fuel burn? Do the return lines go back to the day tank and not the main tank? How do you calibrate fuel usage with all this excess fuel being returned?

To maintain cleaner fuel? If there is a filter between the main tank and the engine how does a day tank make this better.

I'm so confused...
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Old 04-03-2014, 04:07 PM   #2
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What is the purpose of a day tank? To ensure no air gets into the fuel system? Is the day tank mounted above the engine and so uses gravity feed? To accurately calibrate fuel burn? Do the return lines go back to the day tank and not the main tank? How do you calibrate fuel usage with all this excess fuel being returned? To maintain cleaner fuel? If there is a filter between the main tank and the engine how does a day tank make this better. I'm so confused...
On our Nordy you can adjust valves accordingly to where you want the fuel to go, it's wise to leave it returning to the day tank but I can also return to port/stb. our day tank has a water sensor and ball valve with a cap where you can test fuel for sediment and water. Our day tank is below the two bigger saddle tanks so they gravity feed into the day tank, you can also manually transfer fuel to the main tank if you do it that way it polishes the fuel.
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Old 04-03-2014, 04:25 PM   #3
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What is the purpose of a day tank? ...
Traditionally the day tank is where polished fuel is gravity fed to the main engines. On large vessels, the fuel is polished with a centrifuge as it is pumped into the day tank from larger bulk fuel tanks. On the large vessels I worked on, the tank also held about 24 hours worth of fuel (hence the name).

As Oliver indicates, on our N47's the large wing tanks generally gravity feed into a smaller day tank that feeds the engine. The small day tank also has a smaller head section that is calibrated for testing fuel burn rates.

By not feeding directly to the engine from the bulk (wing) tanks, you have a better chance of keeping your fuel clean and can easily switch to an alternate fuel tank if contamination occurs.
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Old 04-03-2014, 04:27 PM   #4
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On our Nordy you can adjust valves accordingly to where you want the fuel to go, it's wise to leave it returning to the day tank but I can also return to port/stb.
Well that makes sense to me, returning the unused fuel back to the day tank, but the way this one I saw is set up the unused fuel goes back to the main tanks.

If you draw out if the day tank but return unused fuel back to the main tank, what is the reason for the day tank? Why not just draw out if the main tank and be done with it?

Also, I'm not sure this day tank sits higher than the engine so that would kill the gravity feed idea, right?
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Old 04-03-2014, 06:29 PM   #5
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Day tanks are really for larger vessels that have large quantities of fuel sitting for a very long time.

Can you apply the same uses to a smaller vessel???? Sure but it does add complication unless you need to transfer older maybe contaminated fuel through a filtering system to ensure you have a clean supply for running for awhile.

Not all day tanks are set up the same or provide the same benefits...as far as returning fuel, some engines like Lehmans return so little it really doesn't matter. On a higher return engine...kinda makes sense to return it to the day tank unless you are worried the tank is so small it can't dissipate heat quickly enough. If you do return it to the larger tanks...it is helping to clean them up but that's where size and your application/use determine more than concept.
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Old 04-03-2014, 07:01 PM   #6
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I can see where on a modern long range trawler with a relatively large fuel volume that does not get used regularly(probably the vast majority) it would be wise to use some hefty bulk type filtration prior to sending that old fuel through the pre motor filters. As mentioned the smaller day tank can be set up for easy fuel monitoring and polishing with a volume that could handle 12-24 hours running time. I think a day tank would be a waste on boats with smaller tankage and a greater turn over of fuel. there, some bulk high capacity filtration prior to motor filters should suffice. I suspect as long range trawlers age the big tanks with lots of old fuel will become problems unless some significant tank maintenance is preformed. Unfortunately fuel polishing is often not the long term answer to a dirty or contaminated tank. Tank cleaning and fuel replacement is the answer. So basically that long winded rant comes down to day tank = stop gap measure for big tank old fuel problem.
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Old 04-03-2014, 08:20 PM   #7
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I donít see why the fuel in a day tank would be any more polished than mine that I polish in bulk. If your 2 micron filter is going to clog it's going to clog. I would hate to run a bit longer or harder than planned and be entering an inlet on day tank fumes than on my main tanks that served me well all day. As for gravity feed... it's why we all have lift pumps!

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Old 04-03-2014, 08:32 PM   #8
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I think it's a belt and suspenders for some folks...but really is much more common on large vessels with numerous tanks of unknown quality of fuel.

Like any fuel situation...you must refuel or transfer before it becomes or could become an issue.

I'm guessing some have a "day tank" that probably is just an additional smaller tank added for more range/capacity...I really have never seen a stock one on any production boat except the higher end, long range boats. ...but I haven't been on many upper end trawlers either.
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Old 04-03-2014, 08:49 PM   #9
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I actually wished I did have a day tank, as it makes fuel issues far easier to sort out. But now that I understand my fuel system pretty well ( I think/hope), it would just add another layer of things to go wrong.

I also agree that being gravity fed, also eliminates points of failure.
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Old 04-05-2014, 06:18 PM   #10
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I think most has been covered, but I'll add one thing. Old fuel is not the only source of contamination in a storage tank. Crud accumulates over time in any tank, no mater how quickly the fuel is used. Normally it's all settled at the bottom of the tank below any pickup tubes and poses no threat, until.... you hit bad seas. As your boat gets thrashed about, all that crap in the bottom starts to get stirred up and starts getting pulled through the fuel system. Your filters will catch it, but can quickly become overwhelmed and plugged, requiring a change in those crappy seas, including bleeding the system and restarting (hopefully). If you read accounts of vessels that become disabled in heavy seas, this is probably the most common problem.

The idea of a day tank is to create a "firewall" between storage tanks and the fuel that's being fed to the engine. The only way fuel gets into the day tank is by way of a filtration system, so it only ever contains clean, known good fuel. If you are plugging up filters, it's more likely the transfer filter than the engine filter. Because you are running on a clean reserve of fuel that you presumably haven't run down too low, you can change the transfer filter while the engine continues to run and your boat maintains max stability.

A day tank isn't for every boat, but if you are doing serious off-shore cruising, it's a way to reduce the risk of fuel contamination, which most experts will claim is the leading cause of disabled diesel engines.
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Old 04-05-2014, 07:04 PM   #11
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Ditto Twistedtree
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Old 04-05-2014, 08:39 PM   #12
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Absolutely twistedtree. And since a purifier was mentioned above, should we discuss the use of the settling tank?
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Old 04-06-2014, 08:06 PM   #13
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My trawler has 2 30 gallon "day tanks". They only get cleaned filtered fuel, from the lowest tank on the boat usually, or either of the main bulk tanks. The lower tank is 250 gallons and can be gravity fed thru Davco filters or pump fed by a 1/3 hp gear rotor pump (also the polishing pump). The mains are the ONLY tanks that take on new fuel, they are about 500 gallons each with access/cleanout hatches in each chamber. Actually, every tank on the boat has had these installed, and they are accessible. I had the tanks built from 5086 aluminum to commercial specs. All are coal tar epoxy coated. The day tanks are above the engines (4/53 DDs) which makes bleeding the system unneccesary, just put the return to the lower tank until the air bubbles are gone. I can power bleed with an inline 12 volt pump if I'm in a jam, or if a filter is clogging at an inconvenient time I can switch on the pump and keep running. I have an overflow from the day tanks that can go to the bulk tanks or to the mid/lower tank. It's handy for polishing as fuel can be sent to the day tanks continuosly by the gear pump. The day tanks have a head pressure switch that can turn on the gear pump at a preset level, usually at the 10 gallon mark. I have a WaterWitch bilge pump counter that keeps track of the number of cycles. A kitchen timer keeps track of time. At one hour I look at the counter (and reset it) X 10 gives me GPH. Simple and effective.
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Old 04-06-2014, 08:28 PM   #14
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Right on the Nose TwistedTree

Quote:
Originally Posted by twistedtree View Post
I think most has been covered, but I'll add one thing. Old fuel is not the only source of contamination in a storage tank. Crud accumulates over time in any tank, no mater how quickly the fuel is used. Normally it's all settled at the bottom of the tank below any pickup tubes and poses no threat, until.... you hit bad seas. As your boat gets thrashed about, all that crap in the bottom starts to get stirred up and starts getting pulled through the fuel system. Your filters will catch it, but can quickly become overwhelmed and plugged, requiring a change in those crappy seas, including bleeding the system and restarting (hopefully). If you read accounts of vessels that become disabled in heavy seas, this is probably the most common problem.

The idea of a day tank is to create a "firewall" between storage tanks and the fuel that's being fed to the engine. The only way fuel gets into the day tank is by way of a filtration system, so it only ever contains clean, known good fuel. If you are plugging up filters, it's more likely the transfer filter than the engine filter. Because you are running on a clean reserve of fuel that you presumably haven't run down too low, you can change the transfer filter while the engine continues to run and your boat maintains max stability.

A day tank isn't for every boat, but if you are doing serious off-shore cruising, it's a way to reduce the risk of fuel contamination, which most experts will claim is the leading cause of disabled diesel engines.
The only thing I would add is that I have a sight tube on my day tank and monitor and top up hourly. I always have 24 hours in the day tank and know precisely what my fuel burn is. On a long leg I can adjust my speed for best time early on.
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Old 04-07-2014, 06:28 AM   #15
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IF the day tank is above the engine and other tanks , finding a leak is very simple.

Finding a suction leak is a real task , and probably can not be done underway.

The easier it is to maintain the boat , the more fun it is to operate it.

Maint is a PIA , but much can be designed out , which results in a far better boat.
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Old 04-07-2014, 07:09 AM   #16
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IF the day tank is above the engine and other tanks , finding a leak is very simple. Finding a suction leak is a real task , and probably can not be done underway. The easier it is to maintain the boat , the more fun it is to operate it. Maint is a PIA , but much can be designed out , which results in a far better boat.


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You can see the front of the genset in this pic and it's sitting on the day tank. Well maybe not directly on the tank but, it's right below it iirc. Looks low for gravity feed for the main's injection pump but maybe not.
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Old 04-07-2014, 07:10 AM   #17
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