Purpose of day tank

[bayview]

I have never had a day tank.

What is their purpose?

 

[Airstream345]

Good overview and discussion about day tanks here: https://www.trawlerforum.com/forums/s3/day-tank-2454.html

 

[sunchaser]

I have never had a day tank.

What is their purpose?

Good question and yes it seems confusing. What are your cruising plans, which vessel and what is current fuel tank arrangement? Therein lies the answer.

 

[High Wire]

We have them at work for the emergency diesel generators. A day tank is a small tank that is the immediate source of fuel for the engine. If the fuel lift pump fails, gravity will continue to feed the engine uninterrupted. The day tank is topped off by very large storage tanks as fuel is consumed. All in all it increases the reliability of the engine.

On yachts, it is fed by filtered fuel from other large tanks. It will have a means of accurately measuring fuel use and a drain to easily remove water and sludge.

 

[Lepke]

Big boats, tugs and ships have day tanks. Fuel in the main storage tanks is pumped to the day tank as needed and used to maintain an even keel. Large diesel engines pump much more fuel than used in combustion. 5x-10x more fuel. This constant circulation thru the filters and engine components ensures clean fuel and fuel heated to a temperature that improves burn efficiency. The circulating fuel takes excess heat away from the injector pump and in engines like Detroit Diesels, cools the injector and tip between firings.

 

[PM]

There was just a thread about an owner installing a day tank in a 32 foot boat. It should be easy to find.

Doesn't make sense to me on a boat so small but it's a free country.

pete

 

[Mako]

As Lepke commented, Jimmies are God’s natural fuel polishers. For a typical pleasure boat I would question the need for a day tank if the main is a Jimmy

 

[FF]

The fuel pickup from a day tank can be higher in the tank, so water or trash that got
by the filters will fall .

A functioning cleanable sump , which every boat needs.

 

[rslifkin]

A day tank, by virtue of being smaller, has less issues with fuel slosh than even a fairly well baffled main tank. So it means a larger portion of the fuel you carry is usable before you risk starving an engine in rough seas.

 

[twistedtree]

There's a great parallel between the purpose of a day tank, and CV as we are experiencing today.


With fuel, the concept is that you assume purchased fuel is contaminated, and needs to be quarantined. That's what your main storage tanks are for.


Then you have a separate "clean zone" for fuel that will actually be consumed by your engine(s). That's the day tank.


And the only way for fuel to get from the quarantine zone (storage tanks) to the "clean zone" (day tank) is by passing through a cleansing process that removes water and other contaminants. This significantly increases your immunity to contaminated fuel.


That's the concept, and the accompanying questions is whether it's worth it in your situation. For most pleasure boaters, it probably isn't. Space is a concern on smaller boats. Plus exposure to bad fuel is a pretty small risk for most boaters, and the consequences if you get disabled by bad fuel are inconvenient, but unlikely to be dangerous.


Where the calculus is different is for boats that cross oceans or other large, isolated bodies of water, or spend time in very remote locations where any sort of help is unlikely. Also boats where contaminated fuel is more common.

 

[FF]

"And the only way for fuel to get from the quarantine zone (storage tanks) to the "clean zone" (day tank) is by passing through a cleansing process that removes water and other contaminants"


A far simpler way is a bailable sump on each large tank to restore it to being a clean zone, as gravity will sink the water and other crap, for free , no filters or pumps to need.

 

[twistedtree]

"And the only way for fuel to get from the quarantine zone (storage tanks) to the "clean zone" (day tank) is by passing through a cleansing process that removes water and other contaminants"


A far simpler way is a bailable sump on each large tank to restore it to being a clean zone, as gravity will sink the water and other crap, for free , no filters or pumps to need.

That’s fine if it’s not being shaken up my rough seas. But regardless, we are talking about a small part of recreational boating, not mainstream.

 

[sunchaser]

TT nails it in post #10. As he says, not all of us are blue water voyagers thus a day tank may not be required.

On our DF 48 we have 4 tanks and a simple fuel manifold. One of the tanks, 80 gallons when full, is the sole supply for our diesel furnace. It also serves as the cleanest and emergency fuel tank for what has never occurred - bad new fuel.

This 80 gallon tank is continually filtered at 10 microns from a low point sump (keeps FF happy) when the diesel furnace is running.

 

[twistedtree]

For what it's worth, Nordhavns, which do some of the most ocean crossings and cruising to far-flung places, have "supply tanks", not day tanks. The supply tank is really just a big manifold with a sump like what FF describes. The intended operation is to gravity feed from one of the larger storage tanks into the supply tank, and the consumers draw off the supply tank. In some ways it might seem like a day tank, but there is no isolation between it and purchased fuel. The sump in the supply tank is a place for water and gunk to settle, and includes a drain valve and a water sensor and alarm.


So there's a whole line of boats specifically built for the most adventurous cruising, and they don't have day tanks.


Now that said, some number of Nordhavn owners use those tanks differently, and operate the "supply" tank as you would a day tank. I would estimate that it's a small minority of owners/operators who do that, but there are a handful. I am one of them.

 

[Martin J]

The origin of the day tank was to ensure clean fuel was available. I used to run larger motor yachts and we would draw our fuel, sometimes (20 tankers worth). We would then pass it through a alfa laval centrifuge to extract dirt and water. Then pass it to the day tank. You would be amazed how much dirt particulate and water we would get after operating for 1/2 Day. The pain is sadly that cleaning the machine after use is a time consuming job. But the end result is clean reliable fuel which gives a piece of mind.

 

[FF]

The origin of the day tank was to ensure clean fuel was available. I used to run larger motor yachts and we would draw our fuel, sometimes (20 tankers worth). We would then pass it through a alfa laval centrifuge to extract dirt and water. Then pass it to the day tank. You would be amazed how much dirt particulate and water we would get after operating for 1/2 Day. The pain is sadly that cleaning the machine after use is a time consuming job. But the end result is clean reliable fuel which gives a piece of mind.

The OTR trucks use centrifuges to clean fuel volumes like our boats and have very simple cleaning setups.

 

[angus99]

We were lucky with the design of our boat. With three tanks already plumbed for transfers (from sumps)—and ~900 gallons fuel capacity—it was easy to designate one as a day tank. Before getting underway each day, I move enough fuel from the saddle (storage) tanks to raise the the aft (day) tank to about 1/4 full. Takes maybe 20-30 minutes and the pump runs while I’m having coffee and doing other prep. The high-volume Fleetguard pre-filters the diesel before it reaches the primaries and secondaries and clean fuel is something I no longer worry about. I know . . . it’s a belt/suspenders/balloons approach . . . but two years after we had the tanks cleaned we had a shutdown from contaminated fuel. So, for me, this approach was a no brainer.

 

[FF]

"We were lucky with the design of our boat. With three tanks already plumbed for transfers (from sumps)"

Sumps are seldom the source of fuel to transfer, they get bailed ,or drained , their contents inspected and discarded.

Good to see your setup sucks fuel thru the filters, the better choice.

 

[rslifkin]

"We were lucky with the design of our boat. With three tanks already plumbed for transfers (from sumps)"

Sumps are seldom the source of fuel to transfer, they get bailed ,or drained , their contents inspected and discarded.

Good to see your setup sucks fuel thru the filters, the better choice.

I'd think the ideal design is for every tank to be sumped. Have a drain at the bottom of the sump, and then draw fuel from higher up in the sump (but still within the sump). That way you take advantage of the smaller sump to reduce slosh, meaning you can draw the tanks down lower if needed without sucking air.

 

[koliver]

I have a diesel stove. I have a separate tank for that stove. It is 2 gal, it is above the stove burner, so feeds the stove by gravity. Its capacity is enough to keep the stove alight for 48 hrs. It is fed by the port engine when running, so stays full, and by a Walbro pump when necessary.
That is the only "day tank" on my boat.

 

[FF]

"It is 2 gal, it is above the stove burner, so feeds the stove by gravity. Its capacity is enough to keep the stove alight for 48 hrs"


That is really efficient , our Dickinson unit takes 2-4 gal per day , the 4 gal is needed when below 0F, About 20,000BTU output.

 

[rslifkin]

Diesel has about 128,000 BTU per gallon, so he must have the stove turned down to not much over 5000 BTU for it to run that long. Your example of 4 gal/day is pretty much right on for 20k BTU.

 

[thorn8272]

All the above are true and useful information. BUT my shrimp boat has a day tank and it is for coolant expansion for the keel cooler. If this is what your referring to. Nomenclature varies amongst everyone in the marine industry

 

[Nickair]

On a pleasure trawler a day tank may help with ballast trim with clean fuel transfer options?

 

[catalinajack]

I rather think not. To provide trim the the tank would have to be installed to one side which would create a list. Or, one would need two days tanks mounted outboard on each side. Centerline, it provides no trim opportunity.

On a pleasure trawler a day tank may help with ballast trim with clean fuel transfer options?

 

[twistedtree]

All the above are true and useful information. BUT my shrimp boat has a day tank and it is for coolant expansion for the keel cooler. If this is what your referring to. Nomenclature varies amongst everyone in the marine industry

Wouldn't that be called an "expansion tank" or something like that? Do you commonly call it a "day tank".

 

[angus99]

I rather think not. To provide trim the the tank would have to be installed to one side which would create a list. Or, one would need two days tanks mounted outboard on each side. Centerline, it provides no trim opportunity.

John, we can keep our boat in trim using the centerline day tank by simply alternating which saddle tank we draw from each morning. If the boat is trimmed side-to-side in the morning, it generally stays that way all day as we run on the day tank. Of course, the same could be accomplished by pumping from one saddle tank to the other. (We don’t have a cross-over hose between the saddle tanks.)

 

[catalinajack]

Yes, and you could do the same thing by transferring fuel from one side to the other without a day tank. Or, run off one tank until tanks are balanced. That is what I do. Simple, effective. Ian, my feelings have been expressed many times on this forum. I consider fuel polishing and day tanks unnecessary. However, I do much covet your Seaboard Marine dual filter head with vaccuum gauges for sequential filtering. In my opinion this is all that is needed and far better than what most folks use everyday aboard their boats, including mine.

As an aside, a few weeks ago I opened the bottom drain on one of my side tanks. I doubt whether it had been opened since a prior owner had the tanks cleaned in 1999, twenty plus years ago. No water and just a bit of crud.

John, we can keep our boat in trim using the centerline day tank by simply alternating which saddle tank we draw from each morning. If the boat is trimmed side-to-side in the morning, it generally stays that way all day as we run on the day tank. Of course, the same could be accomplished by pumping from one saddle tank to the other. (We don’t have a cross-over hose between the saddle tanks.)

 

[Mako]

... my feelings have been expressed many times on this forum. I consider fuel polishing and day tanks unnecessary.

To add to such heretical talk, I don’t believe in have a small “sump” for collecting water and sludge. Instead I believe the fuel pickup should reach all the down to the bottom. Suck the crud continuously.

That’s the way my 40 year old single 1600 gallon tank was designed (with zero access for cleaning) and it was perfect. Of course I had built-in “fuel polishers” in the form of twin Jimmies.

 

[angus99]

Courses for horses, guys. I’ve had enough experience with bad fuel to know I don’t want a repeat. I suspect I may have some fouled injectors from the last batch. I may never get another load of fuel crud, but if I do, it will be manageable.