Day Tank

From time to time I see refferences to a "day tank" in regards to fuel.* I know I don't have one, but what is a "day tank"?

A day tank is a (usually) smaller fuel tank that feeds the engine(s). Fuel from the main tanks is transferred-- by gravity in the case of some boats like ours, or by pump in other boats--- to the day tank. My understanding is that the name is based on the idea that this tank holds enough fuel for a day's cruising. Our day tank holds 60 gallons, which in theory is enough for 12 hours running at cruise rpm.

There are advantages to a day tank system but I don't think they are earth-shattering. Fuel can be passed through a filter on the its way to the day tank, so you get an additional opportunity to remove contaminants. Day tanks are usually mounted on the centerline of the boat. Ours is in the bilge between the engines. So there can be some fuel management and trim advantages in using a day tank. Since a day tank typically holds less fuel than a main tank--- in some cases a LOT less fuel--- there could be advantages if a load of contaminated fuel is taken on board one or more of the main tanks. The smaller day tank will be easier and faster to pump out and clean than one of the large main tanks. And there may be other advantages to a day tank, too.

Our boat was completely re-tanked by it's previous owner the year before we bought the boat. The three original tanks were removed and replaced with a pair of smaller "cube" tanks on either side of the engine room and the day tank in the bilge. The total capacity was reduced by 50 gallons, which is actually an advantage. The four saddle tanks have sight tubes, the day tank has a fuel gauge readout at the helm.

The main engines and the generator pull from the day tank. However each main engine can be valved to pull from the saddle tanks on its side of the boat. The fuel return from each engine can be valved to go to the day tank or to the forward saddle tank on the engine's side of the boat.

The transfer system is gravity, and the outflows from the saddle tanks are in the very bottom of each tank. So each tank can be drained completely dry. In practice we periodically transfer fuel from one opposing pair of saddle tanks until they are empty. We then leave them empty and start transferring from the other opposing pair of saddle tanks. When the second pair of tanks is below a quarter full we think about filling the first pair. After they've been filled, we completely drain the second pair and leave them empty until the first pair is drawn way down. The tanks are stainless so letting them "air out" on the inside is important.

Most of our weekend cruises into the islands year round are perhaps five or six hours round trip. So we can get at least two trips from the day tank before we need to transfer fuel again. The transfer process is very simple and quick, so it's not any sort of inconvenience.

Day tanks are also good for fuel consumption calculations. If they are equiped with a site glass, they make calibrating a flowscan easier. Also, if you develop a fuel leak, it limits the amount of fuel in your bilge. Finally, if you are trying to run a fuel log on a long trip, the smaller tank is much easier to measure daily consumption with.

As already Mentioned, pumping fuel from bulk tanks through a fuel seperator with a sight bowl into a day tank, gives great peace of mind regarding daily fuel quality.

Ted

Many times on offshore boats the day tank will be mounted so it gravity feeds the main engine.

Stops another few sources of engine stoppages , fuel supply leaks are far easier to find , filter banks and last chance on engine filter will gravity fill. Reprimes easier.

Thanks for the answers. I guess for my boat and my use I shouldn't feel bad for not having one. I do wish I had sight gauges or a way to stick the tanks though. The fuel gauges make me nercous and I'm afraid to go below 1/3 or 1/4 on the gauge.

"I guess for my boat and my use I shouldn't feel bad for not having one."

No you should not feel bad, I don't have one either, many boats do not.
If I ever have to re-do my tanks I would likely make them smaller and add a day tank of about 25-30 gallons (single engine).

rwidman wrote:I do wish I had sight gauges or a way to stick the tanks though. The fuel gauges make me nercous and I'm afraid to go below 1/3 or 1/4 on the gauge.

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As was mentioned on another thread here, you can add a single sight glass anywhere in the fuel system before the filter and it will show the level of the selected tank. You do not have to place the glass on the tank.

If you want, you can have all the tank glasses next to each other in a convenient location, each with a back plate marked in gallons, liters, gills, or firkins. All you have to do is make sure the bottom of the glass is at or near the bottom of the tank and the top is above the tank height.

If you are really geeky and have some spare cash, install a pressure transducer on the tank outlet and display the reading on your instrument panel or flat screen. Short of that, buy a Tank Tender system.

RickB wrote:As was mentioned on another thread here, you can add a single sight glass anywhere in the fuel system before the filter and it will show the level of the selected tank. You do not have to place the glass on the tank.
Interresting.* You mean I could "T" into each fuel line and install a vertical piece of clear tubing with a valve at the bottom to let fuel in or keep it out, and a valve at the top to let air in or keep it out?

I think I could mount the tubes directly on the outside of the tanks to make it pretty obvious where the level is.

Do you have a link to the thread or any other information on this?

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*

You don't need a valve at the top. You can connect all the "tops" together and run a vent line up to the radar arch or someplace high enough above the tank top that an overfill will go out the tank vent rather than your new vents.

The valve on the top of a sight glass is to prevent the glass becoming a blow torch if there is an engine room fire. All you need for what I described is a valve at the bottom of the tube.

Sure, you could mount the tubes directly on eacht tank but it read like you wanted sight glasses but didn't want to put them on the tanks.

Sorry, no link ... it is so simple and basic the location and fittings should be obvious. If not then have someone help you.

-- Edited by RickB on Saturday 25th of December 2010 10:14:55 AM

I'm surprised the blue water sailors on the list have not stressed the benefit of having a more easily managed supply of clean fuel from a day tank when cruising off shore, particularly in a single engine vessel. When you are caught in the big water off shore, every bit of gunk in your tanks is put in suspension and subject to clogging the fuel filters. Filtering fuel in and out of a day tank will provide added protection against engine failure due to dirty fuel filters. The internal cleanliness of a 60 gallon tank is much easier to manage than the cleanliness of the two 360 gallon tanks on my boat. If I due blue water cruising in this boat, I will install a day tank.

RickB wrote:

You don't need a valve at the top. You can connect all the "tops" together and run a vent line up to the radar arch or someplace high enough above the tank top that an overfill will go out the tank vent rather than your new vents.

The valve on the top of a sight glass is to prevent the glass becoming a blow torch if there is an engine room fire. All you need for what I described is a valve at the bottom of the tube.

Sure, you could mount the tubes directly on eacht tank but it read like you wanted sight glasses but didn't want to put them on the tanks.

Sorry, no link ... it is so simple and basic the location and fittings should be obvious. If not then have someone help you.

-- Edited by RickB on Saturday 25th of December 2010 10:14:55 AM

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I understand the concept and I have the skills.* I have saddle tanks with the fuel pickups at the bottom rear so I could "T" anywhere on the fuel lines between the pickups and the filter.* I suppose that means I could put both glasses near the filter if I wished.* I'm not at the boat right now so this is all from memory.

I might be able to vent the glasses outside the boat and above the tops of the tanks.* I'm not sure.

What would be ideal would be a push button, normally closed valve.* I don't suppose these are available.

BTW: This is not something I would need to use every day.* I have electrical gauges.* It's just that I can't tell when the tanks are full because the fuel docks don't have automatic shutoff nozzles, and I'm not sure how much fuel is left when the gauges read 1/4 tank.

It's pretty cool that I could do this for a few dollars and a couple hour's work without emptying the tanks.

Thanks for the idea.

*


-- Edited by rwidman on Saturday 25th of December 2010 10:57:16 AM

OK, somebody stop me before I go overboard!

Since I have Garmin MFDs with video input, all I have to do is aim a cheap video camera at the sight tubes and I have a very accurate fuel gauge.

Next question:

I don't see the bronze fittings I would nee in the WM or Defender catalogs.* Is brass OK for diesel fuel?* Is plastic OK after the valve (the sight tube would be plastic anyway)?

-- Edited by rwidman on Saturday 25th of December 2010 12:29:08 PM

Brass is OK with diesel fuel. You can use a plastic tube fittings after the valve but since you probably want to use 3/8 tubing or something large enough to be easily viewed and respond quickly when changing tanks brass barbed fittings would be my choice. Secure the tubing with little hose clamps.

12 VDC solenoid valves are fairly inexpensive on the surplus market, expensive from retail sources. Check out Ebay or the industrial surplus sales shops and you should be able to get one for less than $50. Just make sure they are rated for oil service and are not the diaphragm type that requires pilot pressure. Those are the ones with a large square base with the solenoid above. The kind you want are more or less cylindrical in shape. Try and get 1/4 inch or larger so the* level responds rapidly.

I like the idea of viewing them with the camera.

Correct on keeping the sight tube valved off. Still, open it to fill the tank and or calibrate the electrical gauge. Or, electrical valve. Lots of options but first, to get to the boat and see what I need in the way of fittings, then to find them. An auto parts store has been suggested for fittings.

reefdrifter wrote:

On another thread it was discussed to keep the site gauge valves closed except when you want to see how much fuel is left in the tanks so I closed my valves for safety purposes.

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An acquintance bought a 32GB and was very impressed with the minimal fuel use of the FL120.* He kept checking the sight gages and figured his boat must be manufacturing fuel as the gages never went down. When the*engine quit his wife suggested that perhaps they'd run out of fuel.* When he finally figured out to open the valves at the bottom of the sight gages sure enough bone dry tanks.

*

Duh
enough said.

Self closing spring loaded valves are the only way to go.
Gauge glasses plumed in to bottom and top of tank.

Benn

Tidahapah wrote:

Duh
enough said.

Self closing spring loaded valves are the only way to go.
Gauge glasses plumed in to bottom and top of tank.

Benn

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OK, where can I find the self closing valves?* Web searches on the subject are not doing well.

*

By the time you find self closing valves in the size you would use and ship them in from th eUK or somewhere you could have installed radar level gauges in the tank tops.

There is a reason why the only place you will find those valves on sight glasses is on SOLAS ships. They are great if you can stand in front of a tank and step on the valve lever while taking a level reading but I don't think you have that kind of room.

More and more large yachts and ships are using magnetic indicating sight gauges. These don't require self closing valves (that's why those valves are so hard to find and so expensive) are fireproof by their nature, and can be read from great distances.

Self closing valves are not required on small unclassed yachts. ABYC writes standards and recommendations, not laws. The practical way to work around this is to install manual valves and use them to isolate the unprotected non fire resistant plastic tubing when not actually taking a reading.

The idea to install solenoid valves and camera viewing performs those functions. Installing pressure transducers on each tank or using a Tank Tender is much more elegant. Draining the tanks and welding on valve bosses in order to install the usual plastic tubing sight glass works too. You have several options, including keeping a log of how long you ran the engines and a guess about how much fuel was burned.

Thanks for your input.

My electrical gauge and sensors are far more reliable than keeping a log. If the push button valves were relatively inexpensive and readily available, that's what I would use. If not, I won't.

I looked at the Tank Tender but I couldn't mount the tubes without cutting holes in the deck of my boat. It's expensive and as I said, I already have gauges.

I'm going down to the boat today to see what I will need to tap into the fuel lines and install valves and plastic tubing. If it's easily doable, I will do it. This will allow me to "calibrate" my existing gauges in the sense that I can know what they read when the tanks are full and close to empty and places in between. That will serve my needs just fine.

Example, I took a trip that was about 115 miles each way with no fuel available at the destination. In theory, I could have made the round trip on what fuel I had on board. To be safe, I stopped halfway there and topped the tanks off. I spent a lot more on fuel there than in my home area. With a more precise knowledge of how much fuel I actually had on board, I might have made the trip without stopping and fuelled up when I returned. It was an extra fuel stop, an extra tip, and an extra $.40 per gallon.

rwidman wrote:

To be safe, I stopped halfway there and topped the tanks off.

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To rules which are absolutely indisputable.* In a plane, the runway behind you is of no use whatsoever, and in a boat, the tank volume above the level of the fuel in the tank is of no use to you, either.

*


-- Edited by Marin on Sunday 26th of December 2010 03:27:06 PM

Another fuel management rule; 1/3rd out, 1/3rd back and 1/3rd reserve for bad weather.

Well, the tanks connect at the rear of the engine compartment where it would be difficult to operate valves.* I am considering inserting "T"s in each fuel hose, then running either fuel hose or tubing to a location further forward where I could install and operate valves and view the clear tubing.*This would be four or five feet for each tank.* Thoughts?

-- Edited by rwidman on Monday 27th of December 2010 10:12:08 AM

We HAD a day tank in our vintage '86 Albin trawler. About 60g in size and was actually 2 tanks with an adjoining hose.

I need a place to put our house bank of batteries, 6-L16 AGM and we moved the 2 8D engine batts right next to them. All in the center line below the deck in the engine room.

Thank goodness we did the removal as those tanks had maybe another yr before the bottom was completely rusted out & I had 30g of old fuel in the bilge. I had closed off the day tanks shortly after buying the boat, not knowing what was in there.

The PO and only other owner had not used the boat much, less than 500hrs on 2 Ford Lehman 135 naturals when we bought her in '10.

We used our boat initially as a summer cottage, vacation home on the Chesapeake but in '16 started the Loop. Didn't feel the necessity of a day tank and haven't missed it.

I use the sight glass on each tank and fill to the same mark each fill up with gal & Hobbs meter readings so I have a pretty good idea of usage.

I think having a day tank is age dependent, do they build them into boats anymore?

Have any of you guys run dry on a day tank? Just curious. I've never dealt with one on a boat.

But I did deal with issues from dirt-based daytanks at an industrial site. A lesson learned.

Power got knocked out to a wafer fab in Albuquerque (separate story.)
The backup gennies kicked in, but somebody forgot to switch them off the day tanks.
Day tanks ran dry, gennies dropped off line, fab went into meltdown, 1100 bunny suits in the parking lot!

Nepidae said:

I think having a day tank is age dependent, do they build them into boats anymore?

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Can’t speak to newer boats, but most ‘80s Defever 44s have a 250 gallon aft centerline tank and two 350-gallon saddle tanks. We use the aft tank as our day tank.

GoneFarrell said:

Have any of you guys run dry on a day tank? Just curious. I've never dealt with one on a boat.

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Never came close. I installed a good fuel sender and gauge that I can read in the ER. Each morning, I bring the tank up to about 30 percent full maybe 75-80 gallons. We would have to run for ~20 hours to bur that much.

GoneFarrell said:

Have any of you guys run dry on a day tank? Just curious. I've never dealt with one on a boat.

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I doubt anyone custom building a passagemaker trawler would build it without a day tank, they are just too important for a true offshore boat. The also are just plain handy. We ran ALL our fuel through a racor 1000 then into the 100 gallon day tank. It had a sight glass marked in 5 gal. graduations and was damn accurate. From the day tank into a select able twin racor 500 setup, then to the generator and main engine, both had secondary filters. Needless to say I never had a fuel issue, the day tank also had a drain sump. The Nordhavns I have been on used one of the tanks in the same way, and it also works well. I done have one on my current Ocean Alexander ( has 2 main tanks ) .. and I miss the security .


HOLLYWOOD

GoneFarrell said:

Have any of you guys run dry on a day tank? Just curious. I've never dealt with one on a boat.

But I did deal with issues from dirt-based daytanks at an industrial site. A lesson learned.

Power got knocked out to a wafer fab in Albuquerque (separate story.)
The backup gennies kicked in, but somebody forgot to switch them off the day tanks.
Day tanks ran dry, gennies dropped off line, fab went into meltdown, 1100 bunny suits in the parking lot!

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It's a risk and needs to be dealt with. We had an alarm on ours that would wake the dead. It went off once, and from then on my wife was always asking me if the day tank needed to be topped up. She did NOT like that alarm.


All of this is one of the arguments against more sophisticated fuel systems. Actually against more sophisticated anything. They can be more capable, but bring along with them more operational complexity, which in turn introduces new operational risks.


It's stating the obvious, but you need to know how to operate whatever thing you are operating. And if you don't understand it or know how to operate it, you will always argue for "simpler". Everything we build or use has to find the right balance for the application.

"I think having a day tank is age dependent, do they build them into boats anymore?"#23

I would expect a day tank is more required on a new boat with electronic injection than any older boat.

Most fuel delivered from a marina is NOT clean enough for these engines so a grand on board fuel cleaning setup is required.

Once finally clean enough to use, storing enough to operate for a day seems a good idea.