Diesel Fuel Day Tank

Polaris, the sailboat we looked after for a month in Greenland, is fitted with a diesel day tank that’s installed so the bottom of the tank is higher than the intake on the main engine and Refleks heater. (The Webasto heaters require a combined metering and lift pump, so their position, relative to the tank, is not an issue.)

There are several advantages to this system:

  1. Finding an air leak in the fuel intake line to a diesel engine, which will quickly stop the engine, can be one of the most frustrating tasks known to humankind. That whole problem goes away with a day tank like this one since the fuel system is under slight pressure from gravity, making a leak both less likely—pressure tends to seal fittings and gaskets, vacuum has the opposite effect—and obvious if one does occur.
  2. Fuel plumbing is simplified since a boat like Polaris, with four separate diesel tanks and without a day tank, would need a feed and return (where required) line for each diesel burning device on the boat to each tank—a plumbing nightmare. On Morgan’s Cloud we have two tanks and three user devices, together with a fuel polish system, which results in a fuel valve system that looks as if it should require a nuclear plant operator’s license.
  3. Changing filters on the engine is easier since they will automatically fill by gravity and priming the engine will be easier too. (On Morgan’s Cloud we have a small electric fuel pump in the system that provides the same benefit without a day tank.)
  4. The day tank can be more easily cleaned, and water as well as sludge more easily drained from the bottom, than the main tanks.
  5. Since the fuel is filtered when being pumped from the main tanks to the day tank and then again as it goes out to the user devices, a polish system is not really required, particularly when you take into account point four above.
  6. It is much easier to measure and monitor the fuel consumption of each device using a day tank. For example, we filled in a log of the fuel used by the heater each day while caretaking Polaris.

Hutting, builder of Polaris, has done an installation job on the day tank that is, like all the systems on the boat, a work of art.

The front of the day tank (just above the twin filters) is what you are looking at in the photo. (The smaller black tank in the left middle ground is the engine cooling header tank and nothing to do with the fuel system.) The blue pump on the upper left fills the tank from whichever main tank is selected on a separate manifold (not shown) when the black button on the control box to the right is pressed. The pump also starts automatically if the day tank reaches half empty. The red button stops the pump, which also stops automatically when the tank is full. There is also a manual pump (green) in case the electric one fails. Under the tank there is a deep drip pan to catch the inevitable spills that will occur when the tank is being cleaned or worked on. There is also a simple sight-glass gauge that Michael (owner of Polaris) has accurately calibrated.

Drawbacks to the system—there are always drawbacks since everything on a boat is a compromise—are the substantial amount of valuable space used by the day tank and related plumbing, and the chance of running the engine dry if the automatic refill system fails or overflowing out the vent if the automatic stop fails.

On balance, we would say that a day tank makes a lot of sense.

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Meet the Author


John was born and brought up in Bermuda and started sailing as a child, racing locally and offshore before turning to cruising. He has sailed over 100,000 miles, most of it on his McCurdy & Rhodes 56, Morgan's Cloud, including eight ocean races to Bermuda, culminating in winning his class twice in the Newport Bermuda Race. He has skippered a series of voyages in the North Atlantic, the majority of which have been to the high latitudes. John has been helping others go voyaging by sharing his experience for twenty years, first in yachting magazines and, for the last 12 years, as co-editor/publisher of AAC.

5 comments… add one
  • Denis Dec 19, 2009, 3:59 pm

    I have installed a similar day tank system to my Fisher Freeward 30 Freewind, the main difference being that, instead of a high/low switched system I have installed an overflow that returns excess fuel to the main tanks. The day tank is fed from the engine’s mechanical pump which draws diesel from the low points of the two tanks through a water separator/fuel filter. This way all the fuel is constantly circulated with the engine being gravity fed from the day tank. Should the fuel pump fail I have about 20 litres of fuel in the day tank and my fuel is constantly ‘polished’ and de-watered. I have had this system in use for about five years with no problems so far, touch wood!

  • Jim R Sep 9, 2016, 11:55 am

    1. 35 gallon day tank
    2. 555 gallon / 2 main tanks
    3. A transfer / polishing system of 150 GPH

    I would like to use it to fill the day tank (e.g. just include day tank on same manifolds). Any concerns that the fill rate will be to high?

    • John Sep 9, 2016, 2:53 pm

      H Jim,

      That depends on the venting of the day tank. Also, what you decide to do with any overflow from said day tank. That said, 150 gal/hour seem to be a lot for this application since if anything goes wrong the mess at that flow rate is going to be pretty bad. If it were me, I would use a pump of about 40 gal/hour. That’s what we use for our fuel polish system. http://www.walbro.com/fr-series-fuel-pumps/

  • David Irons Nov 24, 2018, 9:40 pm

    Thanks to the information about avoiding diesel fuel degradation above and on other sites, I have developed the most resilient fuel system I can imagine for a modest sized cruising boat and an equally modest budget to pay for it. The main differences between my system and Denis’s is that the header tank is the hub of the fuel delivery system where the fuel level remains almost constant. A separate electric fuel pump fills the tank whether or not the engine is running. This permits the fuel to be polished more frequently when considered desirable. It also supports diesel fuelled heating with a gravity fed supply.

    By hub, I mean that fuel flows from the header tank via a secondary water separator/fuel filter to both the engine and the cabin heater. Fuel is pumped into the header tank from either one of the two fuel tanks (or out of a jug) through one of the two primary filters by the electric fuel pump at a rate of about 80 litres (20 US gals) per hour.

    The electrical digital timer (TM-619-series) that controls the fuel pump can be configured to set the pump’s on/off frequency during operation as well as its cyclical on/off periods each day of the month. These configurations include: ‘pulling fresh fuel from a jug’, ‘fuel polishing only’, ‘heater on only’, ‘engine on only’ and all their possible combinations. The configurations can be manually overridden when required.

    The fuel returned from the engine, when it is running, flows into the header tank for cooling and dilution before recycling through the secondary water separator/fuel filter to the engine’s mechanical fuel pump and filters. A permanent overflow from the header tank is directed to one of the two fuel tanks, both of which have access hatches fitted for manual cleaning. The breather on the header tank is connected to the breather line from the fuel tanks to a deck-mounted water lock. A 1½” diameter filler cap is fitted to the header tank for manual filling if the electric pump fails.

    The level of the fuel in the header tank is displayed on a gauge mounted near the engine control panel. You can see that the fuel supply is faulty when the level falls below 2/3 full. At that level, however, the header tank contains sufficient fuel to motor at cruising speed for at least another 8 hours.

    In short, my research into how to deal with diesel fuel degradation on a modest sized cruising boat is to fit an electric pump on a timer that circulates the diesel oil at least once every two or three days through the best filtration system you can afford and/or find space for. I found that people who run their boat’s diesel engines frequently don’t seem to have fuel degradation problems. Checking the quality of the fuel before putting it in the tank is also a sensible precaution.

    • John Nov 25, 2018, 9:19 am

      Hi David,

      Sounds like a good system. We too have a polishing system, although no day tank, and have found it very useful over the years.

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