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:
- 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.
- 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.
- 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.)
- The day tank can be more easily cleaned, and water as well as sludge more easily drained from the bottom, than the main tanks.
- 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.
- 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.