Day tank for diesel fuel

[rigormortis]

Fluid engineers:

From what I understand, diesel engines enjoy sucking air before fuel. Does it not make sense to have a day tank (5-10 gallons) mounted above engine level in order to assure gravity flow and reduce air entry into the system? I realize not all boat and engine layouts would allow such a convenient setup but are there other reasons for not doing this? Or am I in error in assuming you would reduce your engine's desire to suck air with such a day tank installation?

Pete.

[gonesail]

makes sense to me. if you let the 10 gallon day tank go empty then you would still have air in your diesel.

[GordMay]

Yes, with some important provisos.

Robert Beebe said it well, in “Voyaging Under Power”
Goto page 76:
Voyaging Under Power - Google Book Search

[Pblais]

A day tank does come in handy when you have more than one tank or some boats have even more. It can simplify the plumbing of supply and return lines and make bleeding the system easier. It means each tank only needs a vent, fill, and a supply line but no return line. The day tank would then be the only tank with a return line. The process of switching between tanks can be a little more complex.

You then have to transfer fuel into the day tank and can choose which tank to pull it from and hopefully balance the boat more.

Any fuel pickup can suck air. A tank near empty can do it easier should the fuel slosh around. A well positioned day tank can make it harder since the tank is smaller.

As far as gravity flow I don't see that as a huge advantage you still use a lift pump and the line still must be bled.

Our boat has two tanks and the process of switching tanks requires a time consuming process and manually flipping several valves that are not easy to quickly. I've thought about eliminating the dual plumbing and just adding a transfer pump to move fuel from tank 2 to tank 1. The trade off is then I have to manually make sure I don't over fill tank 1.

Our prior boat had two tanks and a nice switch mechanism to switch over. Just flip two valves that were easy to get at. One 30 gallon tank was outboard and the other 30 gallon tank was mid line. I did on the very first cruise trip suck air out of the outboard tank because the tank was only about 1/8 full and being outboard in some very nasty weather is sloshed the fuel and sucked air.

If I had only one tank I don't see why a day tank would help except for one application. If you have a diesel heater you can mount a day tank high enough that you don't require a fuel pump or a return line. It does not work that way for an engine though.

[Latitude9.5]

I have a 14 gallon day tank, i also polish the fuel when it's pumped into the day tank, so it works out well.

[Chief Engineer]

Every Tug I ever worked on ran off a day tank.....you were guaranteed clean fuel as it was filtered well during transfer......you just have to be cognizant of fuel consumption as well as your oppo if you are cruising for an extended period.