Day Tank

[SV Allie Rose]

I've acquired, and decided, to install a day tank on my 1972 Hallberg Rassy Rasmus. Photos of the day tank are included below. Note that since I took those photos, I've welded in two (2x) 3/8" NPT ports into the top of the tank. It's 10 gallons, and has an internal baffle.

I intend on running fuel from the primary tank to the day tank w/ 3/8" A1 fuel hose via a RacorP510MAM. It is an integrated fuel pump/filter, a "polisher".

I am intending on using a three way valve to allow the current fuel pump to select from pulling fuel directly from the main fuel tank (bypassing the day tank and P510MAM), or from the day tank itself. I have a length of metal tubing that I will use to draw fuel from the tank and into the day tank until I haul the boat later this spring to install and secondary tap.

QUESTIONS:

1) I can't figure this one out. Since the Racor P510MAM is both a filter and an integrated pump, where/how would I install a pressure/vaccume gauge? Would I do that on in input side of the filter? The output side just goes to the day tank, so I doubt it would have any type of a pressure reading... It does not have the capability of tapping into the top of the filter like on other RACOR products, because that's where the pump sits.

2) The day tank is 13" deep, and has 2x 3/8" NPT ports on the top, and 1x 3/8" NPT port on the bottom. Should I use a "pick up tube" on one of those top ports to deposit fuel into the day tank, or just allow the "input" line to drop the fuel directly into the tank? Secondly, can I run fuel hose, with some kind of a check valve, from the other port to a splice with the primary tank vent? The day tank will be in a helm locker, so I could always use a mushroom vent or other similar apparatus too.

3) Do I need check valves anywhere within the transfer system?

Thanks for your feedback. Especially curious about the pressure gauge. Photos attached of the tank.

[SY Harmony]

The vacuum gauge will need to go in after the selector valve. Which means you really won’t get the filter monitoring when using the main tank position.

As far as filling the main, i see no reason to run the fill pipe to the bottom.

[noelex 77]

A fuel day tank is commonly used in long distance power boats that are very dependent on engine reliability. They are not common on yachts for some strange reason. So kudos for fitting one.

I don’t really understand the plumbing of your system, but a few points:

Polishing requires frequently filtering of the fuel multiple times rather than just once.

The fuel return needs to be connected to the tank where the engine draws the fuel.

A bypass enabling fuel to draw from the main tank rather than day tank is a good idea, but this should really have its own filter.

Drawing a plumbing diagram would help.

[skipperpete]

A day tank is one of the best additions to a marine diesel engine fuel supply you can possibly make if you have enough space to fit one in. There are a few unforeseen benefits and potential problems as well as several variations on the basic design that you propose so this might help.
Depending on what engine you have, the return line needs to be considered, some engines send a lot of fuel back to the tank and a day tank eliminates return fuel from heating up the main storage tanks. This is relevant to common rail engines, Detroit diesels and larger Cummins diesels.
There are a variety of fill ( and overfill ) options to consider. Manual pump fill, electric pump fill with manual switch , automatic fill with float switch, and an overfill line directly back to main storage tanks.
A sight glass or level indicator is a necessity.
One of the major advantages of a day tank is the elimination of air in the fuel system since the line from the day tank to the engine always has positive pressure....... the downside of that is that if a lift pump diaphragm fails the engine lube oil system will fill with fuel oil and an isolating valve is needed to change the secondary filter or do any work on the fuel system.