Day tank

What is the purpose of a "day tank?"
My guess, based on other postings, is that it serves to provide well polished fuel to the main. I run a racor 500 plus two secondary filters on my Ford Lehman 135. All my fuel regardless of which tank it comes from passes thru these filters. Therefore fuel drawn from my "day tank" is not filtered any differently than the fuel coming from the other tanks. It's much easier to use fuel from whichever tank I choose rather than having to transfer fuel into the day tank to feed the main. How do most of you manage your tanks?
BTW-what micron size filters are usually used in a polishing system?

On a Lehman, if the day tank is higher than the fuel inection pump, the lift pump can fail and be bypassed. Of course if your regular tanks are higher, then no help.

While not the main function...it is a nice bennie.

On an old yacht that I used to sail on, there was a smallish tank mounted higher than the engine - you had to hand pump fuel into this tank from the main. It held roughly enough fuel for a days running (thus the name). The engine was a Lister and it didn`t have a lift pump - gravity from the day tank to the injector pump. No lift pump was, I guess, one less mechanical thing to break although if you forgot to top up the tank it was a pain.

The point of a day tank is to provide isolation between the fuel you burn and the fuel you buy and store. The only way fuel can get from the storage tanks to the day tank is via a transfer pump and filter. It's not essential, but provides an extra measure of insurance against fuel related problems.

Without a day tank, all fuel still gets filtered by racors and on engine filters - that's very true - and under normal circumstances it is just fine. But if you take on a crap load of fuel, or get in rough water and stir up settled crap from the bottom of your tank, all that is going to get filtered real time by your main filters, over and over again, as long as you run your engine. You have no control over it, and the likelyhood of filters plugging up and requiring replacement while underway goes way up, including the probability of plugging the on engine filter requiring a shutdown and bleeding. And will you have enough filters to clean the fuel, or will you run out when there is still crap in fuel? If that happens, you are kinda screwed.

With a day take, your racor and on engine filters only ever wok on daytank fuel, never on whatever you bought or stirred up. So they are no more subject to blockage when you have crap in your storage tanks. And you have complete control over how much and how fast you transfer and filter the crap fuel as you transfer it to the day tank for consumption. The most you have to filter is whatever you will consume in a day. Chances of running out of filters goes way do.

So depending on the type of cruising you do, it brings a distinct extra measure of fuel protection. But it involved more work, so a tradeoff each will have to make for themselves.

I've heard the term used to describe a tank that carried a sufficient quantity of fuel for some reasonable period of use. Folks who day-sail/day-trip and don't care to maintain a full fuel tank are happy with a smaller supply of fresh, clean fuel and a clean, empty main tank.

Obviously, all sorts of questions arise, like piping, valving, filling, venting, mounting, safety of a perhaps unlisted tank.

We had an 80 gal. tank of diesel on the NE 38 and would burn about 40 gal./year; I figured that that was enough dilution to keep the stuff fresh enough. I'd fill it once a year in the fall, and top it off just before the boat was pulled for the season.

We had a 15 gal. tank of gasoline on the Morgan 27 and my dad, who owned the boat for the first 20 years, had several episodes of jellied gas to clean out of the tank.

Thank you for your replies. Should I embrace the day tank concept of fuel management what level of filtration would be required of the fuel transferred to the day tank from my other tanks. What specific filtration (raycor 500? with 2 micron filter? or? for example) would you recommend to be plumbed into the transfer/polishing circuit?

Another question....if I wanted to plumb the transfer/polishing circuit to act as a lift pump in the event of lift pump failure, where in the fuel system would I connect such a supply line?

petdoc4u said:

Thank you for your replies. Should I embrace the day tank concept of fuel management what level of filtration would be required of the fuel transferred to the day tank from my other tanks. What specific filtration (raycor 500? with 2 micron filter? or? for example) would you recommend to be plumbed into the transfer/polishing circuit?

Click to expand...

We polish fuel at 30 microns, which is the refinery standard, and transfer fuel to a 100 gal day tank at 10 microns. The engine draws the fuel from the day tank, passes it through a 2 micron Racor, then the OEM filter, then the return line sends polished fuel back to the day tank. Since this process filters the fuel to 2 microns, it probably doesn't help that much to transfer the fuel to the day tank at a finer filtration than 10.

"Should I embrace the day tank concept of fuel management."

For offshore where a clean fuel supply is required,,, yes.

For inshore multiple filters should do fine.

On advantage to a day tank is the fuel pickup can be a bit up from the tank bottom and a bottom drain can be used to clear the water that got past the filter bank.

I installed daytanks in my previous boat, a Grand Banks 42 with twin 120 hp Ford-Lehmans. When running with the daytanks I was able to simultaneously polish the main tanks while the boat was in motion. This way the polishing was more likely to pick-up any stirred-up crud.

I have always being skeptical of polishing fuel while in a slip as the water and crud just sit in the bottom of the tank and are not picked up and filtered, unless the tanks have a sump for the polishing pick-up point.

The fuel filter should be before the pump as protection. Also you can install vacuum gaugeson filters so you can momonitor the fuel filter, and double filters allow quick change to a clean filter and or change a filter. Also have a small fuel can so you can fill or top of the filter.

petdoc4u said:

Thank you for your replies. Should I embrace the day tank concept of fuel management what level of filtration would be required of the fuel transferred to the day tank from my other tanks. What specific filtration (raycor 500? with 2 micron filter? or? for example) would you recommend to be plumbed into the transfer/polishing circuit?

Click to expand...

Our process is pretty close to delfins, but with slightly different filtering.

From storage tanks to day tank filtering is 30 micro plus water separation from the Racor. The goal is to get rid of the sea gull, cats, dogs, trash, and water. 30 micron accomplishes this. I think 10 would be fine too. We picked 30 so we can carry only one type of Racor for everything.

Consumption of fuel goes through another 30 micron Racor, 10 micron on engine OEM, then 2 micron on engine OEM. The Racor is 30 because that's what Deere says to use. But because the day tank contents are pre- filtered and dewatered, these additional 3 filters are always acting on clean, civilized fuel.

Thank you, all.
I have an additional question. As previously mentioned I have an unsophisticated fuel polishing/ transfer setup which draws from a chosen supply tank and returns to the same or a different tank. Basically a supply/return loop. IF I wanted to integrate, through a valving system, this setup such that it could be used as a backup lift pump, at what point would I connect to the engine fuel system? before engine filtration? after? before the lift pump? after? etc.
Thanks for your responses.

You definitively do not want to connect it after the secondary filter ("engine filtration") as you should not bypass this filters(s).

If you connect it before the lift pump I believe you run the risk of pumping diesel into the crankcase in case the problem with the lift pump is a broken membrane. This would dilute the engine oil and could do serious damage to the engine.

So the best place would seem to be after the lift pump and before the secondary filter. I would think even better is just to carry a spare lift pump and use it when needed.

Having said that, one advantage of doing what you propose is that you can use the transfer pump to bleed the the system in case you need to purge air, for examble, after changing the fuel filters.

Another thing I give Art Defever credit for. Our 44 has a third "fuel box" with a capacity of about 200 gallons that some owners use as a day tank. The inspection port is on top and ours looks pristine inside. Transferring enough filtered fuel to it for a day's run seems like a small price for dramatically improving the odds against clogged filters.

Also, you need to be careful the fuel transfer pump is appropriate as a substitute lift pump in terms of pressure and thruput: too much and there could be consequences.