Whys and Wherefores of Day Tanks

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Old Bumbulum

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Day tanks.
What is the purpose? Are they a practical or useful thing on a yacht, assuming it has the space available?
How big should one be in terms of hours of usage?
Position, above engine for gravity feed?
Refill automatically or manual switching?
Additional filter necessary/desireable?
What are the downsides?
 
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I can't see the point, TBH. Depending upon how much your engine returns to the tank, only half the day tank might be usable unless you also plumb in a switchable return.

It's much easier just to look after the fuel in the main tank. :)

Richard
 
I think it is a good way to go for a long distance cruiser with a serious fuel capacity.
For a Solent/Channel cruiser-racer, not really much need.

If you've ever bought a shipload of dirty fuel in Africa, being able to run from cans and a day tank while you sort it out can be a real bonus.
Knowing your engine is fed from a small tank of clean fuel can be re-assuring. It's a lot easier to be sure a small 5 gallon tank is clean. You filter the fuel as it comes from the main tanks. If those filters clog, no need to stop the engine. No need to pump while in rough water that stirs the tanks.
Those I've experienced, the returned fuel goes to the day tank.
There are the normal filters between day tank and engine, but they last forever.

For many people it's never going to be worth the complication of course.
 
I think that a day tank would solve a lot of problems for some boats. An ideal fuel tank (like mine) has a dirt/water sump fitted with a drain valve. This ensures that there is no dirt or water to move around in the base of the tank, with its associated problems of choked filters and diesel bug. I appreciate that for many boats, this ideal is impossible to achieve. A day tank makes it easy. If it is feasible to mount it higher than the engine, giving a gravity feed, so much the better.
 
We have had a fuel day tank fitted to a new yacht.

The fuel is filtered as it is transferred to the day tank, then filtered from the day tank to the main engine. The advantages are:

1. The tank is higher than the Reflex diesel heater. Therefore the heater can be operated via gravity. A continuously running electric feed pump is not needed. The diesel heating consumes almost no electricity.

2. As the day tank is higher than the engine, the diesel lift pump can fail and the engine can still have fuel.

3. A gravity fed fuel supply makes filter replacement and bleeding easier.

4. The day tank has a very large easy to drain sump to trap water.

5. A fine 2 micron filter can be used between the main tank and the day tank without the inherent risk of low fuel flow if this was to be used between the tank and the engine. As the fuel is filtered to the 2 micron level before reaching the day tank, the engine primary and secondary filter should almost never clog.

6. If the day tank develops a fault (leak, dirt, water) the engine can be fed directly from the main fuel tank. This is a completely seperate feed with separate fuel lines, valves and filters so it provides a redundant fuel source for the main engine.

7. The engine has effectively three filters (rather than the more conventional two) before the fuel is fed to the injectors. However, as the day tank provides a reservoir of fuel the extra filter does not restrict the fuel supply to the engine.

Many, or even perhaps most long distance trawlers have a day tank system. These vessels are of course completely dependent on their fuel system for propulsion. On yachts the main engine is only a secondary propulsion source so a day tank system is overkill. However, it does add redundancy, helps ensure the engine has clean fuel and adds a little to the overall tankage capacity.

The only drawback is the need for a pump to fill the day tank, but an electric diesel pump is helpful to be able to transfer fuel between the main tanks or as a means of polishing the fuel, so the transfer pump can have multiple uses.
 
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RichardS said:
I can't see the point, TBH. Depending upon how much your engine returns to the tank, only half the day tank might be usable unless you also plumb in a switchable return.

It's much easier just to look after the fuel in the main tank. :)

Richard
Click to expand...

Have you a reason why the return wouldn't go to the day tank?
 
RichardS said:
I can't see the point, TBH. Depending upon how much your engine returns to the tank, only half the day tank might be usable unless you also plumb in a switchable return.
Click to expand...

The return is normally to the day tank so there is no loss in capacity or need to switch the return.
 
noelex said:
We have had a fuel day tank fitted to a new yacht.

The fuel is filtered as it is transferred to the day tank, then filtered from the day tank to the main engine. The advantages are:

1. The tank is higher than the Reflex diesel heater. Therefore the heater can be operated via gravity. A continuously running electric feed pump is not needed. The diesel heating consumes almost no electricity.

2. As the day tank is higher than the engine, the diesel lift pump can fail and the engine can still have fuel.

3. A gravity fed fuel supply makes filter replacement and bleeding easier.

4. The day tank has a very large easy to drain sump to trap water.

5. A fine 2 micron filter can be used between the main tank and the day tank without the inherent risk of low fuel flow if this was to be used between the tank and the engine. As the fuel is filtered to the 2 micron level before reaching the day tank, the engine primary and secondary filter should almost never clog.

6. If the day tank develops a fault (leak, dirt, water) the engine can be fed directly from the main fuel tank. This is a completely seperate feed with separate fuel lines, valves and filters so it provides a redundant fuel source for the main engine.

7. The engine has effectively three filters (rather than the more conventional two) before the fuel is fed to the injectors. However, as the day tank provides a reservoir of fuel the extra filter does not restrict the fuel supply to the engine.

Many, or even perhaps most long distance trawlers have a day tank system. These vessels are of course completely dependent on their fuel system for propulsion. On yachts the main engine is only a secondary propulsion source so a day tank system is overkill. However, it does add redundancy, helps ensure the engine has clean fuel and adds a little to the overall tankage capacity.

The only drawback is the need for a pump to fill the day tank, but an electric diesel pump is helpful to be able to transfer fuel between the main tanks or as a means of polishing the fuel, so the transfer pump can have multiple uses.
Click to expand...
Top post Noelex, +1 to everything
 
Luckily we had a day tank, when our main tank had debris within it (boat is over thirty years old), we would switch to day tank which had filtered fuel for manoeuvres which required 100% assurance that the engine stop without us wanting it. The other plus side when we changed our pre filter system it also allowed gravity feed to allow pre bleeding, if you have space why not. Ours did require hand pump refill 10 mins max for a four hour use.
 
NormanS said:
Have you a reason why the return wouldn't go to the day tank?
Click to expand...

You could, of course, send the return directly to the day tank but that's even more plumbing required. As Noelex says, on yachts the main engine is only a secondary propulsion source so a day tank system is overkill. :)

I reckon that time and money would be better spent on looking after the diesel in the main tank.

Richard
 
Completely agree with the majority of posts so far, probably way overkill unless you're heading off to cruise further afield. Then it's just a matter of time before you get dirty fuel. So new engine install a few years ago I put a day tank in with a few 3 way valves as below, certainly much simpler in operation than it looks!
But can pump through a filter from main tank to day tank, either tank to/from a canister so you're not messing about on the side deck in a holie or to get some fuel to yer mate in the anchorage who's low when there's no fuel anywhere near. Day tank only ever sees filtered fuel & that filter is easy to get at so filter to engine has an easy life.

Cost of a decent meal with some wine & couple afternoons to fit. Very pleased I made the effort. As ever got a load of stick from a few naysayers on here for doing it ;)

https://imgur.com/rlBUC1N
 
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