What's a day tank?

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My boat has 2 main fuel tanks and a third tank aft which the manual describes as a 'day tank'. It has it's own deck filler and it is connected to the main tanks although it can be shut off from them by a valve.
I've heard the phrase 'day tank' before, mainly in connection with displacement boats, but can anyone tell me what the purpose of a day tank is?
 
Mike,

A day tank is where fuel is pumped or gravity fed for consumption by the engines. They are normally fitted low down to reduce weight and close to the engines. We certainly fit them into some of the steel versions of the Van der Valk's and the two larger main tanks feed into this tank directly.
They often have an additional filter or two so by the time the fuel gets from the main tanks through he day tank to the engine it should be very clean.

Not sure where the idea came from but assume it is something to do with a long range vessel needing to store huge amounts of fuel and to maintain the ships stability and trim this would have been pumped from the various storage tanks about the ship into a day tank for use on that day?
Only guessing here so open the doors to a more informed person.....ex RN perhaps?
 
The terminolgy is often used in large engine based power station applications where the fuel is stored in large bulk storage tanks before being pumped into the power station then often through fuel treatment plant before delivery to the day tank which is mounted at high level to permit gravity feed to the engines. There is a day tank for each engine and has a capacity to allow the engine to run for several hours in the event that the fuel transfer pumps or fuel treatment plant goes down. In the event of such a failure the plant continues running while someone sorts out the problem.
Does your "day tank" provide this type of gravity feed to both engines if the main tanks are at a lower level? Best I can think of!!
 
Some 'day tanks' were installed because the fuel pumping systems would suck air from low levels in wide tanks.

Have a look at the return to tank line on the engine pumps, follow this back and you might find a diverting valve to re direct the return fuel to your day tank.

Others have been installed because the fuel tanks are so low or far away the engine has trouble getting enough fuel, a day tank installed above the engine had little effect on the vessels stability and solved the fuel starvation problems.

Hope this helps.
 
search on "Diesel fuel day tank " will find detailed story of Day Tank use and design for a large marine engines. Such a tank generally has capacity for 24 hr at cruising speed and can be located at suitable height to supply the engine with well dewatered, filtered, and warmed -or cooled to suitable temperature for the injectors.
I am fitting one to my 31 ft sail boat as main fuel tank is in long keel under the engine and with infrequent irregular use I have problem pumping out the sludge - water and diesel bugs. or just stirring it up which was rapidly clogging fuel filters.

Darmog
 
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The term day tank is I believe regularly used in warships. In commercial ships it is known as the service tank. Depending on fuel type a ship may have two or three stages of fuel storage. Firstly there is the bunker tanks where most of the bulk fuel is held. They are then pumped to the settling tanks. This is where the fuel is setteled out so as any water can be drained out of the bottom and other muck skimmed of the top. This is especially important with heavy fuel oil (bitumen) but less so the purer the fuel. From the settling tanks the fuel is passed through filters and centrifuges etc and into the service/day/daily run tank. This tank is where the engines draw from. So basically you use the day/service tank to hold filtered, clean and pure fuel ready for use.
I'm not sure its really required on yachts etc with very small bunker capacity but it does give you a way of isolating good fuel from potentially not so good fuel.
 
Days tanks seem to be less common but are still used on some smaller sailboats and seem to be pretty common on larger high end motor yachts. I think jfm has one on Match II. One common reason for them on small boats is eberspachers or similar where a day tank means they won't crud stirred up from the bottom of the main tank - also they can be mounted higher up to allow a gravity feed, etc.
 
We have a day tank on the Princess 85 that I skipper.

We transfer automatically from main tank via a simple filtering system. All machinery, engines and generators go off day tank. I can isolate if necessary.

Our day tank holds 900 litres and main tank hold 7600. In essence at our cruising we can get 9 hours run out of tank.

If you have a day tank I would suggest you install a basic fuel filtering system and use it, this add another layer of protection from dirty fuel in engines..
 
grumpy_o_g said:
Days tanks seem to be less common but are still used on some smaller sailboats and seem to be pretty common on larger high end motor yachts. I think jfm has one on Match II. One common reason for them on small boats is eberspachers or similar where a day tank means they won't crud stirred up from the bottom of the main tank - also they can be mounted higher up to allow a gravity feed, etc.
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Fitted to the BT Challenge yachts, manually pumped from main tank.
 
We are getting around 11 knots on average at 100lph. With stabs on it is very comfortable...this summer I have done 5500 NM around the Med hence the displacement speed.
 
Lozzer said:
this summer I have done 5500 NM around the Med
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Wow, that's some cruising. Averaging 11kts, that's 500 hours, hence 50 tons of fuel.
I can definitely see the reason for choosing the slow cruising style, I don't dare thinking what she would have burnt at 25kts... :D
 
25 knots we are eating circa 600 lph..... WOT we are just shy of 800lph.

So range goes from circa 800nm to 280nm

If we cruised at 25 knots we would burn around 132 ton of fuel.

No brainier.
 
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