Three fuel tanks… why?

[nicholasdrayer]

Hi all,
I have a new-to me boat, a Nordia 49 feet built in 1990. It has a Perkins 4.236 aka M90. It works very well.
We (my son and I) are perplexed by the diesel tank system. As is the experienced diesel mechanic at the boatyard here in Greece.
We have:
a main tank, 750 liters
a day tank, higher than the engine, automatically filled from the main, guess about 100 liters
a mysterious third tank under the engine room floor, about 50 liters

We are trying but having trouble tracing all the fuel lines between these 3 tanks, the engine, the generator, filters. The main engine ran out of fuel while switched to the day tank system. Due to the third tank being empty after about 60 miles. We switched to using the main tank directly, and all is now ok.

I’m hoping someone can shine a light on the third tank, and how such a system should work (and why not just only use the main tank…).

Thanks for reading!
Nicholas

[wrwakefield]

Nicholas, in our case, we have an extra tank for supplying #1 fuel oil to the diesel heaters on board.

Both heaters perform better and maintenance intervals are extended [by 2x+] by not using the #2 diesel [blend for temperatures down to -25°F; -32°C] preferred by the main engine and generator. Our #2 tanks also contain a lubricity additive which seems to contribute to further fouling of the heater systems…

Otherwise, on past boats I have added additional, smaller tanks in otherwise unusable spaces just to increase cruising fuel tankage.

Our two main tanks for engines [130 gal; 492 liters each] are low and outboard port and starboard. We can transfer fuel between them to help balance the boat- which is also useful if on a long passage on single points of sail.

A day tank is nice for tracking/verifying fuel consumption.

In case any of this relates to your 3 tanks…

Cheers, Bill

PS: Another reason for separate tanks on a cruising vessel is in case one becomes contaminated. [e.g., bad o-ring on deck fill that allows water ingress, etc. Same applies to potable water tanks…] In which case the 3rd tank could be considered an emergency spare- like extra fuel cans.

[nicholasdrayer]

Hmm, yes… none of those ideas, valid as they are, can be the reason here. We know for sure that the generator’s return line goes into the third underfloor small tank. And that the main engine’s supply comes from that same tank. And the actual day tank gets filled automatically from the main tank via a sensor and a refill pump.
We’re going to have to open the floor and walls and start tracing lines, I guess… not trivial!!

[wrwakefield]

Originally Posted by nicholasdrayer

Hmm, yes… none of those ideas, valid as they are, can be the reason here. We know for sure that the generator’s return line goes into the third underfloor small tank. And that the main engine’s supply comes from that same tank. And the actual day tank gets filled automatically from the main tank via a sensor and a refill pump.
We’re going to have to open the floor and walls and start tracing lines, I guess… not trivial!!

I guess the only consolation is the improved understanding of your boat after reverse-engineering some else’s undocumented efforts!

My condolences…

Cheers, Bill

[pjShap]

If a return line goes into the 3rd tank and a feed line comes from the 3rd tank, and the 2nd tank is automatically filled from the main tank, maybe the 2nd tank above the engine and the 3rd tank below the engine are connected together as a single functioning day tank?

[noelex 77]

A day tank is a significant help in ensuring clean fuel for the engine, so the boatbuilder put some thought into the fuel system.

The third 50L tank is obviously a puzzle. Do you have diesel heating on board? Some boats have a seperate "heating" tank. It and enables them to use old, or even water damaged fuel.

Some diesel heaters (such as the drip models) will quite happily burn fuel that should not come near the main engine. Hence it is not unusual to fit a seperate tank to isolate fuel that while OK for heating should be kept seperate from fuel sent to the engine.

[Statistical]

The 3rd tank is likely a day tank. Fuel can be polished and put into the day tank. The engines and genny run only off the day tank.

[ChrisJHC]

My immediate thoughts have been well covered by Bill above.
Alternatively, have you tried contacting the previous owner?

[Bowdrie]

While a day tank can be used to accurately gage fuel consumption, (if it's plumbed properly with the necessary valving,) and assuming there are no stoves/heaters that rely upon a gravity source of diesel, the main purpose of a day tank is to allow continued, (for a time,) engine operation in the event of a failure of either a primary feed pump or the engines own lift pump.
A failure of a feed/lift pump at an inopportune moment, (crossing a bar, maneuvering into a slip, etc.,) could have a serious outcome.
They are generally plumbed so that only fuel that has already been thru the primary/secondary filters is used to fill them.

[nicholasdrayer]

Well, it cost us our Sunday, but we’ve figured it out! I’m very happy. For anyone interested or because they run into a similar problem:

The main tank connects to the day tank via a circulation pump, filter, and a ‘debugger’. From the day tank it overflows back into the main tank. The main engine and Vetus heater tap from the daytank, with the main engine return going back to the daytank.
The generator is only directly connected to the main tank (feed and return).

There’s a pair of valves that allows switching the main engine directly to the main tank, bypassing the day tank circulation system. I.e. for when the circulation system is broken, as ours currently is.

The day tank is placed slightly lower than the main engine. The previous owner told me the system was put in place to clean the diesel (there’s a debugger, a filter and a separator in the circuit). Because the main tank is virtually impossible to clean out.

Now for the surprise: what looks like the day tank is nothing of the sort. It’s hydraulic fluid, tied to the main engine that turns the bow thruster’s hydraulic engine. Now I know I feel a bit stupid, but in our defence two diesel mechanics assumed it was the day tank too… It was critical to take the floors and wall panels off to make a diagram.

All in all a good use of a Sunday for my son and I!

Also learned from the diesel mechanic: our Perkins 4.236 is very difficult to impossible to bleed using the manual handle on the lift pump. He showed us how to use the starter motor briefly, to go through each step of the bleeding. In particular once all the injectors produce diesel, to then attach 3, see if the motor actually tries to fire, and then quickly tighten the 4th one. He also blocked the air intake with his hand while using the starter motor, to save the battery.

[Cheechako]

With roughly 240 gallons of fuel storage You should ask yourself how much fuel you need for your type of boat use. If not crossing oceans under power, you may want to just abandon one or two of those tanks.
If your 4-236 burns about a gallon an hour you have maybe 800-900 mile range now.
If you dont motor a lot and often, your fuel is getting stale from non use.
KISS

[Dockhead]

Originally Posted by nicholasdrayer

Well, it cost us our Sunday, but we’ve figured it out! I’m very happy. For anyone interested or because they run into a similar problem:

The main tank connects to the day tank via a circulation pump, filter, and a ‘debugger’. From the day tank it overflows back into the main tank. The main engine and Vetus heater tap from the daytank, with the main engine return going back to the daytank.
The generator is only directly connected to the main tank (feed and return).

There’s a pair of valves that allows switching the main engine directly to the main tank, bypassing the day tank circulation system. I.e. for when the circulation system is broken, as ours currently is.

The day tank is placed slightly lower than the main engine. The previous owner told me the system was put in place to clean the diesel (there’s a debugger, a filter and a separator in the circuit). Because the main tank is virtually impossible to clean out.

Now for the surprise: what looks like the day tank is nothing of the sort. It’s hydraulic fluid, tied to the main engine that turns the bow thruster’s hydraulic engine. Now I know I feel a bit stupid, but in our defence two diesel mechanics assumed it was the day tank too… It was critical to take the floors and wall panels off to make a diagram.. . .

Glad you got it figured out!

As others have said, a day tank is a very valuable feature. I envy you. Likewise, a fuel polishing system is great to have.

I don't believe, however, that this system is well configured, and I would suggest changing it.

1. Fuel polishing system should return the fuel right where it got it, and NOT into the day tank. The whole point is to circulate and recirculate through filters and water separators to get it clean.


2. Day tank should be filled from the main tank by a transfer pump. Make sure the pickup for the fuel polishing system is as low as it can be, lower than the pickup for transfer to the day tank.


3. You should not need a main or generator pickup in the main tank. I would keep it switched off. It defeats the purpose of the day tank.


4. The day tank should NOT be automatically filled. You should be filling it yourself and watching the fuel as you do it. It should have a big inspection hatch, maybe a glass one, so that you can clearly see what the fuel looks like. That's the whole point of a day tank.

To make it work like that, most likely only small mods would be needed.

[Wotname]

Originally Posted by nicholasdrayer

....... He also blocked the air intake with his hand while using the starter motor, to save the battery.

It's great you have solved the tank arrangement.
I wonder about the 'saving the battery' statement though - it seems strange to me and hard to believe - unless I have missed something.

[skipperpete]

It’s a diesel mechanics party trick, little to no air going in so less to compress with a consequently higher cranking speed for bleeding. I’d prefer to find out why the batteries needed saving and fix that first. I wonder if that diesel mechanic knew that the lift pump needs to be on the back of the cam for succesful operation?
Glad it all got sorted, well done.

[Wotname]

Originally Posted by skipperpete

It’s a diesel mechanics party trick, little to no air going in so less to compress with a consequently higher cranking speed for bleeding. I’d prefer to find out why the batteries needed saving and fix that first. I wonder if that diesel mechanic knew that the lift pump needs to be on the back of the cam for succesful operation?
Glad it all got sorted, well done.

Well if it results in a higher cranking speed then it must be true. I was thinking about how much energy is lost in trying to pull a vacuum. If the cranking rpm is higher, then the energy sum between pulling a vacuum and compressing additional air must favour the method.

I prefer an engine with a decompression system .