Fuel line between tanks

[MichaelB1969]

Does this work for anyone? I've had the tanks at different levels and have left this line open for hours and have seen zero change. Could obviously be blocked, but also it's a pretty small tube.

Anyways, wondering if others have the same issue or not.

Thanks,
Mike

 

[Comodave]

My understanding is that it takes a pretty large hose to actually equalize two tanks.

 

[RT Firefly]

Greetings,
Mr. 1969. Many moons ago we we had a 34' MT with dual side tanks. The crossover line (1/4" I think) was always open. To the best of my recollection, the tanks would equalize IF the boat was trimmed properly. Meaning, if one of our dual outboard aft water tanks was full and the other empty that would impart a list that would not allow the fuel tanks to equalize. Hope I've explained properly...

 

[porman]

The cross-over between our two tanks is about 3/4 inch, the size of a garden hose, and it will even out the tanks over a couple hours.

 

[alormaria]

Same here.

 

[Maerin]

Could obviously be blocked, but also it's a pretty small tube.

Anyways, wondering if others have the same issue or not.\

Pretty small ?? What diameter is it?

If there's a pathway between two tanks that is routed below the liquid level of both tanks, physics says it will allow the liquid to seek equilibrium, even if it's a pretty small tube; more rapidly if it's pretty big. If the path is routed higher than the liquid level (highly unlikely) it will probably not work unless it is piped from the bottom of both tanks, is filled with fuel, and cannot allow air to displace the contents of the hose/pipe.

Typically, an equalizing line is connected to both tanks at the bottom, with both connections valved, and the line is generally larger than the other fuel lines, but not always. If there's no equalization taking place, either one or both valves are closed, or there's an obstruction in the line (kinked?). The line could also have a section that is physically higher than the lowest level of the fuel in the tanks that could create an air lock, but that wouldn't be normal, and would only create a problem if the tank levels were very low.

My boat has a 3/4" hose connecting the tanks, ball valves at each connection, and takes an hour or more to level out, depending. Note that the hose must be rated USCG-A1. That rating must be visible on the jacket of the hose. A surveyor might catch that and require it to be corrected if it's not.

 

[GoneFarrell]

Michael,
Check your tank vents for clogging, assuming they are somewhere on freeboard, usually just below the gunnels/railings. I have found that they can gunk up with green stuff. Cleaned them out with a small bottle brush/blast of contact cleaner.

I have crossover line between 200 gal tanks, works fine (when the vents can flow air!)

 

[boater3]

make sure you have the valves open on both ends

i thought it wasn't working for me either but it just takes a long time

if i want to equalize i do it overnight, you should not run with it open

also do not rely on your fuel gauges to tell you they are equalized. my gauges are about 5% off from one another

 

[drb1025]

make sure you have the valves open on both ends

i thought it wasn't working for me either but it just takes a long time

if i want to equalize i do it overnight, you should not run with it open

also do not rely on your fuel gauges to tell you they are equalized. my gauges are about 5% off from one another

Why not run with it open?

 

[rochepoint]

Why not run with it open?

Mine is always open..........

 

[fryedaze]

Mine is always open..........

I run with mine closed. I discovered that allowing the tanks to equalize just gives me more stb list. As holding tank fills, the boat lists to stb. That makes the port tank higher which intern transfers more fuel to the stb tank, increasing the list.

 

[diver dave]

Why not run with it open?

You would introduce a single point of failure that could result in all the fuel leaking into the bilge. Not a large risk if the hose/tube/pipe and fittings are good.

 

[boater3]

You would introduce a single point of failure that could result in all the fuel leaking into the bilge. Not a large risk if the hose/tube/pipe and fittings are good.

yes that is one reason. the other reason that i read about is while running they may not equalize depending on sea conditions and angle. i think most open up the transfer valve when on hook for extended periods of time and running genset as it draws from one tank.

 

[boathealer]

yes that is one reason. the other reason that i read about is while running they may not equalize depending on sea conditions and angle. i think most open up the transfer valve when on hook for extended periods of time and running genset as it draws from one tank.

Also be sure that you know where the return fuel is going. On my previous boat, return fuel (a lot of it with the Cummins 380 wrt what is burned) was returned to only one of the two tanks, while drawing from both simultaneously. Over a long run, if the valves are closed and that one tank starts out nearly full, well.......

 

[boater3]

Also be sure that you know where the return fuel is going. On my previous boat, return fuel (a lot of it with the Cummins 380 wrt what is burned) was returned to only one of the two tanks, while drawing from both simultaneously. Over a long run, if the valves are closed and that one tank starts out nearly full, well.......

Sorry but what is return fuel?

 

[boathealer]

Sorry but what is return fuel?

Also be sure that you know where the return fuel is going. On my previous boat, return fuel (a lot of it with the Cummins 380 wrt what is burned) was returned to only one of the two tanks, while drawing from both simultaneously. Over a long run, if the valves are closed and that one tank starts out nearly full, well.......

A diesel engine typically only burns a fraction of the fuel that is pulled from the tanks by the fuel pump. Some of it is to aid in cooling of the fuel system. A side benefit is you get a bit of polishing through your fuel filters.

What is not burned directly by the engine via the injectors is returned to the tanks via the return fuel line - typically deposited back into the top of the tank(s).

 

[boater3]

Interesting and thank you for the explanation! I thought I had traced all of the raw water and fuel lines and didn't see a return fuel line. I will have to look again!

PS. to ST44 owners, I just found there is a hatch above the sea water strainers for the engines you can access from the salon... did not know that existed until I removed the snap-in carpeting!

 

[Maerin]

Sorry but what is return fuel?

Fuel that the diesel engine returns to the tank. There is a fuel pump, usually referred to as a "lift pump" that provides pressurized (a few psi) fuel to the injector pump. It's more volume than the injector pump needs even at its maximum fuel burn. The injector pump provides very high pressure to the injectors that meter and atomize the fuel and inject it into the combustion chamber. What the injector pump and the injectors don't use continues to the return line and back to the tank. Ergo "return fuel". That's a simplified view, different engines may use a variation on that theme.

 

[MichaelB1969]

Great info everyone. As for: "Typically, an equalizing line is connected to both tanks at the bottom, with both connections valved, and the line is generally larger than the other fuel lines.."

Yes, this is how it's setup. I think the line is 1/2".

Valves are open.

I'll check the vents for blockage.

Mystery - how are you checking actual levels if the gauges are not accurate?

Thx,
Mike

 

[AusCan]

A vent blockage is possible, but I'd expect that you'd have trouble filling the tanks if the vents were blocked.

It could also be that sediment has collected in the crossover line. You may need to remove the line and blow it out.


I have a sight glass connected to the cross-over line on my boat. It allows me to check the level of both tanks with one sight glass. There is also a water drain point tee'd into the lowest point.

I normally keep the crossover line isolated, especially considering the extra fittings attached.

 

[SOJOURN4]

All -
I keep the crossover line closed.
On my ST 44 without the genset running both engines burn almost the same.
If genset is running port side will take a bit more fuel.

 

[rclarke246]

I’ve kept the crossover valves open but the gauges never show equalization. I chalk it up to a malfunctioning gauge on my starboard tank.

I recently decided to close the crossover because I see no benefit in keeping it open.

 

[boater3]

Great info everyone. As for: "Typically, an equalizing line is connected to both tanks at the bottom, with both connections valved, and the line is generally larger than the other fuel lines.."

Yes, this is how it's setup. I think the line is 1/2".

Valves are open.

I'll check the vents for blockage.

Mystery - how are you checking actual levels if the gauges are not accurate?

Thx,
Mike

I don't know actual levels (I don't think anyone with a boat fuel sender does unless they top off) but what I do know is when I let the tanks equalize I saw a 5% discrepancy on average. If you are daring enough, you can fill up to the top (listen closely!) on each tank and get the gallon readings from the pump. That is if filling to the top does not some how automatically clog vent lines with diesel. What I did was I let the tanks equalize over night, I went to fuel up, and one gauge hit 100% quicker than the other. I took a guess that the one that hit quicker could take more fuel and I kept going. So far no issues but it is possible that the other tank that hit 100% later could also take more fuel. I was just not in the mood to really push the limits and clean up diesel on my deck.. I also noticed when I do this the discrepancy between the two as I run stays around the 5%. Before it was getting to be as much as 10% but I am guessing that is because I stopped filling up when the gauge that hit 100% first showed that 100%.

 

[Ole Hank]

Another reason to close is of you are in an area where fuel may be sketchy. Only add fuel to one tank.

 

[sunchaser]

Do the Bs in question have tank sight tubes? For non integral tanks this would seem a builders obligation. Then no more guess work once calibrated.

 

[AusCan]

Besides the safety aspect, another reason not to leave the valve(s) open on the crossover line is that it will make a list worse. Not your to-do list, but the boat leaning list.
If you start with a perfectly level boat and then add weight on one side, the boat will list slightly to that side. With the tanks crossover line open, diesel then starts flowing to the lower side, making the list worse, then flowing more, and listing more.
On a hard chined boat the problem is slight, but with a more rolly round bilge type boat, this issue is quite pronounced.

Use your crossover line to correct a list in a controlled manner, but don't leave it open and forget about it.

 

[Bay Pelican]

I usually have kept the crossover between the starboard and port tanks closed. However on our last trip there were three/four jumps of more than 20 hours. This would have resulted in 40+ gallons of fuel being drawn from one side. While not a significant weight for a Krogen 42, I preferred having the fuel drawn from both sides so I kept the crossover open.

 

[KSF]

On my 2006 ST42, we have a crossover line. When we bought the boat, the valves were open, but didn't seem to work.

We found a lot of black tar in the port Racor fuel filter so did a fuel polish. They used the pickup tubes as the intake for the fuel polishing system, and the lower crossover as the return to "stir up the bottom of the tank during polishing".

When they took the crossover hose off, they opened the valve to check and nothing came out of either. The hose too was clogged. A bit of compressed air into the fitting, and we got a flow of fuel. They ran the polisher 12 hours on each tank, then switched the intake and output (so they were drawing from the bottom) to get the rest of the crud as the pickup tube only goes down to about 2" from the bottom. Then ran the polisher another 12 hours. (They had two pumps and filter systems so we got it done in 24 hours.)

They had to change the filters three times during the first couple of hours in each direction. When they drew from the bottom of the tanks, the first filter clogged within about 15 minutes! All gunky and nasty.

An interesting note, the setup was obviously "self-engineered" (mounted to a plywood panel), there were two stages of filters, the first used a roll of paper towels as the filter media! The second was standard Racor filters. They started with 30 microns and each change went down. The "paper towel" filter was commercial and they said that it was common on over the road trucks. Who Knew!

One other note, I've been changing my Racors (30 microns) every 100 hours with my oil. I've been about 250 hours since the fuel polishing and was replacing a cracked injector line and decided to drop the secondary (on-engine) fuel filter and have a look. After only 250 hours, it was so clogged, that it had sucked in the sides and bent the metal caps! The other engines fuel filter was almost as bad. After replacement the boat responds much better and my idle RPM went from about 560 to 700.

I'll start changing those filters MUCH more frequently!