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Old 04-22-2018, 09:02 PM   #1
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Fuel line between tanks

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
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Old 04-22-2018, 09:15 PM   #2
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My understanding is that it takes a pretty large hose to actually equalize two tanks.
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Old 04-22-2018, 09:16 PM   #3
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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...
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Old 04-22-2018, 09:21 PM   #4
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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.
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Old 04-22-2018, 09:32 PM   #5
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Same here.
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Old 04-22-2018, 10:05 PM   #6
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Quote:
Originally Posted by MichaelB1969 View Post
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.
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Old 04-22-2018, 10:10 PM   #7
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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!)
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Old 04-22-2018, 11:51 PM   #8
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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
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Old 04-22-2018, 11:58 PM   #9
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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?
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Old 04-23-2018, 12:00 AM   #10
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Why not run with it open?
Mine is always open..........
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Old 04-23-2018, 04:15 AM   #11
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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.
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Old 04-23-2018, 07:19 AM   #12
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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.
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Old 04-23-2018, 08:16 AM   #13
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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.
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Old 04-23-2018, 08:28 AM   #14
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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.......
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Old 04-23-2018, 08:34 AM   #15
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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?
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Old 04-23-2018, 08:45 AM   #16
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Sorry but what is return fuel?
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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).

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Old 04-23-2018, 08:54 AM   #17
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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!
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Old 04-23-2018, 08:56 AM   #18
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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.
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Old 04-23-2018, 03:51 PM   #19
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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
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Old 04-23-2018, 04:26 PM   #20
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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.
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