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Old 02-01-2017, 10:05 PM   #1
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Fuel tank selection

I know this has been discussed before, but I can't seem to find the thread. Mainship 34 has 2 110 fuel tanks one port one starboard. Alternating between them requires lifting the floor panels to change tanks and return line. Not the most fun in using the boat. Has anyone opened both fuel tank valves and run that way, and what was the result. I still would have to shift the return line, which my boat has that capability, but wouldn't have to be done quite as often. If both valves are open does it draw from each equally or does it draw from the tank with the highest fuel level? In other words is this a viable alternative?
Thanks in advance.
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Old 02-02-2017, 07:07 AM   #2
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It would be easy to say yes, but reality shows that some boats draw too unevenly to use regularly down to low fuel.

The alternatives are to move the valve or to replace the valve with a remotely operated one.

There are valves that switch both feed and return in one valve..
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Old 02-02-2017, 08:48 AM   #3
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My boat has one fuel line from each tank and one return line to each tank (single engine diesel). Normal procedure is to run with all four valves open. I've never had a problem running this way. If you fill both tanks equally to start with, they should stay balanced.


If you can re plumb your return line to feed both tanks, you should be able to operate the same trouble-free way I do. I haven't touched my fuel tank or return valves since I've owned the boat.
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Old 02-02-2017, 09:08 AM   #4
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There have been plenty of reports on this forum, my boat included that running with both open can lead to pretty noticeable fuel imbalance.

You can try it with full tanks and see how it goes. Perfectly even...then try it all the time....but....

The trouble is if one goes lower than the other and your engine sucks air.....that's a different issue than just flipping a valve....especially in dangerous conditions or strong currents in confined quarters.
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Old 02-02-2017, 09:18 AM   #5
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There have been plenty of reports on this forum, my boat included that running with both open can lead to pretty noticeable fuel imbalance.

You can try it with full tanks and see how it goes. Perfectly even...then try it all the time....but....

The trouble is if one goes lower than the other and your engine sucks air.....that's a different issue than just flipping a valve....especially in dangerous conditions or strong currents in confined quarters.
If you consistently have that problem, you can partially shut the valve from the tank that seems to deliver the most fuel.

As for sucking air, yes, that could be a problem. My solution is to just not let the tanks get that low in the first place.
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Old 02-02-2017, 09:31 AM   #6
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It is sometimes impossible to get equal draw unless you are willing to redo tanks and a lot of plumbing....fixing or changing valves in my mind was/is easier....by the time you figure out where to set that valve partially open....you can replumb it to a more accessible area and have that benefit.

If you tanks draw down equally, then no problem filling up before you use up most of your available fuel....but again not always possible.

There are reasons to run off one tanks at a time....so making tank switching should be made easier...not just accepting running off 2 all the time is a good idea.
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Old 02-09-2017, 08:57 PM   #7
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my 390 has a crossover line between the two tanks. i run with all valves open including the crossover. have never had a problem.
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Old 02-09-2017, 09:12 PM   #8
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What WesK said! I was able to fool with the return valves to even out use.

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Old 02-10-2017, 09:10 AM   #9
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What WesK said! I was able to fool with the return valves to even out use.

Rob
My flybridge has three seats. The center one for the operator and one on each side. If I'm cruising all day and one tank seems to getting lower than the other, I ask my wife to sit on the other side for a while. It's easier than messing with valves.

Docked or anchored, the tanks equalize themselves overnight.
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Old 02-11-2017, 11:34 AM   #10
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The only valve I close is the crossover when fueling - to make sure I get both tanks full - after which I open it. I've never found the list to be pronounced enough to bother with the valves or plumbing.

From a physics POV, it would seem to me that if the feed and return lines were the same (identical restriction, length, diameter, etc.) then the higher tank would have more head and tend to draw more and return less.

I'm pretty sure what list I have isn't fuel induced; it comes from having all of the cabinets on the port side of my 2002 model. I just put all my tools, dive weights, spare anchor, ect. on the stb side.

I only have a single return to the stb tank, which I learned the last time I played with feed tank selection. After a fill up, I closed the x-over and stb feed to try running just on the port tank. I quickly learned that my return only feeds the starboard when fuel started spewing out the stb vent - which happened just as I was passing the Coast Guard station. Fortunately I noticed and corrected (opened all the valves) before I spilled much fuel).
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Old 02-13-2017, 10:21 AM   #11
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I have the same two feeds and one return.
The balance line finally evens everything out after everyone has left for the day.
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Old 02-13-2017, 10:46 AM   #12
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If cross tie line taps into bottom of tanks, you can be pretty sloppy with selection valves. Crosstie will keep things fairly even side to side. Provided valve is open!! Problem is when only taps are on top like on my ride. Then it is sketchy.
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Old 02-13-2017, 10:52 AM   #13
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It didn't work for me but it would be pretty easy to try it and see what happens.
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Old 02-13-2017, 10:57 AM   #14
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My boat has no dedicated crossover line but since the two fuel lines join before the filter, they are connected to each other and they self equalize over time. The same goes for my water tanks.


I can see where drawing fuel from both tanks but returning the excess to just one tank could cause overflowing and uneven use. I'm surprised a boat would be built like that and if mine was, I would be looking to add whatever is necessary to return it to both tanks equally.
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Old 02-16-2017, 09:09 AM   #15
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I've ran my 82 Mk1 with both inlet valves open, and yes it draws more from one side than the other. Mine has no valves on the returns. Always thought that running when heeled over that would also certainly affect it as well. I just don't let fuel levels get real low in either tank.
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