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Old 08-21-2019, 11:07 AM   #1
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Unbalanced Fuel Tanks

I titled this for lack of a better way to refer to the issue. This issue has been going on since I bough the boat 3 years ago.

I have 2 150 gallon aluminum fuel tanks and a single Yanmar 6 cyclinder.

The engine draws from both tanks. There are two fuel feeds and two returns and the valves are open. There is also a passive cross-over between the tanks.

Whenever I fill the tanks they appear to have equal quantity according to the gauges. Within about 10 minutes of running, the port tank starts to read lower than the Stbd tank.

This weekend I was running with less fuel than normal. I was down to about 1/3 in each tank. After running for about 2-3 hours, I checked fuel when I was noticing a list to Stbd. Port tank was almost empty and the stbd tank was just under half.

I raced around and shifted weight (anything I could find), including sliding the couch from the stbd bulkhead and the cooler, to as far port as I could. I eventually introduced a list to port.

Shortly after introducing the list to port, fuel began filling the port tank. I headed straight to the fuel dock.

While I see this issue, it is diminished the closer to full the tanks are. When the tanks were low, I inadvertently almost introduced a condition where all fo the fuel from the port could drain into the starboard and cause the fuel system to start sucking air and stalling the engine.

It almost seems like the fuel is being pulled from both tanks but favoring the starboard on the return. I suspect the passive cross-over attempts to equalize the condition, but does so at a bit of a deficit because it doesn't quite keep up with the flow of the return.

Has any see anything like this? What can or should be done about it?
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Old 08-21-2019, 11:11 AM   #2
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Maybe a plugged vent?
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Old 08-21-2019, 11:50 AM   #3
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The owner of my old boat just had a very similar issue. He's still on the med after a leg injury and I was in town, so I went over to give him a hand.

In his case the boat was a 1977 42' Californian with 4x 125 gallon tanks for a total capacity of 500 gal. There are 2 tanks to starboard and two to port. There are supply (low) cross-overs between the two port tanks, between the two starboard tanks, and from port to starboard. I had recently replaced all senders and gauges.

What he observed is that every other tank was emptying. And, every other tank was staying full ?!?!?!

So, the first thing we did was slide the beds forward to get access to the senders and pull them out. We then dipped the tanks with a wooden rod to find their true level. And, looked down into them with a flashlight, just to get another measure. We also exercised the float senders outside of the tanks. In so doing, we verified that the gauges were reading correctly and that the sender-gauge pairs worked correctly across the entire range. We also noted the senders were in good condition with no signs of rust. More importantly, we noted some sludge at the bottom of the tank as we poked at the bottom with the rods. I went down and checked his Racor's and noticed sludge in the bowls.

I then crawled down into the laz and checked that all valved were open -- and they were (as they should be). I then opened and closed each valve to check that each felt right. And, each did.

I then closed all of the supply valves (vs return valves), placed a couple of diapers underneath the supply valve on one of the tanks that was showing full, and removed the cross-over hose from it. I got a large bucket ready, placed a solo cup under the valve, and opened the valve. Nothing came out. I bent a wire zip tie to give the end more curl, slid it in, and around the 90 degree turn into the valve and some fluid came out. I poked more, and a bunch of black sludge and black flakes came out along with what looked like it might have been water. Then bright red diesel came rushing out clean. I emptied a couple of solo cups worth into the bucket as I went (closing the valve each time). Then, I opened the other return valve. Nothing came out. I disconnected the hose on that side, repeated the process on that side, and got the same result. I blew through the hose and it was clean. I reconnected it. I then went to the other two pairs of cross-overs and did the same for them. They turned up clean and flowed smoothly. I put everything back together and cleaned up.

The tanks then leveled for the first time since he'd owned the boat. And, maybe for the first time even since I owned the boat. My typical cruise was about 6-7 hours and I topped off each time. So, if leveling was slow, I'd never have noticed.

I checked the o-rings on the deck and they needed replaced. I checked his Racors and they needed cleaning. His mechanic took care of both the next day. I understand he is back to happily cruising the boat -- but is keeping a close eye on the gauges and the filters until he gets the chance to get the tanks cleaned. Having said that, he also isn't sweating it -- because the boat carries so much fuel, so it hadn't been a problem in hundreds of hours of use.

Our best guess is that some prior owner was probably only running with the two tanks, leading the sludge to grow into the valves on the two that were off. And, when they were turned on later, they were already clogged or close to it.

At any rate, I guess the bottom line is that one or more supply/cross-over valves (or hoses) might be clogged (or broken).
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Old 08-21-2019, 12:00 PM   #4
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I have the same issue.


No plugged lines (brand new and verified clear) no clogged vents (verified clear), no sludge ( new clean tanks).


Guessing just tiny bits of more or less resistance from tiny things like length of lines, filters and list, etc add up to cause it. Other than that, I have not found a reason.
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Old 08-21-2019, 12:21 PM   #5
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I'm considering installing a fuel manifold on the port (maintenance) side of the engine compartment. It would allow me to close the fill and return valves easily, allowing me to isolate the running tank. Currently, my valves are on the tank fittings, which makes getting to the Stbd tank very difficult. I'd have to move the couch and coffee table to pull the Stbd engine hatch and access those shutoffs.

I was considering alternating the tanks each day or run.
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Old 08-21-2019, 01:25 PM   #6
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Why not just isolate each tank with valves and separate filters then run off one tank at a time? Mine is set up that way. I also have a polishing system which allows me to transfer fuel from tank to tank. This way I can keep the boat relatively balanced.
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Old 08-21-2019, 01:39 PM   #7
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The problem is typical in multiple tank setups. Fuel flows to the path of least resistance. Also over time the tank levels are affected by the boats balance and fuel will flow to the down side.
One solution is a day tank the engines draw fuel from and that tank is filled as necessary from the main tanks. That's how I have done it in all my boats. Another is to slightly close the valve on the tank losing fuel. And adjust as necessary. Eventually you will find a close spot, but it will change depending on the fuel level. A globe valve works best for regulating flow. Or try permanently shifting some weight to the port side.
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Old 08-21-2019, 01:48 PM   #8
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It sounds like the passive crossover line only makes matters worse. As psneeld points out, the inherent differences in flow restrictions in the supply and return lines are bound to cause the tanks to empty at different rates. Once the variation in rates is sufficient to cause a list, the passive cross over line will allow the tank on the high side of the boat to flow into the lower side of the boat. An isolation valve in the crossover won't solve the issue but it would certainly lesson the severity. If the isolation valve doesn't help enough, a reversible fuel transfer pump in it's place with controls to the helm (and a timeout switch) would be handy. A fuel manifold would allow you to control things precisely but is one more thing to keep track of.
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Old 08-21-2019, 03:01 PM   #9
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It may be an issue with the crossover lines blocked, as gkesden suggested. Its worth checking.
Regardless, why leave them open and depend on passive equalising?.

Most of us have tankage that allows us to run on one tank for days or weeks. Just run on whichever tank is on the high side. Same with the water tanks. Once it is level, just alternate once a day.

Opening/closing valves is much easier than moving furniture.
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Old 08-21-2019, 03:13 PM   #10
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We had a similar issue, turns out it was a fuel filter. Replaced a month ago and tanks have been balanced since.
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Old 08-21-2019, 05:30 PM   #11
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Pass over/ cross over, I wouldn't want one. One tank gets contaminated and both tanks end up contaminated. I feel you're on the right track with the fuel manifolds. Keep them close so when switching feeds you can switch returns. My idea from reading about diesel fuel systems is each tank filtered 20m to 10m before the fuel manifolds and have a 5m on the engine. This is my plan. My boat will have three tanks though. Two mains and a day tank.
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Old 08-21-2019, 07:00 PM   #12
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I have a equalizing line and it keeps both tanks at the same level.
My ex an old Mainship 34 did not have one. I had to draw and return to one tank, then switch sides to keep the levels balanced.
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