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Old 05-31-2023, 09:28 PM   #1
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Mainship Pilot 34 Lateral Fuel Imbalance

Our 2005 Mainship Rumrunner Pilot 34 has a strange quirk that I can only attribute to uneven fuel consumption from the two tanks. When the tanks are full, the boat sits largely on its lines with no list in either direction. As fuel is consumed, the boat lists increasingly to starboard to a very noticeable degree.

In theory, the fuel should drain evenly from both tanks because there is a crossover line between them, correct? So then why would fuel empty from the port tank first. This is corroborated by the fact that I have to put far more fuel in the port tank when filling than the starboard. Any suggestions on how to keep her on her lines as fuel is consumed? Has anyone else experienced this?

Thank you all in advance,
Doug
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Old 05-31-2023, 09:37 PM   #2
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Where does the return fuel go?
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Old 05-31-2023, 09:38 PM   #3
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Uneven venting will cause a fuel imbalance. One vent could be partially plugged.
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Old 05-31-2023, 09:43 PM   #4
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Yes, a blocked vent is my first thought. Going to investigate that in the daylight tomorrow.

Doug
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Old 05-31-2023, 10:57 PM   #5
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What engine is in the boat?
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Old 06-01-2023, 06:41 AM   #6
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our 34 pilot only returns fuel to the starboard tank.
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Old 06-01-2023, 07:08 AM   #7
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More details

Dave,

Our Pilot has a single 370hp Yanmar. I believe that the return line goes to the starboard tank.
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Old 06-01-2023, 09:58 AM   #8
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I had the same experience last year. Had fuel polished and replaced filters, problem solved. There is likely junk partially blocking fuel line in starboard tank.

Good luck
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Old 06-01-2023, 12:31 PM   #9
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Quote:
Originally Posted by Duetto View Post
our 34 pilot only returns fuel to the starboard tank.
That could account for the starboard list. If the crossover is plugged or obstructed then the tanks wouldnít be able to equalize.
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Old 06-01-2023, 12:45 PM   #10
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My 2004 Pilot has a valve that sits on the dividing wall near the AC strainer/pump that lets me select my port or starboard tank as primary fuel return. It was turned towards starboard return when I just crawled in my bilge to look.
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Old 06-01-2023, 12:59 PM   #11
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Depending on the engine if you draw fuel from one tank and return fuel to a different tank you might overflow the tank with the fuel being returned to. I much prefer a system that returns fuel to the tank supplying fuel. Some engines return huge amounts of fuel. Some hardly any return fuel.
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Old 06-02-2023, 10:31 AM   #12
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While my 30 Pilot II was in the water awaiting repair of my boatlift, I noticed that it would take a list, usually to port. Since I had installed a sight gauge in the crossover hose between them, shutting off the tank valves one at a time showed me the exact level in each tank, and I found that a very small amount of fuel imbalance when tanks were partially full resulted in a couple of degrees list. I had previously changed the crossover hose to ensure there was no clogging there and tested the tank vents with a vacuum cleaner set to blow air into the hull side vents blowing a piece of paper off the open fills. The one thing I could not do was select which tank fuel returned to (12 GPH at 3000 RPM cruise on my single 315 HP Yanmar) per my Floscan. That modification was quick and relatively easy, and now I normally return to both tanks, but can shut one side off if things begin to go akilter as noted at the helm fuel tank gauge (another mod to allow me to read either tank there). Once the lift was repaired, I force out side-to-side fuel imbalance by ensuring the boat is on an even keel in the lift, but the selectable return is available to adjust the list when away from home.
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Old 06-02-2023, 11:16 AM   #13
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Problem Found

After contorting my rather large frame into the forward starboard corner of the engine compartment, I found the starboard vent line. It seems that the diagram from the boat manual indicates that the port and starboard tanks each vent separately to their own sides. However, at some point my starboard vent was routed to the port side. This likely happened when the holding tank and associated plumbing was changed from Vaccuflush to a standard electric head. The holding tank now vents to starboard instead of port as manufactured.

So, examining the starboard vent hose there is a definite kink in the hose likely blocking airflow and effective venting. I cannot undo the link underway right now as it will require either an elbow or a longer vent hose. This does explain the increasing list to starboard and the uneven fuel consumption between tanks despite being plumbed together. The short term option is to vent the starboard tank at the filler periodically which does level the boat after a fashion. I have done this twice at night when not underway.

The boat runs fine but I would not want to let the fuel level get too low so as not to suck air into the system from the port tank. For the duration of this trip (Erie Canal to Maine coast) I will vent from time to time and keep the tanks pretty full.

Thanks for all the input!

Doug
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Old 06-02-2023, 01:37 PM   #14
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Glad you found the kink. Once you get the kink cleared you shouldn’t have any more problems. It may take an hour or so for the tanks to balance but with the vent working they should stay balanced. The equalization line does this.

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