Uneven fuel usage

Why would I be using twice the fuel from my port fuel tank than the starboard? 2005 Mainship 400, single Yanmar 370 150 gal fuel each side.
I'm going down the ICW and for the last three fills have taken double on the port side.

fuel returns set up correctly?

If you fill the port side first there could be a crossover which is filling the starboard side as you fill the port side.

bwalz said:

Why would I be using twice the fuel from my port fuel tank than the starboard? 2005 Mainship 400, single Yanmar 370 150 gal fuel each side.
I'm going down the ICW and for the last three fills have taken double on the port side.

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Genset draw off the Port side?
HOLLYWOOD

psneeld said:

fuel returns set up correctly?

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+1

psneeld said:

fuel returns set up correctly?

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+1. Been in the ER lately and accidentally hit a ball valve?

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Clogged anti siphoning valve on the full tank causing fuel to be sucked from the empty tank.It usually causes vacuum lock but if the lift pump is strong enough,it's possible.All tho they should eventually settle over time to the same level.

At least once I had the wrong valve turned. Luckily, I noticed the overflow from the vent before more than an ounce of fuel spilled.

Been filling starboard first.

Bay Pelican said:

If you fill the port side first there could be a crossover which is filling the starboard side as you fill the port side.

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Do you have a cross connect that stays open? Do you notice any list on your boat? If you have a list to your boat the transfer of fuel from one tank to the other will feed on itself. The low side will gain fuel, thus gaining weight increasing list angle and causing more fuel to transfer to the low side.
Eventually the boat just rolls over

I had the exact same problem and found my fuel tank selector valve failed. It is a ganged type valve with a single stem so when you turn the valve handle it turns two valves one for the feed and the other for the return. In my case the return portion of the valve failed and I was feeding from one tank but returning fuel to a different tank.

bwalz said:

Why would I be using twice the fuel from my port fuel tank than the starboard? 2005 Mainship 400, single Yanmar 370 150 gal fuel each side.
I'm going down the ICW and for the last three fills have taken double on the port side.

Click to expand...

Our Mainship 350 with the same tanks, different engine, does the same. Do you have dual fuel gauges to monitor each tank? I simply shut off the fuel feed from the port tank when they get about a quarter tank apart. The return line stays open and the tanks slowly equalize. My boat does have a starboard list which I assume causes the port tank to empty quicker.

Tricky to run a single engine on two tanks simultaneously. Unless the two tanks are joined by a large diameter low level cross tie, the tank levels will vary as the return flow may "prefer" one tank over the other.

I have the same arrangement on my boat. I've gotten into the pattern of running on one tank for a day, switching to the other tank for next day.

I have tried running with both tanks valved in at the same time. Sometimes it works fine. Another couple times engine stalled, and I found one tank empty and all the fuel shifted over to other tank.

I would try lining up one tank to supply and return for one day, then switching to the other tank for the next day or equivalent number of hours. Then try a fillup and compare.
Another thought: Are you measuring tank fill by sightglass or something else? Are both tanks actually being completely refilled?

If your boat is the same set up as mine (I have a 2005 400 Mainship though it has twin yanmar 240) there is a simple ball valve fuel line that connects the two tanks. Each of my motors pulls from its own tank, so I keep it closed. Perhaps it is open, allowing the fuel to flow to the lower side?

Also, and this seems most likely to me, the Gen-Set pulls from the port side. The fuel gauge sender is only on the Stb tank, which seems really dumb to me, so I am really careful to fill up often.

I have four tanks with a single engine. Unless you have exactly the same length suction and return lines you will never even get close to an even use. My manifold is on the port wall so favors the port fwd tank (shortest route). I therefore use that as a day tank. It is the only one with fuel gauge and inspection/cleaning port. This makes it easy to monitor and although I have a transfer pump/separate filter, I fuel under way by just changing the suction to whatever tank I want and leave the return to the "day" tank. It doesn't take the big DD to transfer the fuel and the fuel gets filtered before arriving into the "day" tank.

dimer2 said:

I have four tanks with a single engine. Unless you have exactly the same length suction and return lines you will never even get close to an even use. ...

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Ditto. We only take fuel from one tank at a time. When walking the dock, I check the Coot's lean (if any) to determine whether to switch tanks (as well as checking the levels of the four fuel tanks as needed.)





Having a genset only able to take fuel from one tank in a multi-tank boat is an issue since it effectively reduces the potential range if multiple engines take fuel from different tanks, unless there is the possibility of transferring fuel among tanks. (No genset, no problem.)