Port and Starboard Fuel Tanks not Equalizing

[Gypsy Diver]

My 36 foot Island Gypsy has a 225 gallon fuel tank on each side. There is a hose coming out of the bottom of each tank, which should connect them as one tank if the valve is open at each end. The valve on the starboard side also runs a line through the filters to the main engine.

The tanks always used to equalize, with the same amount of fuel in each tank. But lately the port tank is not draining into the starboard tank. My port tank is completely full even if the starboard tank is half empty. I disconnected the connecting hose at the starboard side and there was definitely a lot of diesel coming out of the port tank through the connecting hose, and there was diesel coming out of the starboard tank where I disconnected the hose.

What else could be stopping the flow into the starboard tank from the port tank if the connecting hose is not blocked?

 

[Clark]

Have you added any weight to your boat recently? Check to make sure with equal fuel in both sides and the crossover valves off that the boat is statically balanced. If there is more weight on one side, when the crossover valves are opened, the fuel will run the the heavy side making the boat even more unbalanced. If this is what you have, shift weight to correct. Water tanks can do the same thing.

 

[Gypsy Diver]

Thank you! That's a real possibility because at one point I let the port tank get full while I still had a lot of room in the starboard tank. I thought the full tank would drain into the partly empty tank, but you are saying that it's actually the opposite result.

 

[Comodave]

Are the vents plugged?

 

[Gypsy Diver]

I will check that. But would that matter if the deck filler caps are off and the tanks still aren't equalizing?

 

[Comodave]

No, if the fill caps are open the venting should not matter.

 

[Clark]

No, it should drain in that case you mention, one full and one empty (unless your boat is seriously on it's side). If you fill both tanks or empty both tanks or make them have equal weights of fuel with the crossovers closed, and there is still a list to one side then the boat is not balanced. In that case, if the crossover is opened, the list will become worse.

 

[BYNW]

Check your return manifold too.

 

[KnotYet]

If your fuel pick-up enters the tank from the top, the fuel may not automatically flow by itself.
Any air in the fuel line can interfere with the siphon effect. Filling both tanks could help.
A fuel pump can overcome this if you want to be able to transfer the fuel at will.

 

[gerilla]

Does your boat have a generator? Which tank does it pull fuel from? If tank connecting hose is closed ,a lot of generator usage can cause an imbalance. Also, if you have a fuel fill for each tank, fill up separately with tank connector closed to see if boat is level. If it lists to one side try moving stuff around to balance . If you can't level with tanks full, don't expect it to level with tanks connected.