Modifying a fuel system for cross feed between tanks

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My boat has two diesel fuel tanks. Fuel for the engine is selected by appropriate feed valves with the unused return fuel also piped to the tanks via valves. Normally, fuel is returned to the same tank that is feeding the engine.

On many occasions when I have been very short of fuel, I have wanted to completely drain one tank. It is not a good idea to run the tank dry by feeding the engine as the last few gallons are usually dirty with crud from the bottom of the tank, plus, air can be sucked into the fuel system as the boat rolls.

In the original fuel system a small electric pump is fitted to enable fuel to fill filters after they have been changed or to pressurize the system to bleed air. During normal engine operation this pump is off and fuel is drawn to the engine by the engine-mounted lift pump. I decided to use the electric pump to provide cross feed between tanks, with the engine shut down, of course. All that is required is a selector ‘Y’ valve and a check valve to prevent the possibility of any dirty fuel entering the engine side of the return line.

During normal engine operation the system works as before. To cross feed, the engine is stopped and the appropriate donor tank selected. The tank to be filled is selected and the selector turned to the ‘Tank’ position. The electric pump is run until the tank is dry. The in-line filter before the pump prevents the larger pieces of dirt from clogging the pump. On my boat, this is a wire mesh filter that has a glass bowl and is easily cleaned.

As a precaution, you can clean any residual dirt from the lines before running the engine by selecting the more full tank and running the pump for a minute before putting the selector valve in the ‘Engine’ position. Turn off the electric pump and you are ready to start the engine. The check valve is assembled on the tee connected to the selector valve. The parts are available from the McMaster-Carr Supply Company.       

About the author:
Eric Forsyth has completed several circumnavigations aboard his yacht Fiona.

By Ocean Navigator