drain/flush fuel tank

[snewman]

My boat, a 27' A family cruiser has 2 fuel tanks, 50 gal each. The port tank has been giving me a lot of problems with water and dirt(black stuff).
I decided the best thing to do was flush it out. There are 2 lines to the tank. I believe one returns unused fuel to the tank(but not sure) and I can't tell which one is the line to the fuel filter cuz I can't see back there but I am guessing that one must come from the bottem of the tank.
In the engine compartment pulled both lines free but I can't get fuel to drain out of the tank.
Is the fuel coming out of the tank gravity feed or is there a fuel pump? Is there a way to tell which line is supplies the fuel to the engine and is the outlet for that line on the bottem of the tank? In the alternative, any advice on the best way to drain or flush the tank to get the water and crud out?
Thanks,
Scott

[jleonard]

There shouold be a pump on the enigne to pull fuel out of the tank. It won't rely on gravity. If the fuel does in fact feed from the bottom of the tank and it won't drain then it must be clogged. Try to blow into the tank from that line to clear it but remove the fuel fill cap first.
If that doesn't work then you can siphon out the fuel using an outboard engine squeezball rig.
I have removed water from tanks in the past using a squeezball setup. Water goes to the bottom so I was able to list the boat to make the water go into a corner. Then I went in thru the fuel pickup fitting and get the hose to the corner.
I pumped fuel until I had all diesel. I pumped out maybe 2 quarts to get one quart of water out. Problem solved.
The crud is another story. Good luck on that one without removing the tank.

[Tuxedo]

I've never seen a tank feed from the bottom. That would just be asking for trouble. I don't recall the 27 setup, but I'd bet money the feed from the top with a fuel pickup line in the tank that goes down to within a couple of inches of the bottom. I doesn't go all the way to the bottom to prevent it from picking up crud. Sounds like you have an excess of crud that is now up to the level of the pickup tube.

The easiest thing to do is have a fuel polishing company come out and clean the contents of the tank. They stick a pickup and return line in the tank and circulate the fuel through some giant filters, cleaning out all the crud and water in the tank.

[snewman]

My thanks to Tuxedo and Jleonard. By combining their advice I was able to figure out a solution.
I jacked my boat up so it listed to port(it's on a trailer and in a shed this time of year) and was able to syphon an additional 3 gal out of a tank I thot was empty. The first of those three gal was milky colored which I suspect was a water and fuel mixture.
I then put a few gal back into the tank, covered the vents with electricians tape and blew compressed air(about 20lbs of pressure) into the return fuel line and simply forced fuel out of the tank and into a container thru the fuel line. At first I got lots of black crud and then clean fuel so I think the problem is fixed.
Once again I recieved help from A owners. It is so nice to have a source like this.
Scott