Old Fuel Drainage
A.I., How about AI?(AIR INJECTION), I have a ball valve off my air compressor that I hook up to a male or female coupling to fill or drain liquid and air, depending on what is needed. On 55 gallon drums, I took the large bung,drilled and tapped it for a air fitting and it only takes a little air tank I took off of a dump truck to empty the drum, just be careful to not over fill at first with air, you'll know cause it will push the drum lid up under pressure. It only takes a couple of pounds of air pressure to push liquid,so when beginning, make sure you are close to the tank with the ball valve cut-off so the pressure can be watched. Another way would be to disconnect all lines, (which you have to do anyway to prevent backflow, and hook an air linesto a fuel line and blow it out through a hose going to the fuel tank. Does your boat have electric fuel pumps, (like a bilge pump) on the tanks? All you would have to do is disconnect the line at the engine, and put the hose in a drum and turn the pumps on to empty the tanks. FYI, we're having the same problem with bio-diesel, fill the big rigs, and within 20 miles, it knocks all the sludge loose and plugs the fuel filters and shuts the trucks down, a real pain in the arse.