Thread: Emptying an underfloor tank

Gday team. So the past couple day I took the dog and we spent a night down at the boat just pottering around and doing a few jobs on the boat while we were there. The boat has two stainless underfloor tanks. The rear one is shorter than the front one, but deeper and holds about 70 litres. The one in front of it is not as deep due to design of the boat but is really long and I estimate it will hold around 130 litres. They are completely independent of each other with a couple of taps in the transom to switch from one to the other. The old owner had always only used the rear one, and untill now we had also only used the rear tank. I wanted to run the boat on the front tank as it will be handy for longer trips having them both full so I three 75 litres of fuel into it (bloody hell fuel is expensive now!) And switched the tanks and primed it up. Got halfway out of the marina and alarm started going off for fuel/water separator on the engine. I switched back to the back tank and limped back quietly to our berth. Took the fuel filter off the engine and tipped it out and it had a lot of water in it, I have had the floor up before and from what I could tell the tank seemed and sounded completely empty. I took the fuel delivery off the outboard and switch back to the front tank and primed some fuel through again into a bucket and there's a lot of water in it! I'm not even mad that there's water in there, I should have checked better... I am mad that it's now mixed with $200 worth of fuel!
I've resigned myself to the fact that the tank is going to have to be flushed out. It will be very tedious process trying to pump the entire tank empty using the primer bulb. Any one got any flash ideas to make the job quicker/easier? I thought I might flush it all out then put 20l or so of diesel in it and try rock the boat round a bit and flush that through also before trying to flush any fuel through, I could then put say 20 litres of fresh premix petrol in it and run it through my wee aux motor, 20hp two stroke Merc which will probably eat up any impurities better than the injected motor will.