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.

Diesel won't work. Used methylated spirits.

The water won't be mixed with the petrol,it will sit on the bottom,the tank should fall to the outlet.Keep pumping,should not be to much.
,

Originally Posted by muzza

big arsed electric fuel pump - should suck though easily

What he said.
And avoid sparks.

Originally Posted by muzza

big arsed electric fuel pump - should suck though easily

Yea I was just thinking I have a couple of Cdax 12v sprayer pumps I could probably rig one of them up to the end of the fuel line and suck it through

Sounds like you have a good plan. Don't get too carried away though. Once you have emptied the water as much as possible and cleaned up with the meths just keep an eye on your water separator/filter. You'll always have some water in those tanks. Best way to stop the condensation is keep them full.

put an airline into the fill hole, with a rag tightly bunged in (and around the airline) to seal it off.
Turn the air on and it will blow it all through.
If you want to syphon it out through the fill hole, then put a hose down to the bottom of the tank as well as the air line.
Works well for syphoning fuel instead of sucking a mouthful, Just blowing into the tank with another hose is enough to start a syphon.

To be fair you could probably just drain the water and carry on without dropping all the fuel and flushing the tank. Keep a spare filter handy and keep emptying the water bowl.

Originally Posted by Hahn

To be fair you could probably just drain the water and carry on without dropping all the fuel and flushing the tank. Keep a spare filter handy and keep emptying the water bowl.

I reckon.
So long as it is only water and no sediment etc.

the feul can be used on low importance things so it dosent go to waste once it has settled and you syphon out all the water/crud

think lawnmower for petrol (quads handle it quite well too)

and shittly ol tractor for diesel

not airplanes or boats as its a bit hard to walk home

Id go back to just using the clean tank and above floor (20l containers) storage

C Dax pumps work great, really easy to set up safely. And then to be sure, put in a litre of meths with the first full tankful.

Originally Posted by Husky1600

C Dax pumps work great, really easy to set up safely. And then to be sure, put in a litre of meths with the first full tankful.

why the meths mate?

Originally Posted by Bill999

the feul can be used on low importance things so it dosent go to waste once it has settled and you syphon out all the water/crud

think lawnmower for petrol (quads handle it quite well too)

and shittly ol tractor for diesel

not airplanes or boats as its a bit hard to walk home

Id go back to just using the clean tank and above floor (20l containers) storage

Be cool to get that extra tank going though. Pain in the arse having the floor cluttered up.

Originally Posted by Bill999

why the meths mate?

Meths is hydroscopic and attracts water. It also burns cleanly so won't damage the engine. if the water is in a mist it goes through the engine ok. mates have had water/meths spray set ups for boosted cars

You'd be surprised how much water is in aircraft fuel tanks, especially in humid climates. A trick I was taught learning about water testing was when you have your bucket of fuel and it looks clean spit in it. If it sinks you have clean fuel. If it disappears you have a bucket of water. You could still have entrained water, use these if you're worried. shell-water-detector-kit/ draw the fuel through with a syringe.