Old diesel tank - clean or replace? My decision and experience.

[HoneyRyder]

Hello all,

Just thought I'd relay my experience with two old steel diesel tanks when I bit the bullet and decided to tackle the nightmare of what to do -clean or replace?

I'd had problems with fuel starvation when I first bought the boat, it was old and hadn't been used much, so when I went to sea in her I was constantly replacing filters. After a while it seemed to be OK, and I had the fuel "polished" by a professional and was sleeping well at night, thinking it was all good.

I had some time during the summer and took a look at them, two 40 year old 700 litre diesel tanks. I've already substantially renovated everything apart from the engine room and so I wondered about the pros and cons of cleaning versus replacing the old steel tanks with plastic or fibreglass or even stainless.

I've experienced clogged filters while in a Force 7 out at sea, not a nice experience, and deep down I knew the right thing to do was to replace the tanks. But how to do it safely?

Here's what I did:

1. I pumped all the diesel out into some large plastic barrels.
2. Then I part filled the empty tanks with Fairy washing up liquid and water.
3. I emptied a 6kg powder fire extinguisher into the tank from the fuel inlet.
4. I drilled four holes into the side and then used a jigsaw to connect the holes and cut out a 6" x 9" inspection hatch.
5. I then pumped out all remaining liquid residue.
6. Then I used the jigsaw to cut the tank to pieces small enough to fit through my hatch.

I am now replacing the 2 x 700 litre steel tanks with 2 x 150 litre plastic tanks.

The new tanks will act as "day tanks" which will be backed up by auxiliary plastic storage tanks, the idea being that smaller tanks are easier to clean and replace, and I only need to buy and store a large amount of diesel when needed to travel, rather than to stop corrosion of empty steel tanks.

Some comments which others might find useful:

1. The bottom of each tank had 5cms of "residue", almost like black sand - and there is no way that this residue could have been cleaned effectively without opening the tank.
2. The steel at the bottom of the tank was showing signs of corrosion.
3. The steel above the very bottom was immaculate.

Conclusion - Although it was a big job I am very glad I bit the bullet and removed the old tanks. It was surely only a matter of time before the tanks would corrode through at the bottom and start leaking fuel. Removing the tanks gave me a chance to inspect the hull behind the tanks and to paint the hull with epoxy when I painted the entire engine room. With the old tanks gone I was free to design and install a better fuel storage/supply system with flexibility and modern materials. I chose linear polyethylene because it does not corrode. And I was able to resite filters and to replace old copper tubing with flexible rubber hosing. The other big advantage now that the job is done is knowing that I have diesel tanks that are supplying perfectly clean fuel to my filters and engine, so I shouldn't be suffering clogged fuel lines or filters next time I'm at sea in a big blow, and that's worth every minute of time and effort expended on this project.

If anyone else is dreading undertaking a project of this kind then please be assured that it can be done safely, and that it's a very worthwhile exercise if you want to keep your boat for a while and have confidence in it.

If you have any questions about my process then feel free to ask. You want to be very careful opening up a tank like this because diesel vapours can be explosive and the residue at the bottom is waxy and highly flammable too. But with the right procedure and precautions it can be done.

[Cheechako]

Good job. Sometimes we carry far too much diesel. I doubled the volume on one boat at great cost and work. Then never used the two extra tanks after the first time I emptied them! Too much volume = old diesel.
Of course if you are going to do the NW passage or Antarctica then that's different.