Testing integrity of mild steel fuel tank

[lw395]

Indeed it is impossible to tell very much from those pictures.
A pressure test will only show today's leak, passing today does not mean the rust won't go all the way through next week.
The rust often happens from the outside inwards, where the tank sits on a wet bearer.
If you don't want to remove the tank this winter, maybe you could fit another tank in a locker perhaps?

 

[JumbleDuck]

Hi folks here are a couple of photos of the inside of my tank where the surface is rough. And one of the inspection hatch.
It's impossible to tell how bad it is from the photos isn't it?

Sorry, but based on picture 2 alone I would without hesitation scrap and replace that tank. It's knackered.

 

[DownWest]

Second JB's comment. Quite likely there are deep pits leading to pin holes in the near future.
Recently pulled a tank. Wasn't leaking until I cleaned all the crud out, then the mulitiple pin holes were obvious.

 

[Gentleman of Leisure]

Rather than spending your hard earned on test equipment or dubious internal coatings, I'd bite the bullet and get a new plastic tank. Much better value and will serve you well for many years. Having said that, I had an old boat with original VP engine and mild steel tank. It never occurred to me that it might be on the verge of collapse although it was over 40 years old so you could just ignore it and hope for the best.

 

[Crinan12]

I think you are all right- who are saying replace the tank. I just wanted to be told it was fine because I can't be bothered doing it.
Vyv,not sure on thickness,will measure when next at boat.
But unless it's pretty thick I think it's a new one.
Cheers all

 

[eddystone]

An Endoscope like this will help to inspect inside your tank

2M 6LED USB Waterproof Endoscope Borescope Snake Inspection Camera Scope 7MM 799975357706 | eBay

Would you use one of these in a full diesel tank? One I looked at on eBay was IP68

 

[rogerthebodger]

Would you use one of these in a full diesel tank? One I looked at on eBay was IP68

I don't think you would see much in a full diesel tank. I have used mine under water to inspect my prop.

 

[Iliade]

You could use a hired ultrasonic thickness tester from the inside of the tank, thus mostly avoiding paint etc. Unfortunately I think that it would be wasted effort because the pitting need only hide one leak. Rather than bothering to use any fancy test kit, just grub around underneath the tank and look for leaked diesel. If there isn't any, it hasn't got a hole, yet.

If there is a hole it will already have been found by the fuel and adding pressure at safe levels will not expose a pit with a corrosion scab on top anyway.

To 'properly' continue to use this tank I would advise steaming, blasting, steaming and probably then welding. All of which is rather inappropriate for your situation. It does look like an ideal candidate for resin-based treatment as the corrosion looks to be only at the bottom of the tank. This is, however, only a stop gap solution and you'd have to get it clean of diesel first.

Don't use petrol to clean the tank.1, 1, 1 - Trichloroethane is probably the safest solvent for the job as it will not ignite, but must be treated with great care in other regards, especially if anyone is planning to weld in there. It may no longer be legal too, I don't know. (Wikipedia says maybe use Trichloroethylene as a substitute)

If the tank is in a really awkward position and you don't want to waste the season you could use a plastic tank(s) mounted 'somewhere else' and just pretend that the old tank void is inaccessible. Just remember not to pour diesel into the old tank filler ;0)

A portable plastic tank may even be a blessing in disguise if getting fuel to the boat is as difficult where your boat is as it is where mine's moored!

If you can cut a big enough hole in the side, you may be able to use the old tank as a container for a new flexible plastic tank with minimal effort.

 

[eddystone]

I don't think you would see much in a full diesel tank. I have used mine under water to inspect my prop.

On reflection that’s what I expected. A useful thing to have nonetheless

 

[JumbleDuck]

Don't use petrol to clean the tank.1, 1, 1 - Trichloroethane is probably the safest solvent for the job as it will not ignite, but must be treated with great care in other regards, especially if anyone is planning to weld in there. It may no longer be legal too, I don't know. (Wikipedia says maybe use Trichloroethylene as a substitute)

I used to use a lot of Genklene, but it's not longer generally available, probably because it is toxic and an intoxicant and attacks the ozone layer. Although it is hard to burn, it decomposes thermally at fairly low temperatures (200C) emitting hydrogen chloride, chlorine and phosgene as it does so. Never, ever throw genklene-soaked rags on a fire, Guess how I discovered this.

 

[awol]

Engine removal does not have to be A-frames and lift out. Just disconnecting and sliding forward on suitable bearers may give you the access you require.

 

[Assassin]

Crinan, epoxy is fine for such repairs but the trick is getting the correct type of expoy and this is crucial.

From what you have said so far I would do more testing to establish the true deterioration before committing to anything as a tank in poor condition is not worth doing, if the tank is generally in good condition then it may well be.

 

[oldgit]

Was a post on Forum about this dilemma and a successful to date repair.
Poster ensured complete access to top of mild steel tank.
Not sure if existing removable plate on top of tank was enough to carry out job.

A baffle also had to be removed . Source of immediate leaks located on bottom of tank and likely future candidates around welds pinpointed.
Lower part of tank scoured back to clean metal within an inch of its life and the entire tank chemically cleaned and cleaned again with whatever was available and might do the job.
Challenge was to clean the surfaces to allow sealer to stay stuck.
Tank sealed with a sandwich of CT1 and metal plates.
CT1 will NOT adhere to anything unless surfaces perfectly clean, despite claims on the label.

Unknown just how long this might last but it got the boat quickly back into use for a few pounds until the job has to be done or funds/ time enable the engine to be moved and tank replaced at leisure.

Can still remember the aggro replacing a water tank on a Princess 33 over 15 years ago , it was grinding the old tank into small sections in order to get it out that still gives the nightmares.

 

[Crinan12]

I think i would be better putting the effort into just replacing the tank. The thing that's putting me off doing it is having to remove the engine. I'm nervous about it as it's a big job for a novice. But the previous owner said it can be done fairly easily and will slide into the cabin with a bit of effort. I could try epoxy or other repairs but there will be a bit of work in that and ultimately it might not work. I can buy a plastic tank for a £100 so I think that's best option. Thanks