Testing integrity of mild steel fuel tank

[Crinan12]

Folks
Help please. I posted a while back after my engine broke down due to debris in the fuel tank and/or diesel bug. When I drained/cleaned and re filled the tank and changed the filters everything was fine. When cleaning the tank I noticed that there were flakes of metal at the bottom of the tank- which I presume was bits of the fuel tank rusting from the inside due to water lying on the bottom of the tank. At the time I thought I had better replace the tank and fit a plastic one.

I'm trying to find a way out of replacing the tank if i can, especially as it involves removing the engine. And also I'm a novice so it would be a big job for me.

I drained all the fuel again today and took a couple of photos and had a feel about of the inside of the tank and it seems okay - but I don't know if the bottom of the tank could fall apart at any second or if it's solid as a rock.

My question is this. Is it possible to find out just how dodgy my tank is with it in situ ?
Thanks
P.s tank is original- 1980

 

[pvb]

My question is this. Is it possible to find out just how dodgy my tank is with it in situ ?

Yes, but it's basically destructive testing, so you'd have to replace it anyway.

 

[Crinan12]

Haha!
Next question then
Is there any (technical) reason i should not do this job when my boat is back on its mooring at Crinan? I'm thinking it will be much nicer to do up there in the summer rather than in the winter at Dumbarton boatyard. I can't see that it would make any difference?

Thanks

 

[macd]

I can't envisage any "technical" reason, other than the practicalities of lifting/moving the engine, which you've no doubt considered. A-frames, after all, built half of antiquity. But if you are lifting the engine, it won't be a quick job. You'll know better than us whether you're likely to need to bail off your mooring whilst there's no propulsion. (Crinan basin, I imagine, would be OK, moored outside, less so, but you're the bloke there.)

If you do go the new tank route, do offer the details up for suggestion on here. I would strongly recommend that it has a top-exit drain, which is in my view best practice but far from routine.

 

[vyv_cox]

You can use the same technique as the mechanics use during MOT tests. If your tank will take energetic whacks from a hammer it will be ok. If the hammer goes right through........

 

[Bajansailor]

You could do a pressure test on it in-situ.
This would be quite easy to do - just use one long hose attached to the tank (block off all of the other orifices), fill it up with diesel, hold the end of the long hose as high as you can above the tank and continue pouring in some diesel until the level reaches the top of the hose.
If it is (say) 10' up, then you have 10' of pressure head (pressure head = ro * g * h where ro is density, g is 9.81 and h is the height in metric) - which is quite a lot really.
However - if something ruptures, then you have a tank leaking diesel into your bilge.
So maybe not such a good idea.....
(I used this method to test our S/S water tank, and found numerous pinholes everywhere along the welded seams - but it was only water leaking, not diesel).

So, taking into account the fact that it is almost 40 years old and is built of mild steel, it might be best to replace it.
You can give the engine a good clean up (maybe even a paint job as well?) while you have it out. Similarly the bilges under the engine. The boat will love you for doing this.

 

[Findhorn]

If you have access to the underside of the tank it may be possible to use an ultrasonic thickness meter. they can be hired for approx. £60/week from various sources .

 

[NormanS]

When I made the mild steel fuel and water tanks for my previous boat, I pressure tested them using a dinghy pump, a brush, and some soapy water. But of course, I had complete access to the tanks, which made it easier.

 

[JumbleDuck]

If you have access to the underside of the tank it may be possible to use an ultrasonic thickness meter. they can be hired for approx. £60/week from various sources .

A few years ago I cut up an old 1,000 litre CH oil tank. It was generally in very good condition, thirty years after it had been installed, but it had several weeps and leaks at various heights from the bottom, which were very localised (total damaged are around the size of a 5p piece) and which I think I would have been quite lucky to spot with thickness tester.

OP, if I were you I would bite the bullet and do it now. Crinan is a lovely spot, but a floating mooring there is no place to be trying to move tanks and engines. If you have good access to the inside of the tank and if you are reasonably confident that it's not about to go into holes ( I suggest a screwdriver, wielded cynically and viciously) you could use a tank sealant on it: for example POR15 Basic Big Fuel Car Tank Repair Sealer Kit.

Otherwise it's new plastic or stainless tank time. Expensive and time consuming, but you'll be glad of it soon enough.

 

[Crinan12]

Thanks guys interesting replies
I can't access the outside of the bottom of the tank but there is a good sized hatch in the top of the tank - I can fit my hand inside the tank.

The inside of the tank feels smooth except in one section which is the lowest point of the tank where presumably any water would sit below the diesel

I did look at that product jumble duck but I think you are supposed to pour it into the tank then shake the tank so it coats the whole thing which I obviously can't do

I'll give it some more thought. I would like to replace it so i can forget about it.

Thanks

 

[PilotWolf]

I did a boat delivery from London to Brighton for a refit once, when I went back to talk to the owners in the yard he told me that when they went to lift out the fuel tanks his fingers went through the bottom of one of them!

W.

 

[thinwater]

Ultrasound can be very effective, if there is access... but there probably is not. Additionally, it is a pro job.

 

[Assassin]

You can also get epoxy kits to line the bottom of your tank and no removal necessary.

 

[rogerthebodger]

The first is to pressure test the tank to see if there is a leak. An old tyre pump would do the job. Pump up to a set pressure say 20 psi the leave it over night and if the pressure has dropped you have a leak. You can then apply more pressure and search for the leak with soapy water.

Once you have found the leak/leaks you can the decide if you wish to repair or replace.

When I built my steel boat I pressure tested all 3 tanks as they were welded as part of the hull.

 

[Crinan12]

How come a pro job thinwater- Does the machine not just give you a reading ?

Assassin I've read about the epoxy kits quite a bit. General consensus seems to be that it can be very hard to get the stuff to stick to the tank.

Roger my tank isn't actually leaking. It's just that I found pieces of rust in it and in worried that it's about to fall apart !

Cheers

 

[rogerthebodger]

Roger my tank isn't actually leaking. It's just that I found pieces of rust in it and in worried that it's about to fall apart !

Cheers

May be a hammer test is the way to go then. That will at least remove all the loose rust.

I use brick cleaner/ pool acid to remove surface rust before painting so if the tank is very thin the acid would show up any doggy areas.

 

[ean_p]

[ UOTE="Rogershaw, post: 7009488, member: 531"]
May be a hammer test is the way to go then. That will at least remove all the loose rust.

I use brick cleaner/ pool acid to remove surface rust before painting so if the tank is very thin the acid would show up any doggy areas.
[/QUOTE]

How come a pro job thinwater- Does the machine not just give you a reading ?

Assassin I've read about the epoxy kits quite a bit. General consensus seems to be that it can be very hard to get the stuff to stick to the tank.

Roger my tank isn't actually leaking. It's just that I found pieces of rust in it and in worried that it's about to fall apart !

Cheers

I think that you need to establish how rusty the inside really is and how thick is the plate. I no expert but would think it unusual for a diesel tank to perforate from the inside out! How much rust did you have and over what area? I seem to remember that an eighth of mild steel equates to am inch thick of rust if not disturbed or there abouts. Over reaction could be as costly as under reaction!

 

[AndrewB]

How come a pro job thinwater- Does the machine not just give you a reading ?

I'd also favour Ultrasound testing if it's possible, but you must be able to get all around the tank, particularly the bottom.

Not many yacht surveyors do it, not for lack of an instrument, but (so one of them told me) ultrasound testing is skilled: you have to be knowledgeable to get proper results, specially with steel that is painted or has some surface rust. If you go down this route, check their credentials.

 

[rogerthebodger]

I'd also favour Ultrasound testing if it's possible, but you must be able to get all around the tank, particularly the bottom.

Not many yacht surveyors do it, not for lack of an instrument, but (so one of them told me) ultrasound testing is skilled: you have to be knowledgeable to get proper results, specially with steel that is painted or has some surface rust. If you go down this route, check their credentials.

I do have and have used an Ultrasound thickness tester. It relies in the speed of sound through the material you are measuring. Mine has a set of different materials to calibrate the instrument for different materials. The one issue is that id you have any surface rust of paint you will not get an accurate reading due to the different speeds or sound through the different materials.

 

[JumbleDuck]

I did look at that product jumble duck but I think you are supposed to pour it into the tank then shake the tank so it coats the whole thing which I obviously can't do

Ah, what we used to call a "slosh" sealer. Sorry, hadn't realised. It might be worth calling Frost and asking their advice - they are generally very helpful.