How to cut thick steel?

Otter

Otter

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We have original 1977 steel diesel tanks and as part of the rewire I discovered a water leak has rusted the underneath of one of them. There is no way they can be lifted out whole, they were put in before the engine, so they have to be cut up and removed. I'm guessing that even though the residual diesel film has a very high flash point the only safe way is to use a saw - but what kind of saw and what sort of blade and is it a DIY job? It's a huge job whatever, the steel is very thick :(
 
My commiserations.
Having done it, its a filthy job.
I found the big problem was cutting the baffle plates inside the tank to allow the sections of outer skin to be removed in chunks. A combination of angle-grinder, a "nibbler" in a drill, tin snips and a big cold-chisel were brought into play. The cold-chisel was the most effective tool, but couldn't be used inside on the baffles.
Before starting I filled the tank with water, more to vent fumes than anything and then drained it. There was still a residue of diesel in there but with some precautions (fire extinguisher to hand) the angle grinder was not a problem. Filling the tank with CO2 was suggested, but its impractical unless you are working outside and from the top down.
If you have a glass-fibre boat use old carpets to lay on the hull if using an angle grinder. Specs of hot steel in the fibreglass will ruin you day.
 
How thick is 'very thick'?

I would use a reciprocating saw aka sabresaw or demolition saw.
If access is too difficult for that, a jig saw, but get the best blades you can!

A grinder is not only likely to causes sparks, but also embed rust particles in everything within a 20ft radius.
 
There are no baffle plates inside - which from the sound of it is a real blessing! The steel is about 11mm thick.
 
That measurement was taken from the bolted on access cover, the main tank is thinner I think. Just imagine the weight! The 30 inch side access cover was almost as much as I could carry when we removed to re-bed it.

Once they've gone the old girl will turn into a flyer - time to enter some races.

I've looked up reciprocating saws and good ones are £200 or a weeks' hire is £70. What type of blades should I get - and is this really DIY? How much in the way of stray filings should I expect and should I flood the tanks first?

Thanks for the help, I was absolutely in the dark on this.
 
lw395 said:
How thick is 'very thick'?

I would use a reciprocating saw aka sabresaw or demolition saw.
If access is too difficult for that, a jig saw, but get the best blades you can!

A grinder is not only likely to causes sparks, but also embed rust particles in everything within a 20ft radius.
Click to expand...

Sabre saw is the best bet, a jigsaw will work but even a decent one will be knackered after chopping up a tank made of 11mm steel. Depending on the size of the tank it may make sense to buy or hire a pneumatic one as you can then cool the blade with a trickle or spray of water. Why so thick? Is it a pressure vessel or does it need to be resistant to small arms fire? :D

Given the thickness, it might even be worth looking at hiring a plasma cutter(or hire the services of someone with one) after you have opened a decent sized hole and can clean up remaining diesel; sawing through 11mm steel will take an age even with a powered saw. After the hole is made and the diesel removed the fire risk is negligible(i.e. no problem provided you have someone standing by with an extinguisher), and you can lay down fire blankets to protect surfaces exposed to the sparks.

Editted to add: I missed your 2nd post; if it is much thinner(I would expect a steel tank to be 4-6mm) then the saw is a good bet, although still a few days work in store. The cutting will produce a large amount of steel filings/swarf; saws will make little chips that are as long as the blade's teeth are wide and nibblers tend to make little 3/4 moon shapes 2-3 mm in diameter. The swarf will be hot but probably not hot enough to embed itself, but I'd put down a sheet of plastic and a cotton sheet over that to stop most of it going in the bilge.
 
Last edited:
I'll take an inspection hatch off tonight and look - the shock and awe expressed suggests the tanks are not that thick. Mind you, the scrap might be worth something :D
 
The Lazy Kipper said:
There are no baffle plates inside - which from the sound of it is a real blessing! The steel is about 11mm thick.
Click to expand...

My steel hulled boat is not that thick and my built in tanks are only 5 mm thick.

I think you may find that the edged of the opening may be 11 mm thick but I don't think the tank will be.
 
If you're sure they're scrap anyway, just drill a hole in the side in a few places. I too would be amazed if they're more than 5mm thick. Even 4mm will take a lot of chopping-up, I'm afraid! I'd probably be looking at a nibbler and compressor too, depending on what access is going to be like.
 
Cutting tank

If you have a decent mains supply available I would look at hiring a compressor and a plasma cutter for a half day. Fill the tank with water and some detergent.
Then drain it,this should get rid of any remaining fuel. cover the area around the tank with old wet carpet this will prevent any hot slag doing any damage. Always try to cut from the outside of the tank so that the slag is landing inside what is left of the tank, anywhere where you cannot; always put a piece of 19mm ply between the path of the slag and the hull. The time and money you will save on blades will more than compensate for the cost of hire, about an hours work with a plasma cutter and the tank should be no more.
 
If they are anything like as thick as you think I suspect the easiest by far will be to use the plasma cutter suggested above, or an oxy-acetylene burning torch. I have no experience of the plasma but I have gas welded diesel tanks several times. fill with water and drain is sufficient to remove anything that might burn. I don't suppose it is easy to hire the equipment but almost any truck workshop would do it for you. I can't imagine it being more than a couple of hours' work unless access is very difficult.
 
cutting tank

Hi

just a thought but it sounds like an awful lot of work to fix what is, I assume, a relatively small leak and no doubt of it is as big/heavy as you think this may affect the the boats trim unless your going to replace like for like. So why can you not just plate it by either grinding or cutting back to good metal and welding in a new piece.

Make sure you wash out the tank prior and leave it to vent for a few days before letting in a new plate. Most competent welders would find this a simple task and provided the access is ok getting a welder to do the job for you should be easy.

Regards
 
There's no way a tank would be made out of 11mm plate - most likely you're looking at a flange which would have been fitted to reinforce the edge of an opening.

If it were mine, then I'd remove the inspection plate, completely fill the tank with water, skimming off any diesel residue in the process, then drain-off an inch or two and cut a bl##dy great hole in the top with a plasma cutter or oxy-propane set.
Once that hole has been cut, drain-off the water and dice-up the steel with whatever you cut the hole with.

A 110v angle grinder with a zip disk would also do the job, but it'd be a very messy operation.
 
I'm for the plasma cutter.I use mine regularly and it makes short work of that sort of thickness of steel.Just make sure you wash the tank thoroughly because the very high temperatures at the torch will easily vaporize diesel making it explosive.
 
Are you afloat?
Presumably you have to get a replacement tank in there too at some stage?

I think I would run on one tank for the summer, plus 3 or 4 20l cans, then engine out ( its only a chain hoist up onto scaffold or gantry or a passing tree or a slide forward job into the saloon on a hydraulic jack/ scaffold boards)
then slide tank out. In fact why not both out for shot blasting, weld up and spraying if they are wing tanks? Cant remember the boat, sorry .. Pic maybe for the real experts:)
 
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