When a 23,000 litre (5,000 gallon) water tank starts to rust you end up with a big object to dispose of. It is heavy, it is awkward to handle, it is a dead weight that is always in the way.

We mulled over what to do with ours for a long time before inspiration struck. Vegetable gardens, just like those over-priced ones you see in the gardening centre!

We had a cute idea with concentric circles of garden beds which sounded exciting until we realised that any attempt to bend corrugated iron wouldn't work and would be a waste of time.

I have to acknowledge that my wife had the idea we finally went with: to cut the tank into sections and rejoin them.

This is how we turned our old water tank into 6 boat shaped vegetable garden beds.

Step 1: Dismantling the Tank - Remove the Top

The obvious first step was to dismantle the old tank. Sounds easy but these things are strong. They are designed to hold 23 tonnes of water without leaking. So there are lots of screws and loads of glue.

This tank was about 1.8m tall, 3 metres in diameter and built from 3 rows of corrugated iron. There was a lid on the top and a obviously a bottom that needed to be removed. These were soldered or welded to the sides. I bought a pack of 10 grinder cutting wheels since I knew they were going to get chewed up.

We got off to a bad start. I removed the entire top row with the lid attached, with the idea of pushing it off and then cutting it apart. This was the point where I realised they are big and heavy and awkward and it ended up stuck and twisted and wonky.

In retrospect I would have worked my way around the top cutting off the lid and letting it fall down inside, then removing the top row of iron.

Step 2: Dismantling the Tank - Row by Row

Getting the tank apart from there is fairly straightforward. Remove the screws, force apart the glue and get it apart row by row.

Of course, when you get to the last row, you are going to also have to remove the bottom of the tank. You did get a 10 pack of grinder disks didn't you?

If you have lots of space you can demolish the entire tank and then cut it up, or with more limited space cut up each row as you remove it.

Step 3: Cut Each Row Into Quarters

Our plan was to take each row, which is effectively a circle, and cut it into quarters. Then join two quarters together to make a single bed.

To measure the quarters, I took a long piece of rope and wrapped it around the circumference and then folded the rope in half and half again. Funny how this will be very precise in theory, but in practice it never quite worked and each row needed a little bit of fudging. But the results were close enough for this project.

Then using a plumb line to get a nice vertical, I cut each section into 4 pieces ending up with 12 in total. Once it is cut, it is much easier to manage smaller bits like this.

One thing to be aware of is that if the cut isn't perfectly plumb it will pull either the top of the bottom of the boat in along the sides. This may be a good thing, with a bit of experimentation you can make the top wider than the bottom.

Keep in mind that 8 of the segments will have a single jagged edge after removing the lid and the bottom. Be sure to put these edges at the bottom of your boats to avoid tears and bloodshed down the track.

So we had the idea, we had the pieces...now to test it out...

Step 4: Join the Pieces

Putting two segments together worked!

As a complete surprise they immediately reminded me of boats - this had my imagination running.

Joining the halves wasn't all that easy and needed my wife to hold them aligned while I screwed them together.

I had some scrap light angle iron to join them together. But not enough so off to the hardware where similar material was about $20 per 3 metre length.

Then I found strips of metal used to form corners when plastering. It was $1 a metre and was conveniently pre-drilled. It is a lot thinner than the other corner material so I over compensated with screws.

We ended up with 6 "boats" as we started calling them.

Step 5: How Much Soil?

The 'filling it' mission had been set to happen a couple of times but weather and other events saw it being delayed. Rather than strain my already dodgy back, I decided that moving 9 metres of material by hand was a foolish idea. Decided to hire a mini digger instead because it would make life easier and more importantly they look like fun. I had discovered that my local hire place will only charge for a single day if you pick something up on Friday afternoon and bring it back on Monday (they are closed Sunday). Bonus.

But if you take it the day before Good Friday, because they are closed until Tuesday I got four days for the price of one! Bigger bonus!

How much material would we need? I am proud to say I worked it out myself though it took a day or so mulling over the problem to crack it.

Since we start with a circle, we can easily get the volume of one row using pi x r (1.5m) squared x height (0.67m)

3.1416 x (1.5 x 1.5) x 0.67 = 4.7

Divide that by 4 and we have the volume of one piece of pie (the red lines in the diagram above)

4.47 / 4 = 1.19

Since we know the radius we can work out the area of the triangle bordered by the red lines with the 45 degree green line. Or in 3D, it is half the volume of the cube. Still with me?

1.5 x 1.5 x 0.67 / 2 = .75

Now deduct that from the total volume of the piece of pie and we are left with the volume of the arc shaped piece.

1.19 - 0.75 = 0.44

Since we are butting two of them together, double that value and there is the volume of a boat.

0.44 x 2 = 0.88 cubic metres

I rounded this out to 1 cubic metre per boat.

For a bit of extra drainage we added about 100mm of 10mm gravel to the bottom of each boat to start with.

Step 6: Fill Your Boats

If using a digger, be sure that you don't try to carry too much gravel at once with the bucket raised otherwise you might end up tipping the thing forward. This is guaranteed to happen just as the photographer is lining up a shot.

Fortunately it is not nearly as dangerous as going sideways or backwards. In fact, once we emptied the bucket with a shovel - there must be a 'don't run if not level' switch somewhere - with a bit of effort to get it started, the machine gently eased back onto its wheels, the balance was quite delicate. I had expected a sudden drop as the centre of gravity shifted towards the back.

On top of the gravel base we half filled the boats with 'forest mulch' which is essentially the output from a tree lopper's chipper. It is pretty coarse and will break down over time, but it is reasonably priced and will become nutrients under upper layer of soil.

And that's what goes in next. Fill to the very, very top with some sort of decent soil. We used an 'organic blend' which is mushroom compost and other composted material and other stuff - it is a dark art. I wanted to overfill to allow for settling but I couldn't convince my wife that it is was a good idea. I have a bet that in 3 months it will have settled at least 100mm.

Step 7: Get Creative

It is time to plant out your boats and get creative.

I have had an old oar sitting around for (mumbles) years for some reason. I grabbed a couple of rowlocks and the boat theme started to come to life.

We needed some climbing trellises for beans and snow peas so I inserted a scrap length of PVC pipe as a 'mast' attached a ring up high and brought twine lines down to the sides of the boat. There is also a length of old chain hanging from the front waiting for an anchor.

It looked good but needed something more and I had another inspired moment. I have added ratlines (the horizontal pieces) to the rigging to make it look like a square rigger and it looks great.

Now we need to be patient and wait for everything to grow which might be a while as we head into the Australian winter. Check out my other instructable on turning disposable razors into plant labels, it complements this one perfectly.

This has been an immensely satisfying project that turned a piece of junk into something that will put food on our table. Part of the satisfaction may have laid in it not needing to be perfect, in fact the more rustic the more character.

Hope you have enjoyed this, please vote for it in the homstead/gardening competition.

Step 8: Two Months Later...

Pretty excited about being a finalist in the Gardening & Homesteading competition, thanks for the views and the votes. Thought I would include a follow-up photo of how these garden beds are going.

We have stopped buying salad greens and are close to harvesting our first eggplant. Everything is growing really well despite the fact that we are into the Southern winter. Can't wait for spring.

This project has been a fantastic success and a neighbour has used this instructable to do the same thing...which is what this is all about, right?

UPDATE: WOW!!! What can I say??? This won the Grand Prize in the Gardening Competition. Thanks so much to Instructables and the judges.