Derwent Marine: Replacing the fuel and water tanks

When we bought the boat the old mild steel fuel tanks had seen better days.  In fact as part of the work before completing the sale we found that the two fuel tanks were in fact full of water!  The outsides had been left to rust and so we knew that one day soon it would need to be tackled.   I had tracked a small fuel leak in one of the tanks in Sydney and so it was time.   There are usually horror stories about replacing tankage in old boats and ours like many are in the bilge and under the floor.  We had spent some time figuring out just how much of the floor and fittings would need to be cut out to do the work and had been pleasantly surprised at how little it would take. 

Still it was a big job requiring good carpentry skills and so we asked Don Bailey at Derwent Marine near Hobart to help us do this and also weld up the new stainless gantry for the solar panels.  We went alongside their wharf and were glad we did this job whilst in the water as it was much more convenient than it would have been on the hard.

I had searched around for the options to replace and really it was a 'no brainer' to replace with HDPE (plastic) both on cost and ongoing integrity.  I ended up ordering with Atlas Tanks in Brisbane, mainly because any business that has an actual production schedule gets my vote of confidence on the basis that they must be organised, or trying to be, and possibly even reliable on delivery.  Turns out they were organised, reliable and professional and produced great tanks delivered exactly when planned.  

At this point it was time to move off the boat to a nearby apartment.

Glen who works at Derwent marine is a good carpenter, which is just as well as the first job was to take out the companion way steps and then cut out the main floor below.  We were lucky in that we could do this without resorting to cutting out cupboards and bench seats.    

The two mild steel tanks were built port and one starboard which I never understood as if one only was used the boat would be heavy one side. The water was the same configuration. They were very obviously put in before the coachroof was built as there was no way they were going to get through the hatch space without being cut in pieces.  We set to with jigsaws and a sabre tooth saw which made short work of the job. It took four hours for two of us to cut and clear them out of the hull. 

I had measured up carefully and planned that the replacements would be built to fit across the width of the boat and so be longer and thinner.  The new size allowed them to just fit though the hatch space.  Each tank was 375 litres which was marginally smaller than the originals.  Outside size was exactly the same and the reduction came as a result of the 10mm sheet size of the plastic lowering the internal capacity.

There was a lot of shifting and pulling to get access to the space, this is Glen, not me! adjusting the tanks before cutting them up.  It was pretty clear that the time was right to clear them out as the insides were a sludge of rust and the outsides a mass of flaking rust. 

The bilge had not seen the light of day since the build in 1984 so there was quite a lot of mess and sludge.  I scooped out what i estimated to be 30kg of mess and this explained the old boat smell.  I scrubbed it all back degreased cleaned and ground it all back to clean fibreglass before painting the bilge with a two pack paint.  I also took advantage of the space under the floors to re route all the electric cables through tubes so I know where everything is. 

After taking the fuel tanks out I had a good look at stainless water tanks. they were immediately forward of the old fuel tanks that were now scrap. Again they were the original and it had been difficult to see what sort of condition they were in.  I found four small leaks on one of them at the base welds so there was no question that they needed to come out as well given this was the only time I planned to pull out the floor of our home.  The space made by the fuel tanks departure made enough room to slide them out, cut them up and send them to the scrap yard. I had cut out lots of panels to be used for backing plates for the planned new winches and deck gear.

The new tanks were pretty easy to replace in comparison to the removal.  Glen did a great job of preparing batons to fix them down and ensuring they do not move and then replacing all the joinery.  He re organised the joists and fixings on the companion steps so that it would be a fairly easy task to take them up if the need ever arose in the future.

I moved the tanks around and put the fuel forward and the water aft.  Tiki sits too high in the water at the rear, a legacy of losing a mizzen mast perhaps when converted to a cutter, and we have more water than fuel on most occasions so there is more weight aft now.  We also only use the rearmost fuel tank as 375 liters is enough for most journeys and the forward will be filled only on long ocean trips where we need to have 700L of fuel.

Lots of detailed fitting of new lines, gauges and breathers and realigning access hatches and we had a glass or two of bubbles for a job well done.  We even came in under what we expected to spend which is a minor miracle in marine world! It took just over five weeks, longer that planned because of the need to wait for the additional water tanks that had not been scheduled to be built.