Tuesday, 13 December 2011

Re-Thinking the Blackwater Tank

Well Ufton really got bashed about by strong winds last night.  I’m please I’d found some old fenders in the long grass and placed them between the boats.  Just to make things more interesting the shore-power failed at 8.00pm.  Left in the dark with no heating we decided to go to bed.

Until I was able to actually carefully inspect Waiouru I hadn’t realised there was a steel floor joist 4’6” from the rear bulkhead.  The planned length of the back cabin is 6’3”.  On 10 June I was able to have a quick look at the area and the steel tank Ben Harp intended to install.  His tank was only 4’6” long which would have resulted in the back cabin having split floor levels.  It would also reduce the tank storage capacity.

After discussing the situation with Andy, we have decided to remove the steel joist between the two red lines drawn on the photo below.  We are not going to remove the entire joist as I wanted to leave a “heel” to retain some structural strength in this area.  However the baseplate is 20mm thick so there shouldn’t be an issue. 

My idea was a separate fabricated tank would be manufactured and installed.  However Dennis has suggested we use the 20mm baseplate as the floor of the tank.  The contents of the tank are going to have their work cut out trying to eat their way through 20mm of steel.  Moreover, most corrosion will occur in the tank air void so it will be the upper parts of the sides and the top plate most susceptible to corrosion.

Andy and I have agreed the tank should remain 12 inches high.  This allows for the build-up of “crud” on the bottom of the tank.  Notwithstanding this I’m also looking at the design of the flush pipe to ensure the base of the tank gets well rinsed.

The tank dimensions will be approximately 6x5x1 feet giving a capacity of 186 gallons or 850 litres.  This will provide approximately 60 days storage.  But it’s unlikely we would want to fill the tank as it would probably affect the trim carrying an additional 8 tonnes of ballast.  However it’s good to know we could probably last two months should we every get stranded or frozen in.

6 comments :

Peter and Margaret said...

For what it is worth, I read an article some time back on the pro's and con's of integral steel waste tanks against poly whatever ones that are installed. Some people thought the contents of a black water tank would be detrimental to the longevity of the hull, but a boat builder added his two penn'th by stating that he had once been asked to extend a hull, which involved cutting straight through the integral steel waste tank. Once he had got over the unpleasantness of the task - he noticed that the internals were the best condition steel of the whole boat, the contents had somehow pickled it against corrosion.

Derek Bird said...

I think it would be short sighted not to have a whole tank as you are just storing problems for later

Derek

Andrew Denny said...

When I had a pumpout tank retrofitted to Granny Buttons, I was told the most important aspect was ensuring there was a proper sump, so that it could be drained properly.

That worked fine. But after 8 years, while the tank itself was fine, the pipe descending into the sump corroded away, leaving it unable to suck out.

Tom and Jan said...

Peter,
I had also read that somewhere.

Derek,
The only part of the tank that will be intergral to the boat is the base and that's 20mm thick. Corrosion is more likely to occur at the top of the tank.

Andrew,
A good point. I'm already considering how to route the flush pipes and where the inspection hatch might go. The outlet will be at the lowest point of the tank (starboard/rear corner)

Anonymous said...

850litres will be more like less then 1ton, as 850liters of water would be 850kg.

Good luck with the (re)building of your boat.

Peter.

Tom and Jan said...

Peter,
You're right... my bad maths :-)