The Blackwater Tank

Back to Google SketchUp.  I wrote in an earlier post that the plan is to fit the blackwater tank under the floor of the back cabin. I came to this decision when I realised the tank was going to occupy a large area.  Actually I wanted the tank to be as large as possible for maximum holding capacity.  But I also wanted it as small as possible so it occupied very little room in a small boat. Initially it was going under the bed and then I though of placing it across the stern.  Both options took up potential living  or storage space.  The bilge was an area I hadn’t initially considered, however because I’d specified a baseplate twice the normal thickness there would not be a requirement for the normal amount of ballast.  The tank would therefore probably fit under the floor.   

The following are my rudimentary attempts to provide my idea of what the tank might look like.  The first drawing shows the tank upside down with the base removed to see the internal construction.  I’ve drawn a diagonal baffle plate across the tank.  It doesn’t go fully from to each corner.  This idea of the baffle is to semi-compartmentalise the tank thereby limiting the ability of the “fluid” contents to rapidly move from one side of the tank to the other so that stability is maintained.  

The inlet from the toilet is at the forward, port corner along with the “breather pipe” and the “fluid” level gauge.  At the opposite corner is the suction outlet pipe and the tank flush water inlet .  Placing the outlet connection at the rear on the starboard side means it’s likely to be at the lowest point (boat trimmed bow up – stern down).

 

 

This is the top view and you can see there are horizontal plates welded as ‘ribs’ on the top to provide strength to the top and a fixing point for the floor joists.

The inlet breather is required because as the contents start to break down gases will be given off.  They need to be vented to prevent the tank pressurizing.  We will ‘feed’ the tank to minimise the most serious smells and also connect an active charcoal filter into the inlet breather pipe to avoid Waiouru smelling like a sewage works.

Of course some (or all) of this may be completely impractical so we will have to wait and see what the tank eventually looks like!