Yet more boat design thoughts – Part 1

Sitting in self imposed isolation does provide time to dwell on the past and obviously our thoughts turned to life on board Waiouru. I commented about some of the lessons we had learned back in December 2017. However I don't think I touched on two important subjects. This first is obviously toilet arrangements and the second is increasing the internal dimensions of the boat.

When I designed the interior of Waiouru I considered the trim. However with no practical experience I was more concerned about the lateral trim (level) (port & starboard) rather than the longitudinal (bow and stern). All the boats we previously hired had a passageway down one side, which meant the majority of the static weight was on the opposite side. To counter this ballast has to be added to correctly trim the boat. I think I solved the lateral problem by designing a layout with a central corridor. This meant the weight was evenly distributed between port and starboard. When a boat has a pump out toilet the tank is sometimes located under the bed. This means as the tank fills the weight increases on one side affecting the trim. My solution was to have the tank under the floor in the back cabin. This solved the trim problem. Coincidentally the tank had a huge capacity and we never filled it.

However there were two problems with this arrangement which I had not realised. The first was the distance between the toilet and the tank. It meant a long flexible pipe had to be installed. The second was the pipe had to be run at floor level before rising 10 inches through two 90deg bends to the top of the tank. This meant the pipe was always full of effluent.

If we were to build another boat (in the hands of Lotto) I would still have a pump out toilet but the tank would go in the bilge under the toilet floor.  This would mean only a very short pipe between the toilet and the tank.  a little like an up market dump through toilet The bilge is also where the diesel tank would be situated.

I had read a boat needs a bilge because that is where the condensation will accumulate.  Hence it is sometimes being named a ‘wet bilge’.  The insulation on Waiouru was so well done the boat only ever had a ‘dry bilge’.  This was achieved by ensuring the spray foam insulation was thick and correctly applied.  The insulation also extended under the floor and finally I sealed between the wall and floor insulation to form a cocoon. This prevented the cold (and heat) from getting into the cabin.  As a consequence any condensation came from us or our cooking.  Actually we slept all year round under a lightweight 4.5 Tog duvet.

Original insulation.  Note the 4x2 timber capping on the baseplate cross members.  also note the thickness of the timber framing on the sides of the cabin.

How would I fit a toilet and diesel tank under the floor.  Waiouru actually had quite high headroom.  I could raise the floor by 6 inches.  My 20mm thick baseplate would allow me to have fewer baseplate  cross members.  Even if that wasn’t possible I could cut horizontal holes through them. 

More Interior Space

The original boat builder had fitted 4x2 timber capping to the bilge cross members.  These were removed and replaced with four 6x1 longitudinal planks.  This provided an additional one inch of headroom.

As you can see in the photo below the planks formed a grid with insulation panels in between.    Additional spray insulation has been added to the walls and ceiling to the point where in places it extended beyond the surface of the timber framing.  I cut the foam back flush using a 26” cross-cut saw.

The final step in the insulation was to apply more spray foam to all the seams to create the cocoon.

Finished insulation

Note the black strips in the cabin wall in the above photo.  This is where I create more internal space in the boat.

The above area is the bedroom and we were fitting a crossover bed.  The problem was that this part of the boat was starting to narrow to form the bow.  If I had left the timber framing then the bed would only be 5’10” long.  To resolve this I removed the timber framing replacing it with dense foam tape (the black strips).  By doing this I recovered 2” making the bed 6ft.  The oak plywood wall panels were screwed through the foam into the steel frame.  If we ever built another boat (unlikely) I would discuss the idea of using thinner steel side framing with the shell builder.  I think I could increase the interior volume without changing the exterior dimensions.

My final thought for this post is about the position of the Lockgate diesel stove fuel tank.  The stove fuel is gravity fed and the tank was located in Waiouru’s cratch where it took all of the port storage space.  Obviously you can’t gravity feed from a bilge fuel tank.  However the Lockgate used very little fuel (probably because of the excellent boat insulation) so I’d probably fit a small header tank for the stove connected to the large bilge tank.