After finally settling on the set up I want in the forward ensuite with regards to holding tanks. I have changed my mind a number of times, including not having tanks, having one tank and pumping both to the same tank, and finally deciding I would have 2 tanks than switching the position of them. There is room in behind the toilet cubicle for the black tank and beside the cubicle (and the hull side) for a wedge shaped smaller tank. I was concerned about having weight forward but then realised that there would only ever be weight in the tanks when I was in a no discharge area and at anchor, so when sailing in open water (when I would be concerned about the weight) I would have the sea cocks open and empty the tanks.
The wedge shaped smaller tank would be the grey tank. I measured the gap between the toilet cubicle side and the hull side and cut some 9mm ply. I started by making cardboard templates. Templates because the hull side tapers in toward the bow so the tank has to match the taper and to make it more difficult to fit the wider section is behind the narrow section meaning I cant make the tank the full size or it would not get past that narrower section. (I actually realised this after I had made the tank and tried to fit it and could not understand why it would not go in! In the end I had to trim it a little, fortunately I had only dry made the tank not glued it.) So with the ply ends cut to shape I made up the sides, just parallel pieces of ply 250mm wide. Then it was a matter of carefully (only 9mm to bury them into) screwing the sides to the ends, butt joined then I coved and glassed the entire inside surface of the tank, and an extra tape over the joins. I attached all sides except the top of the tank which will have the inflow and air vent through tank, I will attach the top last and intend to use the air vent as an overflow and have this just below the top in the side as the top join will only be glued and glassed on the outside, I will use the same method on all the tanks I make including the fresh water tanks. I will insert the out hose attachment before attaching the lid.
The next day I removed the screws, routered the square edges round and gave it a sand before glassing the outside of the tank. It was at this point that I realised how heavy this tank was getting. The plywood soaks up a lot of resin adding lots of weight. It was always my intention to make the black water tank from polycore because I felt it would better contain any smells but having noted the weight of the smaller ply tank, there was now no doubt in my mind. I even considered remaking the tank in poly but decided against it, I might save a kilo or 2 but I will settle for the ply tank. I have not properly worked out the capacity of the tank as it is not a regular shape but I guess it to be 50 litres.
The black water tank was much more straight forward to build, being a regular rectangle shape. I had considered making it with either a pointed or rounded base down to a fine point to aid in the evacuation into the hose but in the end thought it not really needed, the motion (no pun intended) of the boat would clear the tank well enough. I had already pre-glassed the polycore both sides so it was a rigid panel. I cut the sides to the size I had settled on (600mm x 500mm x 250mm meaning just under 70 litres) and the ends and butt screwed and glued them together. I then coved and taped the joins on the inside and back filled the exposed edges of the honeycomb edges of the panels. Once set they will be able to be curved with a router, not as neatly as ply or duflex but still doable.
Once set I attempted to router the edges. Messy. Many gaps that would need to be filled with more filler before applying the glass tapes. I managed and got the tapes down well enough to seal the joins well. I am very happy with the polycore as a working material. The 70 litre tank is about half the weight of the smaller 50 litre ply tank and probably stronger. I gave the inside of the tank a coat of white epoxy whilst the tapes were still green and I peel plied the outside tapes. I also gave the outside a thick coat of white epoxy. I think this will ensure no smells can permeate 2 layers of glass, the poly core and the thick white epoxy on both inside and outside of the tank.
Next step will be to make the tops and fit the skin fittings ready for fitting the tanks in the boat. Perhaps I will also test the tanks by filling them and making sure they hold full of water with no leaks.