Some time ago we decided to take the pumpout option when it came to the toilet system. As storage capacity on the boat will be one of the three critical criteria I opted for a vacuum macerating toilet. The idea behind this choice was it would reduce the amount of water used to flush the system.
After reading some of the technical literature on the subject I became slightly concerned about the distance between the toilet and the holding tank. The bathroom is near the bow and the tank is in the back cabin. In the end I reversed the bathroom layout which had the effect of moving the toilet three feet closer to the tank making them slightly over 20ft apart. Additionally, the tank will be under the floor and my thought was that at least the toilet pump would be moving the waste with the assistance of gravity.
The tank will be only six inches high but cover almost the entire floor space of the back cabin. We will need to leave some room either side of the tank for bilge drainage. My rough dimensions are 6’x5’x6” giving a capacity of just over 400 litres. I’ve calculated 5ft for the width to allow for the start of the curve in the swim.
My idea is the inlet pipe and capacity gauge will be in the forward port corner of the tank and the outlet plus rinse pipe will be in the starboard stern corner. The trim of the boat should be bow high which means the tank should pumpout to an almost empty status. I’m not sure at this time whether we will need a baffle plate to reduce the amount of liquid movement when we are underway. The tank isn’t all that high and a baffle plate may just create an obstacle allowing the build-up of solids in the tank.
The weakest structural part of the tank is probably the top. My idea is to either weld vertical plate or box steel to it thereby providing some rigidity. These can also be used as supports for the floor bearers.
I need to talk all this through with the builder.