Changing the pump…

Hi John – We put our holding tank in the head, behind the sink. We could fit a 13 gallon tank, which is rather small but the Lavac does not use much water. We counted one day – if we do the full Lavac recommended pump cycle, that is 10 pumps-wait 5 seconds-5 pumps more, we got 13 flushes so about 1 gallon per flush. We find this number of pumps is not necessary for most situations, so it’s easy to go closer to double that number of flushes before the tank needs emptying. Details of our installation is on our blog here.

We did consider putting it in the v-berth, but I could not find an available tank that would fit the area with sufficient volume to make it worth giving up all that storage space. We considered this long and hard before we settled on putting it in the head where we did. The advantage of the head installation is that it’s a gravity system, so no need for Y-valves etc. We are pleased with the installation after 2 years of use. And we like having the starboard v-berth locker available for storage.

The other major problem with putting a holding tank in the v-berth is that the locker is a very complex shape, roughly triangular in athwartship vertical cross-section but trapazoidal in horizonatal cross-section. I just turned through my notes, and I have 3 different sets of measurements where we attempted to fully spec the size – including the time we took off the top so we could easily get inside the locker (to get to the screws that allow you to remove the entire top requires that you remove first the ceiling boards, then the wall boards, then the v-berth top)I can tell you that the stbd side locker dims are roughly:
– 35″ along upper hull edge from the fwd locker wall to the line perpendicular to the vertical inside locker wall face + another 8″ aft of that point to get into the upper aft corner along the hull, so 43″ overall, but the aft end of the tank does not make a 90 degree corner (hence the measurement about 35″)
– 19″ athwartship upper forward edge
– 22″ atwartship upper aft edge, perpendicular to the vertical support wall of the v-berth. This is not the same dimension as would be measured at the bulkhead, where there is not a 90 degree corner.
– 19″ vertically from the floor to the top lid at the fwd end, inner edge (that is, not along the hull. Consider the hull to be the hypotenuse of the triangular cross-section)
– 22″ vertically from the floor to the top lid at the aft end, inner edge
A picture would be worth a thousand words here, but if I put one together I think I’d be wasting my time because I still would not recommend you trust our measurements enough to order a custom fit tank based on them.

Due to the complex shape, we considered building our own tank in place in the v-berth locker with the plywood stitch-and-glue technique, but then decided this would be more trouble than it was worth. We also ditched the idea of a flexible tank after reading reviews of how little time they last, and how much they cost. We are happy with our final decision about the head location.

So that’s my two cents – I don’t envy you. Putting in a holding tank sounded like such a logical first thing to do after we bought our boat. It turned into a nasty complex job. If you want to know more about the dimensions of the tank we stuffed into the head area, let me know.

Marilyn

I have been considering fitting a holding tank, and unlike Marilyn, decided the stbd. berth the best place, did check out her recommendations, which have much merit, decided to use the under-berth space.
I have so far removed the berth base for access, a new one will have an access cover to get to the finished tank. I have made a cardboard mock up, which fits in perfectly, I want it removable and not a fixture, will make it up from 10mm ply, sealed inside with epoxy, nice moulded corners and finaly finished with a gel-coat paint, recommended to me by a tank manufacturer who use it all the time, so a nice smooth finish, complete with baffles inside. Externally it will be glassed so good and strong.
The size will give me some 60 litres, similar to the one Peter has on Alexina, with a diverter valve when in restricted areas, and a hand pump to empty when offshore.
The below water line challenge does require vented loops, or the closing of the outlet following toilet usage, or both.