We had to cut away the forward wall of the storage bin under the V berth in order to accomodate the Ronco B102 13 gallon water tank. The outlet fitting at the aft end of the water tank peeks through a hole at the bottom of the storage bin divider. Unfortunately, this fitting will be pretty well hidden when the holding tank is in place. The holding tank and its support cradle will just slip into the hole in the V berth deck. The braces that will hold down the tank will also support a plywood cover for the hole.
Rather than build a cradle for the water tank in the limited space available, we squirted expandable foam ("Great Stuff") in plastic bags in order to provide a conformable support for the water tank. Our initial trial was not successful because the foam requires some moisture in order to cure and the plastic bags prevented any moisture from reaching the foam. Despite the fact that the plastic bags prevented a good cure, we were very happy that we had used the bags. Without them, the cleanup of the first trial would have been a monumental task.
For our second attempt, we splashed some water into the bags before adding the foam. We immediately mounted the tank and fastened the prefitted hold down brackets to keep the foam from lifting the tank out of position. The foam appears to provide adequate support although, after a season of use, there is now some play in the mounting. It seems that the foam may compress a bit under load.
I built a cradle for the holding tank from 5/4 x 6 pressure treated decking. It was designed to rest on the hull at the forward end and to be fastened to the V berth structure at the aft end. I had made cardboard templates of the area first, but there was still a considerable amount of cutting and fitting involved before the cradle fit properly.
With the holding tank cradle in place, we could finish the water tank plumbing. This is the last chance (without a lot of contortion) to get the clamp tight on the water tank outlet hose.
Before installing the new water tank and holding tank, we applied the aluminum foil strips that are the sensors for the Snake River 3 tank monitor.
Now both tanks are in place with their hold-down brackets installed and the plumbing completed. This picture shows the starboard side holding tank vent and the two crossing vent hoses.
Each of the forward corners of the holding tank has a vent which is plumbed to a proper thru-hull and seacock just above the water line on either side of the boat. The hoses are crossed to prevent the tank from emptying through the vents when the boat is heeled. This setup allows a small amount of air to circulate continuously through the tank and minimizes holding tank odors.
From what I've read on the subject, it is the anaerobic bacteria that cause the odors. By providing a steady supply of air, the aerobic bacteria dominate and there is not much odor. So far, this simple system has worked well without the need for an air pump.
This is just another shot of the water tank plumbing showing the hold-down brackets. The vent and fill hoses are simply T'd into the existing hoses for the other water tank. I did add separate valves (under the starboard cabin settee) for the outputs of the two tanks. Normally, the valves are both left open. The bow tank tends to empty first since it is a bit higher.
The whole installation worked out pretty much as planned. We have more than doubled our fresh water supply as well as doubling the effective size of the holding tank. And the addition of the Snake River monitor lets us keep track of things so we can stay out longer and not feel compelled to pull in to a marina 'just in email@example.com