Systems+Engines

Gravity Theory Page 2

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To go with this masterpiece of British engineering, I ordered a rigid 14-gallon polyethylene tank from Raritan. On a boat of Ostara’s size (34’) space is limited and the only place to fit the 24” x 10” x 14” high tank would be in the hanging locker in the heads compartment. I was reluctant to sacrifice the space but if I wanted a gravity system there was no other way; at least I would be able to reclaim the locker under the V-berth for the spinnaker, which had been homeless. There would still be room to hang a couple of foul-weather jackets in the locker. As an added incentive, there was already a disused deck fitting (the old water tank filler) in just the right place, obviating the need to make another hole in my deck for the pump out fitting.

I cut a substantial shelf from ” exterior grade plywood and mounted it in the hanging locker on a pair of brackets cut from 21/2” x 21/2”, ” thick aluminium angle . The shelf was stiffened with lengths of 11/2” x 11/2” 1/8” thick aluminium angle bar, which also served to retain the 24” x 10” base of the tank. Combined with a couple of webbing tie-downs, this should be sufficient to keep a potential 100 pounds of sewage plus ten pounds of tank in its place. Aluminum angle bar can be hard to find – I got mine online, from McMaster Carr.

I had to buy threaded 11/2” fittings for the tank inlet and drain, plus a ” fitting for the breather hose. I drilled a hole in the topside above the top of the tank and installed a barbed nylon thru-hull fitting for the breather hose. I had intended to use Sealand Odorsafe hose for the discharge side and bought 12’ of it (the previous system had more than 25’ of hose). At $6.99 a foot this isn’t cheap, and it is also very difficult to work with – it is so stiff that it doesn’t bend easily, and for part of the installation I had to buy some bendier (and considerably cheaper) hose. I bought another 12’ of ” hose for the toilet inlet and tank breather. The ” ID inlet hose for the Lavac must be looped above the waterline. A small nipple (supplied with the toilet) is inserted into it at its high point. This serves to break the vacuum after the toilet has flushed and also acts as an anti-siphon device.

The Lavac’s operation takes some getting used to. All you do (after opening the inlet seacock) is close the lid and give ten strokes on the pump handle, wait five seconds, then give another half-dozen strokes. That’s it. The toilet lid and seat will remain as if glued to the bowl for a minute, until the vacuum releases. The Lavac’s pump is a massive Henderson Mk V, of the type often used as manual bilge pumps. Indeed, were I to install a diverter valve between the toilet and the pump, I could add a length of hose in readiness to act as an emergency bilge pump. But having gone to all this trouble to de-complicate the system, that’s the last thing on my mind. Note that the estimated job time below is just that, an estimate. Each boat will have different requirements. It should be an easy weekend job, structural modifications excepted.

Our new sanitary system is a big hit. The stale-head odors are long gone, and we sleep much better in the V-berth knowing that underneath us resides a nice clean nylon spinnaker instead of some gurgling, seeping monstrosity of a bladder.

Approx job time: 12 hrs
Skill level: Moderate/Easy

MATERIALS
14-gal holding tank $180
Hose fittings $20
Hose $108
Lavac toilet $350
Aluminum angle $50
Plywood $20

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