David Witton converts a caravan tank to make a holding tank for his small boat
Holding tanks are becoming increasingly necessary and it can often be difficult and expensive to provide this facility on a small yacht. Pump-out facilities in marinas, harbours and boatyards are still rare, or inconvenient. In one Turkish marina, we were told we must go 12 miles out to sea to pump out – not very convenient in a winter gale! Our system utilises a lift-out tank, which can be emptied into a chemical disposal point or, if not available, down the marina or boatyard’s toilet. It may not have a large capacity, but it will enable you to comply with regulations and avoid polluting swimming bays.
We obtained a caravan ‘cassette tank’ which fitted into an adjacent locker. Caravan waste water containers are available in various shapes and sizes and are usually much cheaper than marine tanks (ours cost £17). It needs to be installed in the locker with its filler horizontal and towards the top.
A transparent vent pipe is fitted slightly lower and acts as a sight tube, warning you when the tank needs emptying.
A 38mm (11⁄2in) ball valve is fitted to the bulkhead and a short flexible pipe is fitted to extend it, at a slight downward angle into the tank. Ensure that there is sufficient room to move the tank away from the ball valve, so it can be lifted out for emptying. On the toilet side of the bulkhead the normal 38mm (11⁄2in) pipe from the toilet pump is cut and a Whale Y-piece inserted and fastened with Jubilee clips. One branch of this is connected to the tank ball valve, while the other branch is reconnected to the seacock via the usual loop. We originally used a plastic diverter valve, but had continual problems with limescale seizing it up. The Y-piece and brass ball valve work much better.
For normal operation as a sea toilet, the tank ball valve is closed and the seacock open. To use the holding tank, close the seacock and open the tank ball valve. To revert to seawater, reverse the process. To empty the tank, pump a little clean water through and then close the valve. Undo the cap and lift the tank out – it pays to have a cloth handy, to catch any drips. Make sure you never operate the toilet pump with both valves in either the open or closed position.
It is advisable to add a little chemical toilet fluid to the empty tank, to minimise any odours.
If you are using the tank when hauled out, you will need to add a little water to each flush. At sea keep toilet flushing to a minimum if you only have a small tank. It is also a good idea to dispose of paper into a separate container.
38mm (11⁄2in) ball valve and two brass taper threads £22
Whale 38mm (11⁄2in) Y-piece £4
90° tank vent fitting £3
Tank from £17. About 22lt (5gal) is the maximum size if it is to be lifted out of the boat
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