Waste grey water from washing-up and bathing goes down the plug and straight into the canal.
Showers and baths will usually have assistance from a pump as they sit lower than the waterline and the waste water has to be lifted to be evacuated.
Kitchen sinks and bathroom vanity basins tend to be above the level of the canal water so therefore we can let gravity waste the water to the outside.
Standard domestic water traps can be used under plug holes on sinks and basins but are not absolutely necessary. They are useful for the retrieval of rings or earings that may be inadvetantly dropped down the plug hole however!
In any case the standard type of pipe used for waste water is clear PVC hose reinforced by polyester yarn. Widely available from most chandlers
, it comes in varying diameters to suit most plumbing connections and through-hull skin fittings or drain outlets. When planning the position of sinks relative to external drainage fittings it is important to ensure a downward flow from sink to fitting and to try and keep the run of hose as straight as possible. Attach the hose to the through-hull drain fitting and sink outlet with good quality stainless steel hose clips. It's good practise, although not absolutely necessary on inland boats, to use two hose clamps at each end of the hose for extra security. Include periodic checks of all hose connections as part of your maintenance plan. Regular expansion and contraction of plastic hoses can loosen clips over time causing hard to detect leaks into hard to access bilges.
Incidentally the Boat Safety Scheme requires that through hull outlets are at least 10 inches above the waterline.