Fresh water destroys more timber hulls than the sea does. Decay organisms need fresh water; salt suppresses them. So the tanks, plumbing and drainage inside a vessel represent a larger long-term structural risk than the ocean outside it — and they are the systems most often installed for capacity rather than for inspectability.
The design principle: leaks must be visible
Every tank leaks eventually, every hose fails eventually and every fitting weeps eventually. A well-designed installation accepts this and arranges that when it happens, the water appears somewhere a person will see it and drains to somewhere it can be pumped out. A poorly designed one routes the same failure invisibly into the bilge behind ceiling, under sole bearers or into the ends of the vessel, where it will be discovered by a surveyor years later as soft timber.
Inspect everything
Every tank, every connection and every run should be reachable without demolition.
Drain to somewhere seen
Drip trays and drainage paths that lead to a visible, pumpable point rather than into structure.
Keep fresh water off timber
Isolate tanks and runs from structural members wherever the layout allows.
Tank material and construction
| Consideration | Why it matters in a timber hull |
|---|---|
| Removability | A tank that cannot be removed cannot be properly inspected, repaired or replaced without cutting joinery or structure. |
| Baffling | Free surface movement in a part-full tank affects stability and stresses tank mountings and the structure they bear on. |
| Mounting | Tank weight must land on structure, be restrained against motion, and not chafe or trap moisture against timber. |
| Inspection openings | Access hatches sized for real inspection and cleaning, positioned where they can actually be opened aboard. |
| Ventilation and overflow | Vents routed so overflow goes overboard, not into the bilge. |
Plumbing runs
Runs should be continuous where possible, supported at intervals so they do not chafe, and routed away from structural timber and electrical cable. Every joint is a potential leak, so joints are minimised and placed where they can be seen. Hose selection matters — the right hose for potable water, for waste, for engine cooling and for bilge duty are different products, and substituting one for another is a common shortcut with a predictable failure mode.
Label everything
An unlabelled system is a system that will be misdiagnosed at the worst moment. Labelling tanks, valves and runs at installation costs an afternoon and pays back the first time something goes wrong at sea.
Grey water, black water and the shower tray problem
Grey water — from showers, sinks and galley — is warm, contains organic matter, and is fresh. It is close to ideal for promoting decay. The most common single failure is a shower tray that drains slowly or not at all, allowing water to sit and find its way into the sole bearers and the structure below. Shower drainage should be positive, the sump should be accessible, and the pump should be maintainable without dismantling a cabin.
Black water handling depends on the vessel’s operating area and the requirements applying where it works. Whatever arrangement is chosen, tanks and runs need the same inspectability as everything else, and vents need to be routed so that the system does not make the accommodation unpleasant.
Bilge and limber discipline
Water that reaches the bilge must be able to run to the pump. Limber holes blocked with debris are extremely common and turn the area beside every floor into a small permanent pond. Clearing and, where necessary, enlarging limber paths is low-cost work done at refit and almost impossible afterwards. The consequences of getting it wrong show up as soft floors and wasted fastenings, described in hull repair and caulking.
Sequencing
Tanks and major plumbing runs are installed in the systems phase — after structure is complete and inspected, before joinery closes access. Installing tanks early means working around them during structural repair; installing them late means dismantling new interior. The phase discipline is described under refit and restoration, and the practical planning under planning a liveaboard refit.
What to hand over
A schematic of the systems as actually built — not as designed — with tank capacities, valve positions, hose specifications and service points marked. Vessels are routinely sold with none of this, and every subsequent owner pays to rediscover it.
Commissioning and testing
Systems are pressure-tested and run before the interior closes over them, not afterwards. Every tank is filled and left standing long enough for a slow weep to show itself; every joint is inspected under pressure; every pump is run through its full cycle; and every valve is operated to confirm it does what its label claims. This takes a day and it is the last opportunity to find a fault while access still exists.
The commissioning record then becomes part of the handover pack, listing what was tested, how, and with what result. On a vessel that will spend time far from support, that document is worth keeping aboard rather than filing ashore — the person diagnosing a problem at three in the morning is usually the person who most needs to know how the system was actually built.