Why is there water in my boat's hull, and how do I find the source?
I keep finding water under the cabin sole, and it seems to trickle forward from beneath the engine. How can I work out whether it is rainwater, a plumbing leak or seawater, and which fittings should I check first?
Answer
Water inside the hull usually comes from rain entering through deck fittings, leaking freshwater plumbing, or seawater entering through underwater fittings or engine equipment. Dry the bilge, identify the water and record when it returns: this narrows the search far faster than resealing fittings at random.
Check the rate and water type
If the level is visibly rising, check whether the bilge pump is keeping up before tracing the leak. For rapid ingress, put on lifejackets, operate the pumps, close accessible seacocks where safe and call for assistance if you cannot control it.
For a small, persistent accumulation, pump out and sponge the accessible compartments dry. Use a torch and mirror to inspect beneath the engine, tanks and cabin sole. Water can travel along stringers and through limber holes before reaching the main bilge. The wettest compartment may be some distance from the leak.
- Saltwater: suspect seacocks, hose connections, shaft seals, raw-water cooling or deck leaks admitting spray.
- Freshwater: suspect rain, tanks, domestic plumbing, the calorifier or condensation.
- Coloured or oily water: investigate engine coolant, contaminated engine-tray water or another machinery leak.
Use a salinity meter or chloride test to compare a sample with the surrounding water. Do not taste bilge water. At freshwater or brackish moorings, salinity alone will not reliably identify the source.
Trace the first wet point
Lay strips of dry kitchen roll beneath hose joints, around accessible fittings and across suspected water paths. Check them frequently. The first strip to become wet tells you more than a puddle discovered hours later.
Record whether water appears after rain, overnight at rest, with the freshwater pump switched on, after heating water, or during engine operation. Test one condition at a time.
On a Hallberg-Rassy 34, for example, water travelling forward through a starboard limber hole beneath the engine can originate at the calorifier. The hole is a drainage route, not necessarily a hull defect. Do not block limber holes to stop the flow; that simply hides water elsewhere.
Check plumbing and the calorifier
With every tap shut, listen for the freshwater pressure pump. Repeated cycling suggests a pressure loss somewhere, although a faulty pump non-return valve can produce similar symptoms. Inspect tank outlets, pump connections, shower mixers and pipe joints before dismantling cabinetry.
Check the calorifier early, particularly if water appears after motoring or using shore-power water heating. As water heats and expands, its pressure-relief valve may discharge a small quantity.