Why does my boat's bilge pump keep running, and how can I find the cause?
The automatic bilge pump on my 34-foot sailing yacht is coming on every few minutes, sometimes running for much longer than usual. There is always some water left in the sump, but I cannot see where it is coming from. How do I tell whether I have a leak or a pump fault?
Answer
Your bilge pump keeps running because water is entering, the pump cannot discharge properly, or its switch is staying on. First check whether the bilge level falls while the pump runs and whether water comes out of the discharge outlet; these two checks separate the main causes.
Check water level and discharge
Lift the sole boards, light the sump and mark the water level with a pencil or tape. Watch a complete pumping cycle, including what happens immediately after the pump stops.
- The level falls and stays down, but the motor continues: suspect a stuck float switch, fouled electronic sensor or manual override.
- The level hardly falls and discharge is weak: suspect a blocked strainer, damaged impeller, obstructed hose or low voltage.
- The pump clears the sump, then water returns immediately: suspect discharge-hose drain-back or siphoning.
- The level falls, then gradually rises again: trace incoming water.
If the level rises despite pumping, put on lifejackets, start the manual pump, close accessible seacocks where safe and call for assistance before continuing diagnosis. Keep automatic pumping available while investigating an active leak.
Check the switch and pump
Check the helm switch first: a three-position switch may have been left on MANUAL rather than AUTO. Once the water level is under control, isolate the pump supply before putting fingers near its impeller or wiring.
Lift and lower a separate float switch. It should move freely without catching a hose, cable or the sump wall. Remove hair, cable ties and oily sludge from the strainer and switch; clean electronic sensors according to their instructions.
Some automatic pumps run briefly at regular intervals to sense water load. Check the instructions for your exact model before treating these short checks as a fault. Continuous running needs investigation.
If discharge is weak, inspect the entire hose for kinks, collapsed sections and blockages. Check voltage at the pump while it runs: a sound battery reading does not rule out losses through corroded terminals, fuse holders or undersized cable. Match replacement hose to the pump outlet; reducing its diameter restricts flow.
Trace incoming water
Pump down, sponge accessible compartments dry and place strips of dry kitchen roll beneath suspect fittings. Follow the first wet patch upstream, checking neighbouring compartments and limber holes through the floors. Water can travel beneath an engine tray or moulded liner without leaving a visible trickle down the main bilge.
Do not taste bilge water. If you need to distinguish fresh water from seawater, use a clean sample and a salinity meter.