The bilge pump keeps cycling: causes and what to do

An automatic bilge pump is meant to start from time to time: a little rain, a few drops from the stern gland, some rinsing water. When it starts running every hour, or in short bursts that never stop, it is telling you something. Either water is really getting into the boat, and you need to find where; or the pump keeps pumping the same water round and round, because a detail of the installation fools it. Seen from the pontoon, the two cases look very alike, and they are not at all equally serious. This guide helps you tell them apart, in order. The manuals of the boat builder and of the pump always have the last word.

1. What you notice

• The pump starts often, every hour or more, while the boat is in the marina.
• It runs for a few seconds, stops, then starts again straight away, in short cycles.
• It never stops at all, even with a dry bilge.
• The house battery is flat when you come back to the boat.
• A bilge counter or alarm shows far more starts than usual.
• There is water in the bilge at every visit, sometimes above the usual level.

2. Safety first

If the water level rises fast, or reaches the cabin sole, the priority is to find where the water comes in, not to repair the pump: close the seacocks one by one, keep a manual pump within reach, and alert the marina or the rescue services if the situation gets away from you. In the marina, a pump that runs a lot can also mean a flat battery tomorrow, and therefore a pump that no longer runs at all. Before touching the wiring, switch off the power at the panel or at the battery. Beware of water mixed with diesel or oil: it must not be pumped overboard, and a smell of petrol in the bilge means you air the boat and keep any flame away before doing anything else. Never disconnect the pump’s automatic switch to “get some peace” when you leave the boat.

3. Likely causes, from most to least common

• Water runs back into the bilge after each cycle — the discharge hose holds a column of water; when the pump stops, that water falls back into the sump and lifts the float switch. A missing, damaged or badly placed non-return valve, or a long hose that climbs high before coming down, is enough to create endless short cycles. It is one of the most common causes of a “pump that runs for no reason”.
• Float switch stuck or badly set — a float held up by debris, hair or a greasy deposit, or a fouled electronic sensor, runs the pump all the time, even with a dry bilge. Mounted too low, it triggers the pump for a few centimetres of water.
• Sump too small for the pump — a very powerful pump empties a small sump in two seconds and stops; the little water left between the floors flows back and starts it again. Not serious in itself, but it wears out the pump and the battery.
• Rain or deck water — blocked scuppers, lockers draining into the bilge, tired seals on portlights or deck fittings: after a shower, the pump works hard. If the starts follow the weather, the cause is often there.
• The stern gland or shaft seal — a packed stern gland should drip a little under way, almost not at all at rest. If it runs in a trickle in the marina, or if a lip seal leaks, water comes in all the time.
• A fresh-water leak — fresh-water pump, water heater, a fitting under the sink: the tank empties into the bilge and the bilge pump sends it out. Taste the water (without swallowing): if it is fresh, the cause is on board, not outside.
• Seacock, through-hull or hose — a split hose, a rusty hose clip, a weeping seacock below the waterline: this is the most serious cause, because it can suddenly get worse.
• The cooling circuit, the exhaust or the marine toilet — an engine or toilet raw-water hose can siphon while the boat is at rest, especially without an anti-siphon loop or with a blocked one (see the marine toilet).
• The hull itself — a crack, a keel or rudder that works, the hull-to-deck joint: rarer, but to consider if everything else is dry.

4. What you can check yourself, safely

• Dry the bilge completely with a sponge, then come back a few hours later: has the water returned, and where does it show first?
• Taste a drop of bilge water (without swallowing): salty, it comes from the sea or from deck rain; fresh, look at the boat’s water system.
• Watch the sump during a cycle: if the water jumps back up when the pump stops, it is running back down the discharge hose.
• Lift and release the float switch by hand, power off then on: it should move freely and drop back. Clean the sump, the strainer and the float.
• Run your hand or a paper towel along the accessible hoses, clips and seacocks, and under the stern gland.
• Check that scuppers and locker drains are not blocked and do not empty into the bilge.
• See whether the fresh-water meter, or the tank level, goes down when nobody has used water.
• If you have a pump counter or log, note the starts for a few days: with or without rain, in the marina or at anchor.

To trace the source of water that keeps coming back, the guide water in the bilge lists the places to inspect, and if the battery goes flat at the same time, see batteries that run down.

5. When to stop and call a professional

Call without waiting if the level rises despite the pump, if the water is salty and you cannot find where it comes in, if a seacock will no longer close or if a hose below the waterline is damaged. A marine plumber or electrician can also rework the pump installation: valve, hose run, height of the outlet, float or sensor, wiring and protection. For a leak around the shaft, the saildrive or the keel, you need a yard, often with a lift-out. Describe what you have seen: how often it starts, fresh or salt water, any link with rain, and what you have already checked.

6. Keeping it from coming back

Keep the bilge clean and the sump clear, test the pump and its float at every visit and before each season, and once a year check hose clips, hoses and seacocks, working them open and shut. Have the stern gland adjusted as the manual says. A second pump mounted a little higher, with its alarm, is a useful safeguard if the first one fails. And include the bilge pump in your checklist for laying up and recommissioning.

In the Hullteam logbook, record every pump check, every valve or float replacement and every seacock or hose repair, with the date: a boat whose bilge is looked after reassures a buyer (see which proof counts).