Bilge pump keeps running: what it means and what to do first

Your bilge pump is supposed to cycle on, move water, and stop. When it runs constantly - or kicks on every few minutes for no obvious reason - that cycle is broken, and the boat is telling you something. Whether you just came back from a rainy weekend or you walked down to the dock on a clear Tuesday and heard the pump humming, the four questions below will tell you in about five minutes whether this is a nuisance glitch or a reason to cancel your trip offshore.

Start here: the four-question diagnostic

Work through these in order. Most owners who panic and immediately start pulling floorboards skip step one and waste an hour.

1. Did it rain recently, or did you run through chop? Rainwater enters through deck hardware, hatches, improperly seated lazarette lids, and cockpit drains that feed the bilge under load. If there was two inches of rain overnight and the pump ran a few times and then stopped, that is almost certainly normal. Check whether the bilge is now dry. If it is, log the incident and move on.
2. Is the float switch stuck or fouled? Lift the float switch by hand. If the pump stops immediately when you lift it and restarts when you release it, the switch is working. If the pump keeps running with the switch lifted, the auto circuit has a wiring issue. If the pump stops but the switch does not return to the off position freely - sticky, coated in oil film, or jammed against the hull by debris - clean it and test again. A fouled float switch is the single most common cause of a pump that runs with a dry bilge.
3. Is the pump actually moving water, or just running dry? Shine a light into the bilge. If the bilge is essentially dry and the pump is running, the float switch is stuck in the on position - fix the switch, not the pump. If there is water and the pump is running but the level is not dropping, the pump may be overwhelmed (a real leak) or the discharge hose has a blockage or a failed check valve letting water siphon back in.
4. Can you identify where the water is coming from? If the bilge keeps filling after a dry spell, you have a source. The common sources are listed in the section below. Find the source before you go anywhere.

The most common water sources (ranked by frequency)

Not all bilge water is a crisis. The table below gives you a triage guide - what to look for, what it costs to fix yourself, and when to haul the boat.

Float switch: the quick test and common fixes

Ninety percent of nuisance bilge pump runs trace back to the float switch. Oil film, a fragment of fiberglass dust, or even a twist tie that drifted in on the last trip can hold the float in the up position. Before you replace the pump, spend three minutes on the switch.

Wipe the switch body and the float arm with a clean rag soaked in fresh water. Check that the float swings freely through its full range. Check that nothing in the bilge - a loose hose, a chunk of insulation - is pressing the float upward. If the switch is more than four or five years old and showing corrosion at the wiring terminals, replace it. A quality automatic float switch costs $15-40, takes 20 minutes to swap, and lasts years if you keep the bilge clean. The wiring for it - fusing, wire gauge, connections - is covered in our boat electrical basics guide if you want to do a proper installation rather than a straight swap.

One thing to know about the wiring before you dig in: per ABYC E-11, the bilge pump circuit should be on its own dedicated fused run that stays live even when the main switch is off. If you find the pump wired through the main battery switch, that is a correction worth making - a pump that cannot run because someone turned off the house switch is a pump that cannot save your boat while you sleep in the v-berth.

Shaft seals, bellows, and the through-hulls: finding the real leak

If the bilge fills even on a clear dry day with no recent rainfall, you have water coming in from somewhere below the waterline or through the running gear. Here is how to work through the likely candidates.

Inboard shaft seal (stuffing box). A traditional packing gland is supposed to drip - about 1 drop every 15-30 seconds while the shaft is turning is the target rate. More than that while tied up at the dock is a packing gland that needs adjustment or repacking. Find it by following the propeller shaft from the prop back into the bilge. Look for a brass or bronze fitting with a large hex nut. Water dripping from that fitting at anchor or at the dock (shaft not turning) is too much. Tighten the packing nut a quarter-turn at a time; if you cannot stop the flow without the nut hitting the end of its travel, the packing material needs replacement.

Sterndrive bellows. On an IO (inboard/outboard) boat, the bellows are the accordion-style rubber boots that protect the universal joints where the drive passes through the transom. Bellows failure is the highest-consequence maintenance failure on a sterndrive - a split bellows lets water into the engine compartment continuously, and boats have sunk at the dock because of it. If your bilge is filling and you have an IO drive, open the engine hatch and look at the transom area around the bellows. Water staining, mineral deposits, or a wet transom plate are your signal. This is a haul job; do not go offshore with a suspect bellows. Manufacturer service manuals (MerCruiser, for example) typically specify bellows inspection at each annual service and replacement at roughly every 3-5 years or 200 hours, whichever comes first - confirm the interval in your specific engine's service manual.

Through-hull fittings and seacocks. Every below-waterline fitting is a potential failure point. With the boat in the water, run your hand around every through-hull fitting you can reach - raw-water intake, livewell, depth transducer, cockpit drain. Weeping around the fitting flange or a seacock that resists movement are the two signs of trouble. The real danger is not just a seacock stuck open - it is a seacock that cannot be closed on demand, whether it is frozen open or frozen in any position. Exercise every seacock quarterly: turn it fully closed and then fully open again; if it stiffens between inspections, grease it before it seizes. A softwood plug of the right diameter, kept in reach of every through-hull, is the emergency fix if a seacock fails while you are afloat. Permanent repair requires hauling and working on the fitting from outside the hull.

When the source is genuinely unknown: the stop-line

If you have worked through all four diagnostic questions, checked the float switch, confirmed the pump is moving water and not just spinning, and you still cannot locate where the water is coming from - do not go offshore. Stay in protected water or haul the boat. A pump that runs continuously against an unknown source is giving you one piece of information clearly: the rate of ingress is keeping up with or exceeding the pump's rated output. A Rule 1500 GPH pump running flat-out moves 25 gallons a minute. An unknown structural leak can outpace that quickly, especially at speed when hull pressure changes.

This is not a "use an abundance of caution" suggestion. It is the practical limit of what DIY diagnostics can accomplish without the boat on the hard, where you can see the outside of the hull and the running gear from below. Get it out of the water.

If you are at the dock and the water level in the bilge is rising faster than the pump can handle, close every seacock you can reach, call for assistance, and keep the engine running to keep battery voltage up for the pump. BoatUS Foundation data shows that a large proportion of recreational boat sinkings happen at the dock, overnight - most of them from slow leaks that went undiagnosed because the pump masked the symptom for weeks before the battery finally died.

Checking pump output: is the pump actually keeping up?

A pump that runs constantly but is genuinely moving water is doing its job - the question is whether it can do its job long enough for you to find the source and fix it. Knowing your pump's rated capacity and comparing it to how fast the bilge is filling gives you a useful working picture.

Pump GPH ratings are measured at zero head (no vertical lift). Real-world output through a discharge hose that exits above the waterline is typically 30-40% lower than the nameplate figure. A pump rated at 1500 GPH may deliver 900-1000 GPH in a real installation. If your bilge is filling a few inches per hour and clearing between cycles, a 500-750 GPH pump is probably keeping up. If the level creeps upward even while the pump runs, the ingress rate has exceeded the pump's real-world capacity and you have an emergency, not a maintenance item.

Pump installation quality - wire gauge, fuse sizing, and discharge hose run - affects how well the pump performs. A pump wired with undersized wire will run slow and underperform its rating. Parasitic current draws that flatten your battery overnight will leave you with a dead pump by morning. Our guide on boat parasitic draw walks through how to find what is draining your battery while the boat sits, which matters here because a pump running on a depleted battery is not running at rated capacity.

After you fix the source: a few things to do before you call it done

Once the leak is identified and fixed, take 20 minutes to set the boat up so you get earlier warning next time.

• Test the float switch manually every few months. Lift it by hand and confirm the pump starts. Let it drop and confirm it stops. This takes 30 seconds.
• Keep the bilge clean. Debris, oil residue, and accumulated sludge are the main reasons float switches stick and foul. A clean bilge is also the only way to visually spot a new leak early - you cannot see a fresh trickle in a black swamp.
• Check the discharge hose annually for kinks, cracking, and a functioning check valve. The check valve prevents siphon-back when the discharge port dips below the waterline at heel. Without it, the pump runs continuously trying to manage water that is re-entering through its own discharge.
• Know what your battery can sustain. A bilge pump running overnight on a single group 24 battery can drain it flat before morning. If the boat sits unattended, a battery maintainer and a functioning panel alarm are cheap insurance. The winterizing process - covering battery care and how to store a boat properly - touches on this in our winterizing guide, because deferred electrical work and a dead battery are exactly how boats sink during a first freeze.

A pump that keeps running is a warning, not a catastrophe. Most of the time the fix is a $20 float switch or a morning chasing rain around the deck hardware. But the pump is also the only thing standing between a slow seep and a full bilge, and it deserves the same diagnostic discipline you would give any other system on the boat.