Bilge Pumps Explained: What They Can and Cannot Do

What a Bilge Pump Is Actually For

Every boat leaks a little. A traditional stuffing box is meant to drip; a hatch seal weeps in heavy rain; the shower drains to a sump; the ice box melts into the bilge; condensation runs off the inside of the hull all winter. On a powerboat with an outboard, spray over the transom and rain in the cockpit do the same job. That water, a few litres a day, is what the bilge pump was designed to move, and it does it well. Left alone it rots the sole, corrodes the wiring and stinks.

The emergency role is different in kind, not degree. The numbers in the table above are not exotic: a 38 mm cockpit drain is a standard fitting, a 50 mm stern tube is what the propeller shaft runs through, and the hose that failed on both was probably held by a single rusted clip. Against those openings, the pump is a stopwatch. It keeps the water off the batteries for long enough to find the hole, and it tells you (if you have wired it to tell you) that the water is coming in at all. The fix is always upstream of the pump: close the seacock, drive in the softwood plug, drop a collision mat or a cushion in a bin bag over the outside, stuff a towel into the stern tube. Every one of those stops more water in a second than the pump moves in a minute. Buy the pump for the drips, and buy the time it gives you with the plugs.

The Four Kinds of Pump

The names on the shelf hide two mechanisms and two power sources. Which one you want depends on the job, and most boats need more than one.

Electric centrifugal

The blue or white submersible on the shelf. An impeller spins inside a housing and throws water outward into the discharge; there are no valves and nothing to prime, which is why it is cheap, quiet and tolerant of a bit of grit. The weakness is pressure. A centrifugal pump generates very little of it, so every centimetre of lift and every corrugation in the hose eats output. Manufacturers publish curves for this: a pump rated 2,000 GPH at zero head typically delivers somewhere around 1,400–1,600 GPH at 1 m of lift through smooth hose, and less through corrugated. Add the 10–15% you lose when the battery is at 12.2 V rather than the 13.6 V of the test bench and the honest figure is about half the box.

Electric diaphragm

A motor flexes a rubber diaphragm; inlet and outlet valves turn the flexing into flow. It creates real suction, so it sits high and dry in a locker and draws through a hose with a strainer on the end, lifts 2–3 m without complaint, and runs dry for hours without harm. It moves a third of the water a centrifugal of the same price does, and the valves object to anything bigger than a grain of sand. Right for a shower sump, for a shallow bilge that a submersible would leave 2 cm of water in, and for a boat whose lowest point is somewhere no submersible will fit.

Manual diaphragm

A Whale Gusher or its equivalents: a big diaphragm, a lever and a person. Rated around 60–105 L per minute at a brisk stroke rate, which nobody sustains for long, so plan on half that from a tired crew member. It needs no electricity, does not care if the batteries are under water, passes debris that would stall an electric pump, and can be fitted so that it is worked from the cockpit with the companionway shut. That last point is why the offshore rules insist on it. The ORC Special Regulations require permanently installed manual pumps for offshore categories, one operable from on deck with the hatches closed; the RYA's guidance for offshore cruising says the same in fewer words. Keep the handle clipped beside the pump, not in a locker under a sail bag.

Engine-driven

A Y-valve on the engine's raw-water intake lets the cooling pump draw from the bilge instead of the sea: the largest pump most boats will ever have, with two conditions. The engine must be running, which stops being certain once water reaches the starter, and the intake must be strained, because the impeller will not survive a bilge full of sawdust and zip ties. Label the valve and work it once a season with the engine running, so the crew has done it before the day they need to.

How Many, and Where

Two electric and one manual is the working minimum for any boat with an inboard engine or a cabin, and the two electric pumps are not interchangeable.

• Small automatic pump, lowest point of the bilge. 500–1,100 GPH is plenty. It runs several times a day and it is the one that gets clogged, the one whose float switch sticks, and the one you replace every few years. Its job is to keep the boat dry, and its counter is your leak detector.
• Large pump, mounted 10–15 cm higher, on its own switch. 2,000 GPH and up. It never runs in normal life, so it is not worn and not fouled, and its switch doubles as the high-water alarm: if this switch trips, something is wrong. Wire a loud buzzer or a red light at the helm to it.
• Manual pump, worked from the cockpit. With its intake in the deepest part of the bilge and a strum box (a strainer) on the end so it does not swallow the same rubbish that stalled the electric pumps.

Boats with more than one watertight compartment need a pump in each, because water does not walk through a bulkhead to reach the pump. Catamarans need the whole set twice. An open powerboat with a self-draining cockpit still wants an automatic pump under the deck, because the compartment below the cockpit floor is exactly where a leaking livewell hose fills unseen. Sizing, as a rule of thumb: about 1,000–1,500 GPH primary for boats under 8 m (26 ft), 2,000 GPH for 8–12 m, 3,500 GPH and above beyond that, and the table above is the reason those numbers are for comfort rather than for damage control.

The Switch: Where Bilge Pumps Really Fail

Ask any surveyor what kills bilge pumps and the answer is the switch, not the motor. The classic float switch is a hinged arm with a plastic float and a mercury or ball contact inside; water lifts the arm, the contact closes, the pump runs. It fails in three ways, all of them in the direction of "stuck". Debris jams the hinge, either up (pump runs forever, battery flat by morning) or down (pump never runs, boat fills). A film of oil or diesel on the water coats the float and glues it. And the float itself absorbs water over the years until it no longer floats. Fit the switch inside a cage or a length of guard, mount it slightly higher than the pump strainer so the pump does not cycle on a film of water, and replace it on a schedule rather than on failure.

Electronic switches have no moving parts. Field-effect and capacitive sensors detect water touching a pad, some ignore oil, and most add a run-on delay. Their failure mode is a sensor coated in grime that either reads "wet" or never does. Whatever the type, the test is the same: make it think there is water and listen for the pump.

And add a counter. A cycle counter on the automatic pump circuit is a few pounds of hardware and it turns the pump into a diagnostic instrument. A boat that cycles four times a weekend in September and nineteen times a weekend in October has a new leak, and the counter tells you before the water on the sole does. A high-water alarm on the second switch is the other half of it: a pump that runs silently is a pump that cannot warn you.

Wiring: Straight to the Battery

The automatic pump has to run when the boat is shut down, locked and unattended, which means it cannot hang off the main panel that you switch off when you leave. ABYC H-22 permits an automatic bilge pump circuit to be connected on the battery side of the battery switch, provided it has its own overcurrent protection close to the battery. So: a fused feed from the battery to a three-position switch at the panel (automatic / off / manual), the automatic position through the float switch to the pump, the manual position straight to the pump. On the common three-wire pumps the brown wire is manual, brown-and-white is the automatic feed through the switch, black is negative. Check the sheet in the box; the colours are a convention, not a law.

Size the wire for the pump's draw over the full round-trip length of the run and a 3% voltage drop, not the 10% you might accept for a cabin light: a centrifugal pump is a motor, and a motor at low voltage moves less water and draws more current doing it. A 2,000 GPH pump pulls roughly 8–10 A at 12 V; on a 10 m run it wants something around 4 mm² (12 AWG) wire. Every splice goes above the highest water level the bilge could ever reach, in adhesive-lined heat-shrink, and the fuse at the battery is sized to the pump manufacturer's figure, not to the wire. The most common bilge pump fault after a stuck switch is a corroded butt connector two centimetres below the water the pump was supposed to remove.

Hose, Loop, Outlet: the Plumbing That Halves Your Pump

Corrugated bilge hose is convenient because it bends round anything, and it costs you roughly a quarter to a third of your flow in friction compared with smooth-bore hose of the same size. Use smooth-bore, at the pump's own diameter (28 mm or 1 1/8 in for most 2,000 GPH pumps), with the shortest run and the fewest bends you can manage.

The outlet goes through the hull well above the waterline, and on a sailing boat that means above the waterline when the boat is heeled too, which on a leeward outlet it often is not. If the outlet can go under, the hose must rise in a high loop above the heeled waterline before it comes down to the fitting, or the sea will siphon back into the boat through your bilge pump. A vented loop, with a small anti-siphon valve at the top, breaks that siphon for certain. Yes, the loop adds head and costs a little more flow. It is worth it.

Then the check valve debate, which is not much of a debate. A one-way valve in the discharge stops the column of water in the hose running back into the bilge after each cycle, which is why people fit them: the pump stops cycling every two minutes. But a check valve is a restriction that costs flow, it is a place for debris to lodge, and when it sticks shut the pump runs against a closed pipe and you find out when the bilge is full. Fix the cycling with a shorter hose, a switch set a little higher, or a diaphragm pump instead. Leave the valve in the shop.

Pump Keeps Running: the Diagnosis

A bilge pump that will not stop is either lying to you or telling you something important, and the order of investigation matters.

1. Look in the bilge. Dry, or a film? The switch is stuck up, fouled or its float is waterlogged. Turn the pump off before the battery is flat, and clear or replace the switch.
2. Water present, level not rising. Watch one cycle. If it pumps down, stops, and the water comes back in a minute, it is the hose emptying back down: the discharge dips below the outlet somewhere, or the outlet is under the heeled waterline. Reroute, do not fit a check valve.
3. Water present and arriving. Taste it. Fresh is a water tank, a pipe joint, rain through a stanchion base or the shower sump. Salt is the sea, through the stuffing box, a seacock, a hose, the head or the engine raw-water circuit. Oily or diesel-smelling points at the engine. Now follow the water uphill, starting with every seacock and every hose on it.

A pump that suddenly runs more often between trips than it used to is the same diagnosis in slow motion, and it is the cheapest warning you will ever get. Salt water in the bilge on a boat that was dry last month is never nothing.

Test Before Every Trip

Lift each float switch by hand and listen for its pump. Flip the manual override and hear the pump run. Give the manual pump ten strokes and feel it draw. Look at the counter and write down the number. It takes two minutes and belongs in the pre-departure checklist alongside the seacocks and the fuel. Once a season, pour a bucket of water into the bilge and watch it go, because a pump that hums when you lift the switch may still be pumping nothing through a blocked strainer. Softwood plugs for every through-hull fitting, tied to the fitting with a lanyard so they are where the hole is, complete the set; they are on every safety equipment list for the reason this whole article has been making.

The Numbers That Tell You the Boat Is Leaking

The bilge pump is the one system aboard whose history matters more than its present. A pump that ran four cycles last trip and nineteen this trip is a leak diagnosis, but only if last trip's number was written down. The same goes for the date the float switch was replaced, the season the manual pump had its diaphragm changed, the day you last poured a bucket in and timed it out. The Marine Logbook app keeps that as a maintenance record per system: pump test dates, switch replacements, counter readings at departure and return, with the trend visible instead of buried in a notebook that lives in the nav table. A rising count between trips is the earliest leak alarm a boat has, and it costs nothing but the habit of reading the counter.

The Bottom Line

A bilge pump cannot save a boat. It can tell you the boat needs saving, if you have wired it to a counter and an alarm, and it can buy the minutes it takes to shut a seacock or drive in a plug. Fit a small automatic pump low for the drips, a big one higher for the day, a manual one for the batteries' sake, wire the automatic feed straight to the battery through a fuse, use smooth hose and a high loop and no check valve, and lift every switch before you leave the dock. Then buy the plugs, because the plugs are the pump that works.