Bilge Pumps on Boats, What They Do and How They Work

The right tool for this job

Automatic Bilge Pump For Boats Auto Float Switch

Built in float switch activates when water rises above the base height. Choose 750 GPH or 1100 GPH. 12 volt pump with a compact footprint for small watercraft and tight bilge spaces.

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Bilge pumps on boats remove water from the bilge, the lowest part of the inside of a boat, and push it out through a hose and a hull outlet. An electric bilge pump sits in the bilge and can turn on when water rises. Water belongs on the outside of the hull, and a working bilge pump is the gear that handles the everyday water that gets in anyway.

It is built for "nuisance water" from rain, spray and small drips. It keeps accumulated water from sitting around wiring and gear, and it can help reveal abnormal water entry. A bilge pump does not replace fixing a leak or emergency damage control equipment.

What Is the Bilge on a Boat?

The bilge is the lowest compartment inside the hull, where water collects because water runs downhill. Its location and drainage depend on the boat's layout. It may sit under the floor or engine area, near the keel on a sailboat, or at the back of a compartment on a small powerboat. Water drains to the lowest accessible area within a connected compartment, along with oil, dirt and debris.

That water can come from several places.

• Rain through a hatch that was left cracked open.
• Spray coming over the bow in choppy water.
• Minor drips from a shaft seal or a plumbing fitting.
• Runoff from washing down the deck.

Left alone, standing water in the bilge corrodes wiring and fittings, creates odors and adds weight low in the boat. For that reason, a dry bilge is the base of a healthy boat.

What Does a Bilge Pump Do on a Boat?

A bilge pump moves water out of the bilge and overboard. It does this automatically or on command, depending on how it is wired. Its main mission is steady, low level water management. Keeping this gear working is part of a well-equipped boat, as covered in the boat safety equipment checklist. For required equipment on your boat, check US Coast Guard and state rules, summed up in what the Coast Guard requires on your boat.

Keeping that space clean matters too, and a bilge cleaner concentrate means less hand scrubbing in the bilge.

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Bilge Cleaner Concentrate

Clean the bilge without crawling into every nasty corner with a scrub brush.

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How Does a Bilge Pump Work?

An electric bilge pump works in three parts. A float switch (or built in sensor) detects rising water. A small motor spins an impeller that pulls water in through the intake. The water is then pushed out through a discharge hose to a hull outlet above the waterline.

When the float rises with the water, it closes the circuit and the pump runs. As the bilge empties, the float drops and the pump shuts off. This is why an automatic pump can watch the boat while nobody is on board.

What Does a Bilge Pump Look Like on a Boat?

A typical submersible bilge pump is a compact, rounded plastic unit mounted flat on the bottom of the bilge. Size and wiring vary by model. A pump may have a grated base or strainer, a hose fitting on the side or top, power wires, and sometimes a float switch attached or built in. The hose runs up to a small outlet fitting in the hull side. For reference, the Better Boat automatic bilge pump has a footprint of 5.8 inches by 3.3 inches by 4.1 inches high.

When to Use a Bilge Pump on a Boat

An automatic pump should be ready to run whenever the boat is in the water, including overnight in a slip. Use the manual setting on the switch panel when you want to clear water on command, for example after a rainstorm, after washing the deck, or when you hear or see water in the bilge. If a pump runs often or never shuts off, treat that as a sign to look for the source of the water.

A pump that keeps running while water keeps coming in is a warning that needs attention right away. See the questions below for the usual causes.

Choosing Your Bilge Pump, Electric vs. Manual

Electric pumps run on the boat's power and can switch on automatically. Manual pumps work by hand and need no power. Each suits a different job, and a boat can carry both.

Electric Bilge Pumps

Electric pumps handle the small amounts of water from rain, spray or a leaky fitting, often automatically. Two designs are used.

• Submersible centrifugal pumps sit in the bilge, submerged, and use a spinning impeller to move water up and out the discharge hose.

• Diaphragm pumps use a flexible membrane that pulses to pull water in and push it out. They are self-priming, so they can be mounted above the bilge with an intake hose run down into the water, and they can suit water with small bits of debris. Follow the maker's instructions for mounting.

For more on the submersible style, see this guide to submersible bilge pump options.

Automatic Bilge Pumps With a Float Switch

An automatic bilge pump is an electric pump paired with a switch that senses water. A float switch is a small buoyant lever that rises with the water to start the pump and drops to stop it as the water recedes. Some pumps have electronic sensors that do the same job.

The Better Boat Automatic Bilge Pump

This 12 volt pump has a built in float switch that activates when water rises above the base height. The grated base helps keep debris away from the intake, and the compact footprint suits tight bilge spaces.

A manual-switch electric pump only runs when someone flips a switch, so it gives no protection when nobody is aboard.

The Manual Bilge Pump as a Backup

Electric pumps depend on battery power. If the electrical system goes down, so do the pumps. A manual bilge pump works by hand and needs no electricity, which makes it a useful backup on a small boat. Check US Coast Guard and state rules for what your boat must carry.

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Manual Hand Bilge Pump

Move unwanted water from kayaks, canoes, dinghies, and small boats when power is not available.

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For paddle craft where a larger hand pump is too much, the Small Floating Bilge Pump is a manual pump with a foam grip that floats and a 24 inch hose.

A boat can use a combination, with an automatic electric pump for everyday water, a higher capacity electric pump where the boat size calls for it, and a manual pump as a final backup. Learn more in why having a manual bilge pump is so important.

Sizing Your Bilge Pump

The big number on the box is Gallons Per Hour (GPH). That rating is measured under ideal conditions, with no hose bends and little lift, so real output once installed is lower.

Understanding Head Height

Head height is the vertical distance the pump pushes water, from the bilge up to the hull outlet. Every foot of lift, every sharp turn in the hose and the ridges inside a corrugated hose add resistance and reduce output. Use the manufacturer's performance chart to see the pump's output at your head height, then measure that distance on your boat.

How Much Pump Capacity Does a Boat Need?

The capacity a boat needs depends on its design and hull type, the head height, the hose routing and how the boat is used. There is no single number that fits every boat of a given length. Start with the boat builder's guidance and the pump maker's performance chart, and check US Coast Guard and state rules. Choose a pump whose rated output at your measured head height meets the boat builder's guidance, and match the wiring, fuse or breaker, hose and through hull requirements in the pump instructions. The Better Boat automatic pump comes in 750 GPH and 1100 GPH versions, so match the version to your boat size and water removal needs.

Bilge Pump Installation Step by Step

Installing a bilge pump is a project a boat owner can do with the right parts and the pump's instructions. Four steps cover it.

Step 1 Locating and Mounting the Pump

Mount the pump at the lowest point of the bilge where water settles, using the mounting method in the pump instructions and approved by the boat builder. Clean the surface of old gunk and sealant. Before drilling anything, verify clearance and material thickness, and check both sides for hidden wiring and plumbing. Never drill into the hull skin unless the boat builder specifically approves it. Where fastening into fiberglass is approved, follow the pump instructions for hole size and screws.

Mounting the pump too high can leave water standing around it, so take time to find the true low point.

Step 2 Routing the Discharge Hose

Run a smooth-bore discharge hose from the pump outlet to a through hull fitting above the waterline. Match the hose size to the pump outlet.

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Bilge Pump Hose Kit

Includes a 6.5 foot hose, through hull fitting, discharge outlet fitting and two stainless clamps. Choose 3/4 inch or 1 1/8 inch hose size to match your pump.

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A bilge pump hose kit sized to match your pump keeps the hose and fittings in one place.

Avoid corrugated hose where possible, because the ridges add friction and reduce flow. Run the hose with as few bends as possible, using gentle curves instead of kinks. Secure the hose with the supplied clamps according to the pump, fitting and hose kit instructions. Add extra clamps only where those instructions or the boat builder call for them.

If the through hull fitting sits close to the waterline or may dip under when the boat heels, a siphon can pull water back into the bilge. A vented loop at the highest point of the hose run can break that siphon. Check the pump maker's instructions and your boat builder's guidance for your setup.

Step 3 Bilge Pump Switch Wiring

A weak electrical connection can leave a dead pump when it is needed most, so every connection in the bilge should be moisture and corrosion resistant. Bilge pump wire colors vary by maker, so follow the instructions that come with the pump to identify positive and negative. The negative wire normally goes to the boat's negative bus bar.

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Bilge Pump Switch 3 Way

Manual and auto modes with red and green LED status lights and a resettable 15 amp circuit breaker. Includes instructions and a wiring diagram.

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• For an automatic pump, a 3 way bilge pump switch panel gives manual and automatic modes from one panel. The Better Boat panel works with a standard float switch or an automatic pump on 12V, 24V and 32V systems, has red and green LED status lights and a built in resettable 15 amp circuit breaker, and includes a wiring diagram, stainless screws, coated crimp ends and a jumper cord.
• For a manual-switch pump, the positive wire runs from the pump to the switch panel, which connects to a fused power source.

Use marine-grade heat shrink connectors, which have adhesive lined tubing that seals around the wire when heated. Always fuse the circuit as the pump and panel instructions call for, and replace a blown fuse with one of the same amperage.

Step 4 Sealing the Through Hull Fitting

Choose the outlet location by following the boat builder, fitting and pump instructions. Inspect both sides of the hull for structure, wiring and plumbing before drilling, and account for the boat's loaded and heeled waterline so the outlet stays above the water. If the location is uncertain, use a qualified marine installer. Drill the hole size the fitting instructions call for.

Before pushing the fitting through, apply marine sealant under the flange as the fitting and sealant directions call for. On the inside, tighten the backing nut without overtightening, since too much force can squeeze out the sealant. Wipe off excess sealant on the outside. The Marine Sealant and Adhesive Caulk is one option for this step. Follow the label directions.

Bilge Pump Maintenance and Troubleshooting

A bilge pump is out of sight, so it is easy to forget until it is needed. Regular checks help catch problems early. For more detail, see these 7 essential tips for maintaining the bilge pump on your boat.

Routine Maintenance Checklist

1. Test the float switch. Use the test method specified by the pump maker. If it does not start or keeps running, the switch may be jammed or failing.
2. Clear the intake strainer. Remove debris such as hair, leaves, plastic and oily gunk, and make sure the intake is clear.
3. Inspect hoses and connections. Look for cracks, brittleness and kinks, and check that the clamps at the pump and the through hull fitting are tight and not rusting.
4. Check the wiring. Look for corrosion, which often shows as green or white powder, and confirm connections are secure.

A clean bilge helps keep debris away from the intake. To clean it, add a measured dose of Bilge Cleaner Concentrate and let boat motion loosen the grime. Follow the label directions, which call for 1 ounce for every 3 feet of waterline length. Water carrying oil, fuel or grease must be collected and disposed of as the product label and applicable federal, state and marina rules require. Do not pump it overboard.

Diagnosing Common Problems

Pump runs but no water comes out

• Clogged intake or hose. Debris may be stuck against the intake screen or inside the discharge hose.
• Airlock. Air trapped in the pump or hose can let the impeller spin without moving water. Switch the pump off, inspect the intake and the discharge routing, and follow the pump maker's troubleshooting instructions before running it again. Running a pump dry can damage it.
• Damaged impeller. A pump that ran dry for too long can damage its impeller. The motor will run but water will not move.

Pump does not turn on at all

• Blown fuse or tripped breaker. Check this first, and replace a fuse only with one of the same amperage.
• Corroded or loose wiring. A bad connection at the battery, the switch panel or the pump will cut power.
• Dead pump motor. If the fuse and wiring check out, the motor may have failed.

If the whole bilge system is due for a refresh, the Bilge Maintenance Kit bundles a 750 GPH automatic pump, a 3 way switch, a 3/4 inch hose kit, bilge cleaner and a marine degreaser.

Frequently Asked Questions

What does a bilge pump do for a boat?

It removes water from the lowest part of the hull and pushes it overboard through a hose. It does not replace leak repair or emergency damage control equipment.

When should you run your bilge pump on a boat?

Keep an automatic pump ready to run whenever the boat is afloat, and use the manual setting to clear water after rain, a deck wash or any time water is in the bilge. Check your pump and switch instructions for how it should be wired.

Why is my bilge pump running constantly?

A stuck float switch is one possible cause, since debris can jam it in the on position. If the pump is moving water and still never stops, or water is entering continuously or faster than the pump removes it, treat it as an emergency. Follow the boat's emergency plan, have everyone put on life jackets, and notify emergency services or the Coast Guard as conditions warrant. A pump that runs with a dry outlet may have an airlock or a clog. Switch it off and inspect it rather than letting it run dry.

How many bilge pumps should my boat have?

It depends on boat size and layout, so check the boat builder's guidance and US Coast Guard and state rules. A boat can use an automatic primary pump at the lowest point, a higher capacity secondary pump set a little higher, and a manual pump as a backup. Boats with separate sealed compartments may need a pump in each.

Can I leave an automatic bilge pump on all the time?

Automatic pumps are designed to stay ready while the boat is in the water, with the float switch starting the motor only when water rises. Follow the pump maker's wiring and fuse instructions, and see the Step 3 wiring section above.

Keep reading

How to Replace a Bilge Pump on a Boat7 Essential Tips For Maintaining the Bilge Pump on Your BoatFoot Water Pump: Your Guide to Manual Marine Pumps