What Is a Bilge Pump and How Does It Work?

A bilge pump is one of the most crucial safety features of a boat. Without a bilge pump, excess outside water or leaks can cause a boat to flood or sink. Consequently, these pumps save lives, making them essential to any vessel. There are many types of bilge pumps on the market, and each discharges water differently. Because of this, selecting a bilge pump for your boat or marine application can be difficult. Below, you can find information about what bilge pumps are, how they work, the types of bilge pumps, and how to perform maintenance on your bilge pump.

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What is a bilge pump?

A bilge pump is a marine pump designed to clear water out of the bottom of a boat's hull, also known as the bilge. This water may result from rain, leaks, splashes, cleaning, or coolers that drain into the bilge. Bilge pumps are rated by the volume of water they can remove in one hour, and each type of pump possesses different ratings at which they are available. These pumps can prevent small leaks from sinking a boat, providing more time for diagnosis and resolution. Even for large-scale leaks, bilge pumps can buy more time for help to arrive.

Learn more: How to choose the right bilge pump for your boat.

How does a bilge pump work?

Automatic bilge pumps use a float switch to activate the pump once water pools above a predetermined level. When the switch is activated, the pump starts to remove the water from the bilge until it is dry. Once the bilge is free of water, the switch deactivates the pump until the water reaches the activation level again. This feature prevents the pump from running constantly while also protecting the boat from accumulated water. Some bilge pumps use rotational energy to force water out of a boat, while others use suction to draw water in and push it out. While some older boats use manual bilge pumps, modern boats utilize automatic bilge pumps. Understanding how each pump type deals with water can help you select the best one for your boat.

Where are bilge pumps located?

Depending on the type of bilge pump in a boat, the pump can be located at the bottom of the bilge or outside of it. Submersible bilge pumps are always located at the lowest point of the bilge because they can operate only underwater. Diaphragm bilge pumps are installed above the bilge, away from the water. The hoses for these pumps are lower than where a submersible pump can be installed, enabling them to remove more water than a submersible pump. As a result, these are often referred to as drying pumps and are frequently used together with a submersible pump.

The most important factor for the location of a bilge pump is its accessibility. Bilge pumps must always be accessible for routine maintenance, inspection, and testing. If your bilge pump is located in a hard-to-access area, consider relocating it to a more accessible location.

Types of bilge pumps

There are three main types of bilge pumps: centrifugal, electric diaphragm, and reciprocating. Each type has advantages and disadvantages depending on boat sizes and applications. Some also come in manual configurations ideal for small recreational boats.

Centrifugal bilge pumps

Centrifugal bilge pumps are submersible pumps that utilize an impeller to increase the kinetic energy of incoming water to pump it out. When water enters the impeller, the impeller's rotational force raises the water's speed. Surrounding the impeller is a diffusion ring with fixed blades. These blades convert the speed of the water into pressure, allowing the water to be expelled from the boat. Unlike diaphragm and reciprocating bilge pumps, centrifugal pumps are not self-priming, and they must remain submerged to operate. The simple design of these pumps makes maintenance easier than for other pump types.

When water is discharged from the centrifugal pump, the pressure inside the casing drops, drawing water into the impeller's eye. This means the pump must be primed with water before it can naturally draw in water. This characteristic also prevents centrifugal pumps from removing all water from a boat's bilge. To eliminate all water, a diaphragm or reciprocating bilge pump should be used.

Centrifugal bilge pumps can handle large volumes of water, and they are typically less expensive than other pump types. Large marine operations, such as big ships, use centrifugal pumps with multiple impellers to discharge the high volumes of water that the boat intakes.

Advantages of centrifugal bilge pumps

• Can handle large volumes of water
• Inexpensive
• Can tolerate some debris
• Simple maintenance

Disadvantages of centrifugal bilge pumps

• Ineffective at pumping water uphill
• Must be submerged
• Not self-priming
• Do not remove all water from the bilge

Electric diaphragm bilge pumps

Electric diaphragm bilge pumps are self-priming positive displacement pumps that use a diaphragm to draw in and displace water. These pumps create suction to draw in water as the diaphragm opens and discharge it when the diaphragm closes. Diaphragm pumps are better at pumping water uphill than centrifugal pumps, allowing them to remove all water from a boat's bilge. Unlike other pumps, diaphragm bilge pumps can be mounted away from the bilge, making them less prone to damage and often resulting in a longer lifespan compared to other types. Various types of diaphragm bilge pumps include single-diaphragm and double-diaphragm configurations.

Single-diaphragm pumps contain one diaphragm that draws water in and discharges it as the diaphragm moves. This is the standard design for diaphragm pumps, but the flow rate is lower than that of double-diaphragm pumps.

Double-diaphragm pumps feature two connected diaphragms that move in conjunction with each other. As one diaphragm opens to let water in, the other closes to discharge water. This design enables the pump to displace more water and generate a consistent flow rate.

Advantages of electric diaphragm bilge pumps

• Self-priming
• Can be mounted away from the bilge
• Better uphill pumping than centrifugal pumps
• Minimal maintenance

Disadvantages of electric diaphragm bilge pumps

• Cannot handle much debris
• Smaller capacity

Reciprocating bilge pumps

Reciprocating bilge pumps, also known as piston bilge pumps, are self-priming positive displacement pumps that effectively pump water uphill compared to centrifugal pumps. These pumps draw water in on one side of the piston and discharge it from the opposite side. Each side of the piston contains a suction valve and a discharge valve. When the piston moves up, the top suction valve opens to allow water in, while the lower discharge valve opens to allow the water beneath the piston to escape. When the piston moves down, the lower suction valve opens, the bottom discharge valve closes, and the upper discharge valve opens, allowing the previous water intake to be discharged.

Reciprocating bilge pumps are even less tolerant of debris than diaphragm pumps. Debris entering the pump can clog the discharge and suction valves and lead to pump failure. To combat debris, a filter must be installed in front of the suction valve. Additionally, reciprocating bilge pumps are less capable of displacing large water volumes than other pump types, which restricts their use to small boats and other marine applications.

Advantages of reciprocating bilge pumps

• Can handle fluids more viscous than water
• Self-priming
• Better uphill pumping than centrifugal pumps

Disadvantages of reciprocating bilge pumps

• Cannot handle debris
• Smaller capacity

Manual bilge pumps

Manual bilge pumps are hand-operated pumps designed for non-motorized boats. Most manual bilge pumps have a maximum output of 20 gallons per minute and should not be used for motorized marine applications. Typical uses include kayaks, canoes, and dinghies.

Advantages of manual bilge pumps

• Ideal for non-motorized marine applications
• Requires no electricity
• Inexpensive

Disadvantages of manual bilge pumps

• Extremely limited applications
• Requires manual operation

Bilge pump maintenance

Regular checkups on your bilge pump are vital for both safety and the pump’s lifespan. Maintenance not only saves money on a new pump but can also prevent costly boat repairs due to flooding. There are several checks you can perform to extend the life of your bilge pump.

1. Test the pump

Before heading out on the water, ensure that your bilge pump is working properly. While this may seem mundane, it can save you from headaches and danger should the pump fail while you are on the water. To test the bilge pump, pour a few gallons of water into the bilge. If the pump activates and removes the water, it is functioning correctly. If the water remains stagnant, either there isn't enough water to activate the pump, or it requires maintenance. Do not go out on the water if your pump is not working properly.

2. Check the wiring and float switch

While on the water, you should be able to hear the bilge pump running. If the pump is running constantly, either too much water is entering the bilge, the switch is faulty, or the wiring is damaged or broken. If the pump runs continuously, inspect all wiring for corrosion or breaks. If the wiring appears functional, ensure the float switch is free of debris. If both the wiring and the float switch operate correctly, check for leaks or other causes of excess water in the bilge.

3. Clean the pump

Water that enters the bilge often contains unwanted materials like algae, debris, and dirt. While some bilge pumps can handle these items, they can wear down a pump over time. Bilge cleaners effectively break down built-up sludge on the inside of a pump to keep it running smoothly. They work while the boat is on the water, so you do not need to rinse the pump before heading out. Additionally, wash the outside of the pump about once every month during the boating season.

4. Check the outlet hose

Outlet hoses are durable, but they should still be regularly inspected for damage and clogs. If only a small amount of water comes out of your hose, it is likely clogged or there is a problem with the battery. If the battery voltage is normal and the hose isn’t clogged, the issue most likely lies within the pump itself.

5. Check the impeller

For centrifugal pumps, an inability for the impeller to rotate indicates a faulty pump. To test the impeller, shut off power to the pump and unmount it from the bilge. Attempt to turn the impeller with your hand. If it doesn't turn and no obstruction is present, the pump needs to be replaced.

6. Check the filters

Filters that prevent debris accumulation in the pump can easily become clogged if not cleaned regularly. Before going out on the water, ensure that all debris is removed from the filters.

When should a bilge pump be turned on?

If you have an automatic bilge pump with a float switch, the switch will automatically signal the pump to turn on and off. If your pump does not use a float switch, you will need to monitor the water levels in the bilge and activate the pump when it must be emptied. Submersible bilge pumps cannot run dry, so if you notice the pump stays on when the bilge is drained, there may be an issue with your float switch or pump.

How long do bilge pumps last?

The average lifespan of a bilge pump is around three to five years. With proper maintenance, it can last longer, but you must ensure the pump remains safe each time you set out on the water. A poor flow rate is a sign that a bilge pump is nearing the end of its life. If a pump is not clogged yet outputs less water than normal, it likely needs replacement.

Do I need a filter for my bilge pump?

A bilge pump inlet screen can protect your bilge pump from damaging debris. These screens are a cost-effective way to avoid an expensive accident, not just for the pump but potentially for the entire boat. Inlet screens are also low maintenance. Before heading out on the water, check the screen and remove any debris to avoid clogging.

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