What are Bilge Pumps & How to Choose the Right One for Your Boat?

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The least thrilling piece of equipment on your ships is a bilge pump, essentially responsible for holding your watercraft afloat, making it your best friend on the water. It’s a critical piece of safety gear that keeps your deck dry, ensuring that all water stays outside of your boat and your passengers stay safe. If you keep your bilge pump in good working condition, you’ll be able to sail through most emergency situations and stop them entirely.

Unfortunately, boaters rarely pay as much attention to bilge pumps as they do to gadgets, deck and cabin upgrades, and recreational accessories. However, they are worthy of the same consideration. Let’s start by learning about bilge pumps and their significance, then look at the various types and how to choose the best one. You can check out the Best Boat Bilge Pumps right away. 

What are Bilge Pumps?

Bilge pumps are marine water pumps that can be used on both large and small ships. They are in charge of extracting water from bilge wells and discarding it overboard.

It’s difficult to keep water and oil out of your bilge wells – which are found at the very bottom of a boat. These liquids typically come from machinery spaces or drainage systems in the large craft. On smaller vessels, anything from rain to large waves will cause the bilge well to overflow.

The primary aim of all bilge pumps on boats is the same. These, on the other hand, are available in a wide range of sizes, styles, and capacities. Bilge pumps are available in two types: automatic and manual. Small plastic boat bilge pumps and heavy-duty cast iron ship bilge pumps are available. The right one for your ship will be decided by a variety of variables, which we’ll go over below. Prior to that, it’s important to understand why bilge pumps are so vital.


What is the Importance of a Boat Bilge Pump?

bilge pump system is your primary line of protection against an accidental sinking, which could result in the loss of your boat, all of its equipment, and possibly the lives of everyone on board.

Selecting a bilge system that is well suited to the size and total capability of your boat is one of the most critical safety considerations. It’s always safer to have a bilge pump that exceeds your regular needs than to save a few bucks and risk getting caught off balance when the chips are down.

Standard bilge pumps are mainly intended to remove nuisance water from your boat that accumulates as a result of waves, wakes, or strong winds. Worst-case scenarios: If leaving the vessel becomes necessary, your pump should provide enough time to locate and plug the leak, or allow everyone on board to put on personal flotation devices.

Consider installing a secondary emergency bilge pump on your boat if you often sail offshore or in deeper, more remote waters. Adding a second bilge pump capable of moving several hundred gallons of water per minute to your craft could prevent it from sinking. These pumps are certainly more costly than regular bilge pumps, but they can prove to be well worth the initial investment.

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What are the types of Bilge Pumps and How do they work?

Not all boat bilge pumps are the same. Aside from size and capacity, there are also different types. The four most common types are:

  1. Centrifugal
  2. Diaphragm
  3. Reciprocating
  4. Flexible impeller

These bilge pumps use different methods to get bilge water overboard. And they all have their pros and cons. If you learn how these pumps work and their strengths and weaknesses, you will have an easier time selecting the right one.


Centrifugal Bilge Pumps

  1. Centrifugal types are the most widely used. They are famous for their high capacities, simple design, ability to handle small debris, and low cost.
  2. In large ships, bilge pumps are almost exclusively centrifugal. When you need to move large quantities of water, centrifugal is the best option.
  3. However, these need priming to operate. Before a centrifugal pump can move any water, you need to fill it with water first because of how it works. 

How Do Centrifugal Pumps Work?

  • Centrifugal pumps move water by turning rotational energy into kinetic energy. Inside the pump, a spinning impeller pushes water into the discharge. This creates low pressure in the pump, which sucks more water in.
  • For centrifugal pumps to work, there are two critical components: the impeller and the volute casing.
  • The impeller is a disk with vanes curving outwards. A motor spins the impeller at incredible speeds, and the vanes use centrifugal force to push incoming water to the casing sides.
  • The volute casing has a snail shape, starting narrow and gradually getting wider towards the discharge. This design builds pressure and forces all the water out through the discharge instead of turning around the casing.
  • When the water leaves the discharge, this creates low pressure within the casing, which pulls more water into the pump. Centrifugal pumps need priming to work for this reason.
  • Because of this, you can never remove all bilge water with a centrifugal pump. There will always be some leftover that will need another type of pump for removal.
  • For big ships, we recommend getting a self-priming centrifugal pump. These can separate the air inside the casing from the water. The water circulates in the pump, but the air discharges – creating low-pressure, which pulls in more water until the pump is full of water.

Types of Centrifugal Bilge Pumps:

  1. There are several types of centrifugal pumps. The two common ones are submersible and horizontal bilge pumps.
  2. Submersible bilge pumps are for smaller vessels. They are entirely waterproof and go straight into the bilge well.
  3. Horizontal bilge pumps are the most powerful option for ships. Instead of going into the bilge well, these pumps are strong enough to suck the water out through pipes.
  4. Because the pump isn’t inside the bilge well, horizontal bilge pumps are versatile. You may also connect them to the ballast system on your ship.

Pros and Cons of Centrifugal Bilge Pumps:

  • Pros:
    • High capacity
    • Simple design
    • Easy maintenance
    • Can handle small debris
    • Affordable
  • Cons:
    • Can’t get rid of all bilge water
    • Needs priming

The Rule 380 GPH Centrifugal Pump is an excellent option for boaters and comes in both 12V and 24V. Mounted below the waterline, it’s completely submersible (should your waters rise too high). It pushes through lots of water (3,800 gallons-per-hour) but at a low pressure (7 – 10 psi). 


Diaphragm Bilge Pumps:

  1. Another common type of bilge pump is the diaphragm pump.
  2. Diaphragm pumps are a type of positive displacement pump. Unlike centrifugal pumps, these aren’t typical on large ships. Their capacity isn’t as high as centrifugal types, and they also aren’t very good at handling debris.
  3. However, for small and medium-sized boats, these offer several advantages.
  4. For one, they don’t need any priming. Diaphragm pumps can run completely dry, whereas centrifugal pumps will get damaged. Because of this, diaphragm pumps can remove all the water in the bilge.
  5. Diaphragm pumps also have an easier time pushing or pulling water upwards. Where a small centrifugal pump might have a hard time removing water from deeper bilges, a diaphragm pump will have no problem.

How Do Diaphragm Pumps Work?

  • Diaphragm pumps work by using a diaphragm and check valves. The diaphragm pulls up, creating a vacuum in the pump that sucks water (or air) through the inlet check valve. When the diaphragm presses down, this forces the water out through the outlet check valve.
  • Because a diaphragm pump can also pull air, you don’t have to place them close to the bilge water. Putting them high above the bilge won’t need priming or as much power as a centrifugal pump in a similar position would.

Types of Diaphragm Bilge Pumps:

  1. The three common types of diaphragm boat bilge pumps are single-diaphragm, double-diaphragm, and manual.
  2. Single-diaphragm is the standard design. They have a single diaphragm pulling and pushing the liquid around.
  3. Double-diaphragms have two diaphragms placed parallel to each other in separate chambers. A rod connects these two diaphragms.
  4. Compressed air takes turns entering each chamber – pushing that diaphragm closed while at the same time pulling the other open.
  5. This design is more efficient than single diaphragms. It also creates a steadier flow of water.
  6. Manual diaphragm pumps are not electronically powered. Use a lever to pull the diaphragm open and push it closed manually. These have the advantage that they work even when your electrical system is down.
  7. Having one manual pump is required in most smaller vessels.

Pros and Cons of Diaphragm Bilge Pumps:

  • Pros:
    • Self-priming
    • Better at vertical pumping than centrifugal types
    • Can remove all water from boat bilge
    • Can be installed above boat bilge
    • Can be used without power (manual)
  • Cons:
    • Lower capacities
    • Not good for debris

The Jabsco’s 36680-2000 Marine PAR Belt Drive Bilge Pump is a good choice for this water pump type. It has an auto shut-off feature for when the pressure builds up and a manual on/off switch in the back. 

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Reciprocating Bilge Pumps:

  1. Like diaphragm types, reciprocating (or piston) bilge pumps are also a positive displacement pump.
  2. They can run dry, remove all boat bilge water, and have an easier time pumping vertically than centrifugal types.
  3. These can also work with high viscosity liquids. That is why some large ships use piston pumps to get rid of sludge in their bilge wells.
  4. However, even the most powerful reciprocating type can’t move as much water as centrifugal pumps.
  5. Reciprocating types are also bad at dealing with debris – even worse than diaphragm pumps. Installing a filter at the suction hose is a must.

How Do Reciprocating Pumps Work?

  • Reciprocating pumps work by using a piston and check valves. The piston pulls up, creating a vacuum in the pump that sucks water (or air) through the inlet check valve. When the piston presses down, this forces the water out through the outlet check valve.
  • Because the piston is air-tight, you also don’t have to put it inside the boat bilge. However, reciprocating boat bilge pumps cannot tolerate debris. Debris will lodge in between the piston and the pump’s walls, jamming or destroying the pump.

Pros and Cons of Reciprocating Bilge Pumps:

  • Pros:
    • Self-priming
    • Can handle high viscosity liquids
    • Better at vertical pumping than centrifugal types
    • Can remove all water from bilge wells
    • Can install above bilge well
    • Can use manual reciprocating pumps without power
  • Cons:
    • Can’t handle any debris
    • Lower capacities

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Flexible impeller Bilge Pumps:

  1. Flexible impeller pumps may look similar to centrifugal types, but they are very different from one another.
  2. Instead of a solid impeller with vanes, flexible impeller pumps use a rubber impeller squeezed by the casing to give it its curved shape.
  3. Flexible impellers are another type of positive displacement pump. They are self-priming and can remove all the water from the bilge well. They are also capable of carrying solids and debris.
  4. However, unlike diaphragm and reciprocating types, you should never run a flexible impeller pump dry. Without water, the friction between the impeller and casing will burn the rubber impeller.

How Do Flexible Impeller Pumps Work?

  • Flexible impeller pumps use a rubber impeller and a cam to function. As the spinning impeller meets the cam, it bends, squeezing its trapped water out the discharge. When the impeller leaves the cam, negative pressure creates a suction that pulls in more water.
  • Because of their material and simple design, flexible impeller pumps are affordable. Their design also allows you to run them in reverse should you want to.

Pros and Cons of Flexible Impeller Pumps:

  • Pros:
    • Self-priming
    • Can handle solids and debris
    • Better at vertical pumping than centrifugal types
    • Can remove all water from bilge wells
    • Can install above bilge well
    • Cheaper than other options
  • Cons:
    • Lower capacities
    • Rubber impeller wears down
    • Needs priming

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How to Choose the Right Bilge Pump?

Taking the time to choose the right pump can spell the difference between sinking and saving your ship. Also, knowing what you’re looking for ahead will ensure that you don’t spend a lot of money on a pump that doesn’t fit your everyday bilge pump needs.

However, with so many different types, choosing the right one can be challenging. There are several things you need to consider before buying a bilge pump:

What Type of Bilge Pump Should You Get?

Between centrifugal, diaphragm, reciprocating, and flexible impeller pumps – which one will suit your boat the best? That depends on the type of boat you have and the condition of your bilge.

Large vessels: They almost always use centrifugal pumps. These boats need the high capacities that only these can deliver. You may use other types of pumps, such as diaphragms, for thoroughly clearing out the bilge well. But for bilge transfer and emergencies, nothing can match the output of centrifugal bilge pumps. You can also connect these horizontal pumps to elaborate pipe systems that use valves to control where the water comes from and where it goes.

Smaller boats: For smaller boats, you have more options. The two most popular types are centrifugal and diaphragm. Submersible centrifugal pumps may have difficulty pushing water up if you place the discharge hose high above the bilge. But they’re also cheap, easy to maintain, and can move a lot of water. If you set up your bilge correctly, these will be perfect for you.

Diaphragm types are more expensive. However, you don’t have to place them down in the dirty bilge for them to work. They are more convenient to set up and clean. Plus, they can remove all the water in the bilge. Reciprocating pumps aren’t quite as popular. These are incredibly efficient when dealing with high-viscosity liquids like sludge, which isn’t common in small boats.

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7 Essential tips to maintain your boat’s bilge pump in an efficient way:

The best ways to ensure you’ll never have a problem with your bilge pump while afloat with a boat full of passengers is to regularly inspect your system. We recommend doing so at least once a month, during the seasons you generally use your boat.

Make sure you actually activate the pump and allow it to discharge water, rather than just flipping the switch on and off and assuming everything is alright simply because you hear the electric motor engage. Disassemble your pump occasionally to check valves and diaphragms for noticeable wear or damage to the pliable neoprene components, and replace them if necessary.

Most importantly, make up a maintenance list (including the pump’s brand, model and the manufacturer’s stock numbers for each separate part). This will allow you to quickly order and replace anything you may need, including a new bilge pump, without expending more time in the repair process than necessary.

1. Check Your Battery

If your pump fails to operate with the helm switch in either the automatic or manual mode, check to make sure your battery switch is in the ‘on’ position. Then try operating a few other pieces of your electronic hardware to see if they are functioning. If they do not respond, it is possible that your battery either needs to be recharged or possibly, even replaced.

The Mighty Max ML50- SLA is a maintenance-free, spill-proof battery that resists shock and provides top performance in both high and low temperatures.

2. Inspect the Fuse Connections

If your pump will not start, but the battery switch is in the ‘on’ position, it’s possible you may have blown a fuse. Before proceeding any further, check the battery connections to make sure they’re well secured and free of corrosion. If they are, continue on to find the point of malfunction, and then replace the defective fuse.

Blue Sea Systems ST Blade ATO/ATC fuse blocks eliminate the tangle of in-line fuses for electronics and other appliances and consolidate branch circuits.

3. Ensure Helm & Float Switches Work Properly

If the manual switch at the helm is on and the pump is also operational, but no water is being discharged for the output, there is a possibility that you may have a problem with your float switch. Clear away any dirt or other obstructing materials present. Then, put the helm switch into automatic mode before checking the float switch to make sure it’s working.

This Seaflo SFBS-30-01 Boat Bilge Pump Switch is environmentally responsible and offers an updated design, which includes a removable base for easy cleaning and servicing, as well as a heavy-duty flow switch designed for the toughest of applications.

4. Double-Check Wiring

Regularly inspect your wiring to assure all cables, wires and other connecting points are clean, unbroken and well secured. Use only crimped connections and spray or paste them with an anti-corrosion agent periodically.

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5. Examine Your Pump Impeller

Your impeller should be able to be freely rotated with your finger. With the power off, make sure that this is still the case and if you detect any noticeable resistance, check the impeller to see if there is any type of foreign object stuck in it that might be creating a jam. If nothing is visible, you may need to replace your bilge pump.

This Johnson Pump Impeller meets or exceeds the quality standards of your boat’s original equipment, and is interchangeable with the GLM 24970, Evinrude/Johnson 311684, 984491, Mallory 9-37822, OMC 311684, 984491, Rochester 17078904 and several other makes.

6. Clean the Screen of Blockage

Always keep your screen clean and free of any blockage to assure that water can move through it freely. Since they are extremely economical, you can afford to replace it regularly.

Pactrade Marine’s In-Line Mesh Filter is a stainless steel strainer that collects incidental debris, rust and scale. It’s easily cleaned within minutes and also helps in protecting your pump from valve damage.

7. Inspect the Outlet Hose for Damage

If everything else is functioning properly, but very little water is being pumped out of the exit port, the problem could be as simple as a crimped, ripped or blocked outlet hose. Be sure to keep several onboard so they can be replaced whenever necessary.

The TRAC Bilge Pump Plumbing Kit offers a durable outlet hose replacement, as well as a variety of additional accessories for attachment and repair.


Final thoughts:

The bilge pump is unquestionably one of the most critical pieces of equipment on your ships. Regular maintenance and cleaning of your bilge pump should remain a top priority regardless of the size of your boat, whether it is a 40-foot yacht, a 28-foot deck boat, or an 18-foot fishing skiff. Failing to do so could result in an untimely tragedy for which you may be held legally accountable. However, ensuring that your system is always running at full capacity will allow you to experience a relaxing and comfortable journey.


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