Marine Bilge Pumps: The Ultimate Guide for Ships

October 31st, 2020

Whether you’re working on a recreational boat or a giant cargo ship, you should never head out to sea without a bilge pump. Without these, there are so many things that could go wrong.

That said, the bilge pump requirements for large vessels and smaller ships are vastly different.

In this article, you’ll get a comprehensive look at marine bilge pumps for both large ships and smaller boats:

  • What marine bilge pumps are
  • Things that could go wrong without them
  • Different types of marine bilge pumps (and how they work)
  • How to select the right marine bilge pump
  • How to install marine bilge pumps
  • How to take care of your marine bilge pumps
  • The best marine bilge pumps

By the end of it, you’ll know everything you need to know about marine bilge pumps.

What are Marine Bilge Pumps?

Bilge pumps are a type of marine water pump found on both large and small ships. They are responsible for removing water accumulating bilge wells and throwing it overboard.

It’s impossible to stop water and oil from collecting in your bilge wells (the bottom-most part of a boat).

In large craft, these liquids usually come from machinery spaces or drainage systems. While on smaller boats, anything from rain to big waves can flood the bilge well.

The primary function of all boat bilge pumps is the same. However, these come in a variety of sizes, types, and capacities.

There are automatic bilge pumps, and there are manual ones. There are small plastic boat bilge pumps for boats, and there are heavy-duty cast iron ones for ships.

Choosing the right one for your ship will depend on several factors, which we’ll have a look at below.

Before that, it’s essential to know why bilge pumps are so important.

Why do You Need a Marine Bilge Pump?

Bilge pumps have been around since antiquity. These were the only pumps ocean vessels carried in the old days – showing how essential they are to seafaring.

Modern ships may have numerous pumps on board, but the bilge pump is still one of the most vital ones.

Here are three reasons why every boat needs a marine bilge pump:

  1. Bilge Pumps Prevent Flooding

    Leakage, drainage, and washing down fluids all find their way to the bilge wells of ships. If left unchecked, these wells will eventually overflow and spill into the engine room. And that can cause a lot of problems.

    If the oily water rises, it may get to machinery and damage them. Also, if one compartment floods, this will affect the balance of the boat.

    That’s why in ships, emptying the bilge wells is vital.

    For recreational boats, the risk is even higher. Without deep bilge wells, all it takes is a little excess water to get to your boat’s electronics. Heavy rain or big waves may be all it takes to destroy your boat’s system.

    But with a working bilge pump, you’ll never have to worry about flooding.

    Most boat bilge pumps come with an automatic float switch. This switch turns the pump on whenever water rises above a certain level. This way, you don’t always have to check your bilge.

  2. Bilge Pumps Can Save Your Ship From Sinking

    Aside from the daily role of clearing bilge wells of water, a bilge pump can also be a life-saver in emergencies.

    If your ship has a small puncture and water is rushing in, a bilge pump can spit this water out faster than it comes in (depending on the hole’s size and your pump’s GPM).

    The removal of the incoming water allows you to repair the hole or make an emergency docking. Note: You should never rely on your bilge pump for staying afloat. If your boat has a hole, repair it immediately.

    Installing a designated emergency bilge pump with the highest capacity possible is highly recommended. You can also use ballast and firefighting pumps to help pump out the water in case of an emergency.

    If the hole is too large for your marine bilge pump to handle, it can at least buy you enough time to abandon ship safely.

  3. Bilge Pumps are Required by Law

    It’s not only unwise to venture into deep seas without a marine bilge pump, but it’s also illegal.

    Maritime laws are strict on ensuring vessels have an excellent bilge system for emergencies – especially on passenger ships.

    Generally, the larger your ship, the more pumps you are required to have. You’ll also need more pumping capacity:

    Passengers: Vessel Length: Bilge Pumps Required: Minimum Capacity Required:
    Less than 49 26-65 ft. (7.9-19.8 m) 1 power pump & 1 portable hand pump or… 10 GPM
    … 1 fixed hand pump and… 10 GPM
    …1 portable hand pump 5 GPM
    < 26 ft. (7.9 m) 1 portable hand pump 5 GPM
    Over 49 + Ferry Vessels < 65 ft (19.8 m) 1 power pump and… 25 GPM
    …1 portable hand pump 10 GPM
    Any Number > 65 ft (19.8 m) 2 power pumps or more (depending on design) 50 GPM

    Chart from

Furthermore, it’s not enough just to have the pumps. They also have to be reliable.

For large vessels, getting a bilge pump that is ABS certified will ensure you’re complying with national and international regulations.

Types of Marine Bilge Pumps (And How 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.

  1. Centrifugal Bilge Pumps

    Centrifugal types are the most widely used. They are famous for their high capacities, simple design, ability to handle small debris, and low cost.

    In large ships, bilge pumps are almost exclusively centrifugal. When you need to move large quantities of water, centrifugal is the best option.

    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 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 which 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

    There are several types of centrifugal pumps. The two common ones are submersible and horizontal bilge pumps.

    Submersible bilge pumps are for smaller vessels. They are entirely waterproof and go straight into the bilge well.

    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.

    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


    • High capacity
    • Simple design
    • Easy maintenance
    • Can handle small debris
    • Affordable


    • Can’t get rid of all bilge water
    • Needs priming

    v2. Diaphragm Bilge Pumps

    Another common type of bilge pump is the diaphragm pump.

    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.

    However, for small and medium-sized boats, these offer several advantages.

    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.

    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 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

    The three common types of diaphragm boat bilge pumps are single-diaphragm, double-diaphragm, and manual.

    Single-diaphragm is the standard design. They have a single diaphragm pulling and pushing the liquid around.

    Double-diaphragms have two diaphragms placed parallel to each other in separate chambers. A rod connects these two diaphragms.

    Compressed air takes turns entering each chamber – pushing that diaphragm closed while at the same time pulling the other open.

    This design is more efficient than single diaphragms. It also creates a steadier flow of water.

    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.

    Having one manual pump is required in most smaller vessels.

    Pros and Cons of Diaphragm Bilge Pumps


    • 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)


    • Lower capacities
    • Not good for debris
  2. Reciprocating Bilge Pumps

    Like diaphragm types, reciprocating (or piston) bilge pumps are also a positive displacement pump.

    They can run dry, remove all boat bilge water, and have an easier time pumping vertically than centrifugal types.

    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.

    However, even the most powerful reciprocating type can’t move as much water as centrifugal pumps.

    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 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


    • 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


    • Can’t handle any debris
    • Lower capacities

    Flexible impeller pumps may look similar to centrifugal types, but they are very different from one another.

    Instead of a solid impeller with vanes, flexible impeller pumps use a rubber impeller squeezed by the casing to give it its curved shape.

    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.

    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 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


    • 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


    • Lower capacities
    • Rubber impeller wears down
    • Needs priming

How to Choose a Marine Bilge Pump

The ocean is not the place to find out your pump doesn’t cut it. 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 and so many GPM options, 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 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.

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.

Flexible impellers are cheaper than the diaphragm and reciprocating types. However, you’ll have to change their soft impeller now and then.

How Much GPM Do You Need from a Bilge Pump?

GPM, or gallons per minute, refers to your bilge pump’s pumping performance or flow. The higher the GPM, the faster your bilge pump can drain your boat bilge. Generally, the bigger your pump, the more GPM it will have.

A good rule of thumb is to get the highest GPM that’s reasonable for your boat. Your bilge pump is what will save you when you’re sinking. That’s why you’ll want one with as much GPM as possible.

All ships are different – there is no one-size-fits-all when it comes to GPM. The law chart above only shows the minimum GPM required. In practice, you’ll need much more GPM if you want to be safe.

That’s why you should get the biggest bilge pump you can get.

One misconception a lot of people have is that smaller boats can get away with a small pump. Smaller boats will need the largest pump they can get since their small hulls will fill up with water much faster than large ships.

That said, you also have to be reasonable. Smaller boats won’t have space for an enormous and heavy pump. Consider the pump’s size and weight when choosing because it might be too bulky for your boat.

How Much Head Do You Need from a Bilge Pump?

Aside from GPM, you also need to check the total head of your bilge pump.

Centrifugal pumps have a hard time pushing water up vertically. To know if your pump can handle your discharge pipe’s height and length, calculate the total head and compare it to your pump’s head rating.

To get a basic idea of how much head you need, use this equation:

Elevation + Pressure + Friction Loss = Total Head

Elevation refers to the total vertical rise in feet. If your discharge pipe’s end is 20 ft above your pump, your elevation is 20ft (regardless of the pipe’s overall length).

Pressure refers to the amount of pressure of the fluid in the pipe. Use PSI to measure this, then convert it to head feet (1 PSI = 2.31 Head Feet).

Friction Loss refers to the loss of flow due to friction in your pipes. The equation for this is quite complicated, but there are several calculators you can use online for this.

Add those together, and you’ll get your total head. You should get a pump that reaches or exceeds that total head.

How to Calculate How Much GPM You’re Getting From Your Bilge Pump

Manufacturers rate a pump’s GPM in ideal conditions, where the water is right next to the pump, and the discharge is horizontal. A 10GPM pump will empty 10 gallons of water in a minute with these conditions.

However, in practice, pumps move far less GPM than they are rated. And this can be a problem for smaller boats.

You may think that your 10 GPM pump is strong enough to eliminate all nuisance water and maybe even save you from sinking. In reality, your 10GPM pump might only be moving at 3GPM.

The GPM goes down because there is about a 20% capacity drop due to voltage (small boat bilge pumps are tested at 13.6 volts while most marine batteries only run at 12), 30% for head height, and 20% from hose resistance.

Those numbers are estimates. But you should consider that unless you have the ideal setup, you may only be getting 30% of your pump’s rated GPM. So don’t take chances. Get the highest capacity pump your boat can handle to stay safe.

Do You Need a Manual Marine Bilge Pump?

Unless you have a giant ship, a manual bilge pump is vital. If water gets to your electric system, your bilge pump will shut off. The only way to save your boat or buy time then is by using a manual bilge pump.

Make sure your manual bilge pump is easily accessible and can operate from a comfortable position. Pumping water off your boat bilge is tiresome, so you don’t want to be cramped or in an awkward position while using your hand pump.

For big ships, if your motor-driven pumps fail, you’re better off abandoning the ship than attempting to clear the water with a manual pump.

How to Install a Marine Bilge Pump

You can have the highest capacity marine bilge pump around, but it won’t save you unless you install it properly.

Nowadays, bilge pump systems on ships require inspection for compliance. However, too many small boaters still unknowingly make mistakes when installing bilge pumps.

Here are some general principles to follow when installing a marine bilge pump.

How Many Bilge Pumps do You Need?

Every watertight compartment in your vessel needs a bilge pump or a pipe connected to a bilge pump. Every room in your boat where water can’t flow to the next one needs a bilge pump.

In large vessels, pipes from every compartment connect to three, four, or more bilge pumps. The draining of every area without installing a pump in each one is possible because of this.

However, that is only possible because ships use large and powerful bilge pumps with elaborate piping systems. For smaller boats, you should keep one (or two) bilge pumps in every watertight area.

Where Should You Install Your Bilge Pumps?

Boat bilge pumps should be at the lowest possible point possible. However, the discharge shouldn’t be too close to sea level (to avoid siphoning).

For ships, you’ll want at least one pump on the starboard and one on the port. You also shouldn’t group the marine bilge pumps close to each other. In an emergency, you don’t want the bilge pumps together because they will all get flooded.

How Should You Power Your Bilge Pumps?

Bilge pumps in ships use electric motors, the ship’s engine, or emergency generators for power. An emergency bilge pump should always have its own power supply.

In smaller boats, follow the manufacturer’s directions as closely as possible – from the battery voltage right down to the wire gauge. Doing this will clear up a lot of potential problems you may encounter.

Bilge Pump Switches

There are several ways to switch on a bilge pump. You could turn them on manually in the engine control room (for ships) or connect them to the battery (for small boats). However, the ideal setup has automatic switches in place.

Float switches are simple devices that turn your bilge pump on when the water reaches a certain level – and turns them off once the water descends. Installing one of these is excellent since you won’t have to monitor your boat’s bilge water levels all the time.

Some ships need to turn on their bilge pump manually. To help with this, you can install bilge alarms to go off whenever the water has risen to a certain level. This way, you don’t have to go and check now and then.

Having a float switch is especially crucial if you’re using a centrifugal pump. Centrifugal pumps can’t run dry. If they do, they’ll get destroyed. That’s why having them switch off automatically before the water has fully descended is crucial.

Ensure your switch is high enough that when the excess water in the pump flows back into the bilge, it won’t reach the switch.

When installing a float switch, make sure it is in a place that can’t get caught while rising. If it gets stuck, it won’t trigger your pump to start, and you may flood your boat.

That’s why you should never rely on automatic switches. Check your bilge well now and then to make sure your pump is working correctly.

Other types of automatic switches include contact switches, air chamber switches, and magnetic read switches. All of these perform the same function as float switches.

Bilge Pump Pipes

As mentioned earlier, large vessels have elaborate piping systems that connect to the bilge pumps. Valves control which pipes are running and which lines are standing by.

Each compartment in the ship must have a bilge pipe. These pipes connect to the bilge manifest, which then takes the bilge water to the pump.

For smaller boats, hoses are used instead of pipes. To lower the hose friction, get a smooth hose. Corrugated hoses increase friction and will further reduce your pump’s GPM.

Follow the manufacturer’s directions when choosing hose size and length.

Where do Bilge Pumps Discharge?

A bilge pump takes bilge water and discharges it overboard. However, in ships where oils and other fluids mix with water in the bilge, this water must go through a filtering process first.

From the bilge pump, the bilge water pumps into several bilge tanks, where gravity separates oil and water.

The oil is taken to the incinerator, while the water (which should be under 15ppm) goes overboard. Ships can do this entire process automatically with switches and sensors.

However, in emergencies, the whole bilge water management system is bypassed. All water goes straight overboard regardless of oil content. But this is only legal in emergencies – otherwise, under MARPOL regulations, the water pumped overboard must be under 15 ppm to prevent pollution.

One problem that is common in smaller boats is siphoning. The discharge should be well above the waterline. However, sometimes, this is not possible. To avoid siphoning, install a siphoning break on your hose or pipe.

What are Mud Boxes?

Though centrifugal pumps can handle some debris, it isn’t ideal since it increases pressure and slows down the flow. That is why installing mud boxes on bilge pipes is a good idea.

Mud boxes or strum boxes filter out debris from getting to the pump.

What are Non-Return Valves?

In ships, bilge pipes must have non-return valves. These valves prevent liquid in the pipes from flowing back into the bilge well.

How to Prime a Centrifugal Bilge Pump

Priming your centrifugal bilge pump is slightly different for each pump. But the general method is the same:

  1. Close discharge valve
  2. Open top vent
  3. Open outside supply line to start filling the pump
  4. Wait for the water to spill off from the top vent
  5. Close top vent
  6. Close supply line

Refer to your pump’s manual to know the exact procedure.

Of course, if you have a self-priming centrifugal pump, your machine will do this independently.

Marine Bilge Pump Maintenance

If you set up everything correctly, nothing should go wrong. You won’t have to worry about problems, especially if you are using a centrifugal-type pump.

However, there are a few things you need to check once in a while.

Bilge Pump Regular Maintenance for Ships

To ensure that your pump is running correctly and efficiently, perform the following checks:

Daily Checks:

  • Visual inspection of the pump
  • Check bearing temperature

Weekly Checks:

  • Check power reading
  • Check discharge pressure
  • Check for vibrations

Monthly Checks:

  • Check coupling alignment
  • Grease coupling
  • Check foundation bolts

Semi-Annual Checks:

  • Check coupling alignment
  • Run emergency bilge pump (if standing by)

Bilge Pump General Maintenance

Aside from the regular checks, perform these maintenance tasks once in a while:

Clean Bilge

A dirty boat bilge is especially harmful to smaller boat bilge pumps. Dirt can clog up the filter and prevent water from getting in.

Make sure your boat bilge is free from dirt and other debris by cleaning it now and then.

Clean Filters

You may find your filter already clogged with dirt and debris. If so, clean it up immediately. If you have mud or strum boxes, it’s also essential to check them once in a while.

Inspect Automatic Switches

One of the last things you want is for your automatic switches to fail. In smaller boats, one failure may allow the water to rise to the wiring of your boat. In ships, switch failure will get oily water into the dry-compartments.

Either way, checking your automatic switches is a must. If you have bilge alarms in place, also test those.

Best Marine Bilge Pumps

When it comes to choosing a marine bilge pump, reliability is of utmost importance. Your bilge pump is your last line of defense. If it fails, your ship is going down.

The best way to ensure a high-quality and reliable marine bilge pump is to choose a trusted brand.

For small boats, brands like Rule Industries, Attwood, Albin, and Johnson Pump make great boat bilge pumps. You can buy these at your local marine supply, or you can also get them online.

However, buying a marine bilge pump is not as easy as walking into a store or placing an order online for large ships.

When the demands are high, you need a pump that’s tailor-made to fit your ship’s needs. You need a pump that can stand up to every test and every international regulation. You need a pump you can trust to save your ship when things go wrong.

For high-quality, custom-built, ABS certified pumps, choose Carver Pump.

Carver Pump is a company dedicated to building American-made pumps with military-grade precision to accommodate highly developed specifications.

The ocean is no place to find out your pump does not make the grade. Trust your shipboard services to Carver Pump, the industry leader in shipboard centrifugal pumps.


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