Getting the water out


A couple of thousand years ago, Archimedes taught us the principles of displacement and the importance of keeping water out of our boats. Knowing this it is often surprising just how little thought most boaters give to their bilge pump systems.
CE rules for European boats require all pleasure vessels be equipped with at least one manually operated bilge pump. The U.S., however, does not require any kind of bilge pump on pleasure vessels at all. We all know keeping water out of the boat is a priority when sailing offshore. Most of the time this is not a problem, but sometimes things can and do go wrong. Having the right equipment to deal with leaks and flooding is your first defense. Leak control and water removal is a multi-part process that can use several different resources depending on the severity of the situation.

The first defense in controlling leaks is knowing you have a leak. ABYC now requires all boats with enclosed cabin spaces to have a high water alarm. A high water alarm is simply a water-activated switch located a few inches above what is considered a normal bilge water level. High water switches can be connected to a simple audio alarm, warning lights and even devices that will send a text message to your cell phone. Audio alarms should be located at or near all helm stations as well as in the cabin.

Finding leaks in calm conditions is a relatively easy matter as the boat is not moving and you can usually trace where the flow of water is coming from to its source. At sea this can be an extremely difficult task with water sloshing around in the bilge space. And the deeper the water gets, the harder it can be to find the leak. Once a leak has fallen below the bilge water level, it is all but impossible to spot visually.

A high water alarm saves precious minutes in locating a leak. A light at the helm indicating when the pump is operating is also a great first warning. If the helmsman knows the pump is running more than once or twice during their watch they can investigate the cause before it becomes a real problem.

A large capacity manual diaphragm pump mounted under floor boards. Note handle secured close to pump for immediate use. Provided there are not restrictions like turns in the piping or significant head pressure, this pump should be able to move large volumes of water.

First line of defense
In any leak situation, the boats primary electric bilge pumps will be the first line of defense to remove water from the boat. The most common type of pump is the submersible centrifugal pump. These pumps work by spinning an impeller at high speed. This spinning creates a pressure differential that will pull water into the center of the impeller housing and then in turn forces the water out of the housing and up the discharge hose.

This type of pump is relatively inexpensive and works well under most conditions. It has the ability to pass small bits of trash, but can become clogged with anything much larger than a pea-sized particle. Because centrifugal pumps are not positive displacement, meaning there is no set volume of water moved with each rotation, they can be affected by restrictions in the discharge hose.

Centrifugal pumps can move large amounts of water quickly, however, the height they have to lift or push the water up can affect their efficiency. This height of lift is called head pressure and is important to keep in mind when designing any system. As these pumps are submersible, they are generally not subject to corrosion and can be located in the bilge.

This type of pump is not self-priming, but this problem is overcome by placing the pump itself underwater. This is why these pumps will not remove every last drop of water from the bilge. As soon as they start to pull air into the pump, they lose prime and stop pumping. As there are no valves in this type of pump, any water left in the downhill side of the discharge line will flow back into the bilge when the pump stops. This can result in cycling when an automatic switch is used.

Centrifugal pumps come in many sizes, but the buyer needs to understand rated flow is not likely to be what they get on their boat. As head pressure and other restrictions in the line can greatly affect the actual flow rate.

For practical, real world results, assume about one-half the rated capacity of the pump for an average installation. If the pump is rated for 2,000 GPH it is more likely to pump closer to 1,000 GPH installed.

It is important to design a system with as few restrictions as possible. Voltage can also affect the efficiency of centrifugal pumps, as the lower the voltage the slower the pump will spin, reducing the amount of water moved.


Positive displacement

This looks like a good setup, but note the sponge right next to the strainerless manual pump hose, it would easily pull the sponge into the hose, causing a clog. Also note the hard 90° where the hose comes out of the centrifugal pump.

The next most common type of electric bilge pump is the diaphragm pump. These pumps are positive displacement pumps, meaning they pump a set volume of water with each cycle. These pumps work by pushing and pulling a flexible piece of rubber mounted on top of a chamber. As the rubber is pulled up, it creates low pressure in the chamber which pulls water in. When the rubber is pushed back down, it pushes the water back out. There are normally two openings in the chamber with check valves at each opening allowing the water to only flow in one direction. Diaphragm pumps are self-priming, allowing them to be mounted up out of the bilge. This has an advantage in that they are more accessible for service. Because diaphragm pumps are positive displacement, they are less affected by head pressure and inline restrictions. They can be a bit more tolerant of trash, but it should be noted that both excessive back pressure and bits of trash can damage or block the check valves reducing efficiency or even rendering the pump inoperable.

A couple of distinct advantages of a diaphragm pump is its ability to pump the bilge almost completely dry as the internal check valves stop water in the discharge line from flowing back into the bilge. This can be important with small bilge sumps to prevent pump cycling when used with an automatic switch. A disadvantage of diaphragm pumps is they do not move as much water as quickly as a centrifugal pump can. Many centrifugal pumps are rated at 1,500 to 2,000 GPH while few diaphragm pumps exceed 700 GPH.

There are other types of pumps available as well for removing bilge water. These are generally referred to as utility pumps and come in different sizes and types. Most of these are either impeller pumps or pressure water pumps utilizing three or more small diaphragms operating in a small chamber. These pumps are not generally recommended for removing bilge water as they do not tolerate any trash or debris normally found in bilges. The impeller pumps in particular are a poor choice since if they are run dry the impeller can be damaged.

Along with electric pumps, every boat should have one or more manual pumps. Should the boat lose power, manual pumps are the only choice. Almost all manual pumps are diaphragm pumps consisting of two basic types, single diaphragm and double diaphragm. The single only has one pump chamber and will pull water in on one stroke and push it out on the other. The double, as the name implies, has two chambers and will pull water into one chamber and push it out of the second chamber on a single stroke. This will increase the pump capacity and improve pumping efficiency. The drawback is a double diaphragm pump requires more energy to operate, increasing operator fatigue.

Often these pumps are mounted so that they can be operated by the helmsman while steering. This is good and bad as the helmsman may not always be able to operate the boat and pump at the same time. For this reason, it’s wise to have at least one additional manual pump. Having a second pump inside the cabin could be an advantage as the operator would be out of any weather.

No matter where the pump is located, the operator should be able to operate the pump for long periods of time under varying heel angles and sea conditions. “Avoid the use of handles that are too long for manual pumps,” said Will Keene of Edson International, a long-time manufacturer of marine pumps. “Although a longer handle will make pumping easier, it can easily damage the pump.” A sitting position with the operator’s legs braced against a solid surface is best.

Although manual pumps may be the last option, they have a major weakness: the “man” in manual. Most people cannot pump for endless hours and if there is a leak it needs to be fixed, taking at least one person away from pump duty. In addition to diaphragm pumps, a good strong bucket can be important. One should never underestimate just how much water can be moved with a bucket. A bucket may be a last resort as they are only really practical when the water is above bilge level. Every boat should have at least one really sturdy bucket.

Crash pumps
The last line of defense is what many call “crash pumps.” These are pumps set up only to be used in emergencies. Like normal bilge pumps, crash pumps come in many types and sizes. A crash pump is usually a high-volume pump often powered by the main engine. Some will recommend having a tee in the engine raw water intake with a line running to the bilge so the engine pump can help remove water from the bilge. I do not think this is a good idea. In an emergency it is important to keep the main engine running to keep the batteries charged and to assist in maneuvering the boat. If the pump should clog, run dry or pick up debris it could be damaged. This would result in the loss of the engine just when it is most needed.

An alternative is a separate belt-driven pump powered by the engine. Yet another interesting option is the Ericson safety pump. This is a large-capacity centrifugal pump that bolts to the propeller shaft just behind the transmission. The propeller shaft turns the impeller inside the housing. If the water should get as deep as the propeller shaft, it will enter the housing through the space between the shaft and housing. This pump is really the Hail Mary pass of pumps as in most cases the water would have to be pretty deep to allow the pump to operate.

No matter what type of pump is being employed, the installation is critical to its efficient operation. For all pumps the plumbing and hose runs are important to the pump’s efficiency. Restrictions in any of the plumbing need to be avoided. Don’t use cheap corrugated hose as this adds resistance inside the hose. Pick a hose with a smooth inside surface and one that will not kink easily. I like to use the white sanitation type hose. For the suction hose, make sure the hose will not collapse when clogged. Avoid sharp turns or the use of elbows or other fittings. Every connection in the hose will add resistance.


Systems should be designed so that check valves are not needed as these can cause as much as a 50 percent reduction in the pump capacity. Check valves can also become a clogging problem.

An electrical terminal block mounted too low and not sealed from water.

If the discharge is below the heeled waterline, the hose must be looped above the waterline and a vented loop installed to prevent back siphoning. Each pump should have its own discharge thru-hull as well; never tee two or more pumps to the same outlet. Pickup hoses should have a screen or strainer on the bilge end that is easily accessible for cleaning. Try to pick a screen that is coarse enough to catch larger particles, but not so fine that it clogs too easily. Without a doubt the biggest reason pumps fail is ingesting trash. Hair in particular is a problem for all pumps. “Every owner should clean and paint their bilges white once a year so they can see how dirty their bilges are,” said Keene. “Pump clogging is a major issue with all types of pumps.”

For electric pumps the wiring is critical as well. Any electrical connections that could get wet should be made as waterproof as possible. Heat shrink tubing is the best, but makes replacing parts and trouble shooting a bit more difficult. Terminal strips are better for replacement and troubleshooting, but harder to get watertight. Whatever method used, the connections need to be checked occasionally.

A double-chamber manual diaphragm pump mounted in the cockpit.

One of the biggest wiring errors in bilge pump installations is using undersized wires. Bilge pump capacity is rated with 13.6 volts, so unless your engine is running and charging the batteries your pumps are likely to be running at a lower voltage. Voltage drop from undersized wiring can further reduce the power to the pump. A #12 wire is the smallest that should be used for all but the smallest pumps. The further from the power supply, the larger the wire needed to avoid voltage drop. With the pump running under load, check the voltage at the power source and then at the pump. If the drop is more than three to five percent, the wire size should be increased. If your electric pumps have an automatic switch, they should be wired so that power to the switch cannot be turned off accidentally.

It should be kept in mind that any dewatering system is primarily for removing water once any leaks have been found and corrected. The first line of defense is knowing you have a leak and quickly correcting it. Bilge pumps can buy time while repairing leaks, but should never be counted on to keep the boat afloat with a leak, even a small leak can quickly outpace most pump capacities. The better your pump system is and the more efficient it is the more time you will have to correct any leaks. Keep the boat’s bilges clean and always test pumps under load prior to any offshore passages.

Capt. Wayne Canning lives on his Irwin 40 Vayu, in Wilmington, N.C. A marine professional for more than 35 years he is now a full-time marine surveyor, freelance writer, and consultant/project manager. Canning also runs websites for those restoring project boats. Visit and for more info.

By Ocean Navigator