All cruiser boats have at least one bilge pump (or they should have at least one!). Depending on the size and configuration of the boat, there may be more than one. Whether those pumps are working or not, now that’s a question!
What Is A Bilge Pump?
A bilge pump is an essential component of any boat. It is designed to manage and remove unwanted water from the bilge compartment, also known as the bilge well, which is the lowest point inside the hull. This unwanted water can come from various sources, including accidental spills, leaks from propulsion systems, faulty pumps, and pipelines, condensation from air conditioning units, overflowing tanks, drips from the stuffing box, and even rainwater entering through an open companionway or engine room vent.
In critical situations, such as when a boat begins to take on water, the bilge pump plays a vital role in removing water and helping to maintain stability. While it cannot completely prevent sinking in some cases, it can provide valuable time to signal for assistance and take necessary actions. Understanding the function and importance of a bilge pump is crucial for ensuring boat safety and effective water management while on the water.
The Bilge Pump System
This diagram shows a boat’s typical automatic/manual Bilge Pump system. Note the anti-siphon loop, which is important on sailboats that may heel over and allow the through-hull to be submerged, preventing water from being sucked into the boat.
- Bilge Pumps
- Automatic Water Sensing Switch or Float Switch
- Anti-siphon Loops
- Fuses
- Battery Connection
- Backflow Preventer
- Auto/Manual Switch
Types Of Bilge Pumps
Bilge pumps come in all shapes and styles, some with distinct purposes.
Manual Bilge Pumps
Manual bilge pumps are suction-type pumps that you operate by hand. Boats between 9 and 12 meters (29 – 40 feet) are required to have a manual bilge pump on board. This can be a portable handheld device or one built into the boat. Most sailboats will have a manual bilge pump installed on them as most have a smaller battery capacity than a cruiser-style boat.
Many cruiser-style boats in the 30-50 foot range have a manual bilge pump, typically built into the transom on the stern of the boat. When everything else fails (the water has come up over your batteries, and you have no electrical power), this is your pump of last resort.
Because these pumps are used very infrequently (if ever), the bellows and valves can dry rot. Inspect the pump frequently, and occasionally put some water into your bilge (add some bilge cleaner while you are at it), and test the manual pump to ensure that it is in working order. It’s better on a sunny day at the dock than when taking on water 5 miles offshore.
To operate a manual bilge pump, you either insert or extend the handle. Then pump like hell! If your pump is the style with a detachable handle, make sure that you label both the handle and the location of the handle on the pump, and keep the handle as close to the pump as you can.
Pros & Cons of Manual Bilge Pumps
- Pro: It does not require power.
- Con: You have to pump it. Potentially when you need to deal with other issues.
- Con: Rarely used, so you can become inoperative without knowing it.
Suction Bilge Pumps
Suction-style bilge pumps, also called Diaphragm Pumps, are not as common as primary bilge pumps, as they have fairly low flow rates. However, they are great at extracting the maximum amount of water from a bilge and are common in “Dry Bilge Systems.” You will also find suction-style pumps in grey water systems (which we will cover later).
A suction pump has both an intake and discharge hose. Because the intake hose is fairly small, it can be put deep into a keel or have its intake end flat at the bottom of a bilge, which allows it to extract water to a very low level.
Suction-style pumps typically have a flow rate between 3 and 10 Gallons per minute. If you put a hole in your boat, this will not keep up with the flow. However, condensation or drips from shaft seals are ideal and can keep your bilge as close to dry as possible. Triggering a suction pump with low water levels can be challenging, as most level switches turn off the pump at between 1 and 3 inches of water. An advantage of diaphragm pumps is that they can run dry for extended periods without damage. However, they are susceptible to clogging from debris, requiring a pre-filter, which must checked and cleaned frequently.
Note: Our Beneteau has a narrow bilge area in the keel so that a regular bilge pump won’t fit into the bottom. We have a diaphragm pump that acts as the first-level pump and takes care of most of the water. If there is a major leak, a high-volume impeller-style pump will kick in to manage larger volumes of water.
Suction pumps typically have a water level sensor that detects the water level in your bilge and turns the pump on. If properly installed, you can also manually activate the pump (see the section on switches below).
Pros & Cons of Suction Bilge Pumps
- Pro: Can remove water down to less than 1/8″.
- Pro: Automatic. No manual intervention is required.
- Pro: Most can run dry and will not damage the pump if run without water.
- Con: Requires battery power.
- Con: Susceptible to damage from debris. Requires a pre-filter, which must be cleaned.
- Con: Low flow rate. Typically less than 10 Gallons Per Minute.
- Con: Pumps are not submersible and must be installed above the high water area.
Impeller Bilge Pumps
These are the most common bilge pumps installed on boats. They are available in many shapes and sizes. Impeller bilge pumps are installed so that the base sits at the bottom of the bilge. An impeller at the bottom of the pump spins and forces water through the hose. Most impeller-style bilge pumps are fully submersible so they can be installed in wet areas like the bottom of your bilge.
Impeller pumps are high-volume and range from 500 to over 4000 gallons per hour! When the water in the bilge reaches the top of the impeller, air is sucked in, and the pumping action stops. So, unlike suction pumps, impeller-style pumps typically won’t remove all the water. They leave from 1/2″ to 2″ of water, depending on the size and style of the impeller.
Impeleller-style pumps are also less susceptible to clogging from debris. The filter grate on most impeller pumps will let through bits up to 1/16 inch, and the impeller will push those through and up the hose. Also, when an impeller pump shuts off, there is usually a bit of “backwash,” water in the hose that has not made it out of the boat, that will flush any debris out of the pump screen. (See backflow valves below) No pre-filter is required.
Like suction pumps, impeller pumps turn on automatically when properly installed and can be run manually when needed. However, unlike suction pumps, impeller pumps can be damaged if they run dry, as the water provides the lubrication and cooling.
In the last several years, manufacturers have introduced low-profile impeller-style pumps. These pumps will fit into small spaces yet have reasonably high flow rates. The output connection rotates so that it can be installed in many orientations. Some models come with electronic water level sensors built into the pumps. This was the perfect solution for my narrow keel bilge!
Pros & Cons of Impeller Bilge Pumps
- Pro: Low maintenance. No prefilter is required.
- Pro: Automatic. No manual intervention is required.
- Pro: High volume. Usually > 1000 Gallons Per Hour.
- Pro: Submersible. It can be installed in wet locations.
- Con: Requires battery power.
- Con: Susceptible to damage if run dry.
- Con: Does not remove all the water from the bilge.
Engine Powered Bilge Pumps
On larger vessels, bilge pumps are powered by the engines or auxiliary power units. These are typically large hydraulic or electric 110v or 240v pumps that can handle large volumes of water on big yachts. They are rare to find on cruiser-sized yachts.
Heard On The Dock: I have heard it suggested that, in a pinch, you could disconnect your engine’s raw water intake from the through-hulls (after closing the valve!) and stick them in the bilge. With the engines running, they will suck the water out of the boat. If you are 50 miles offshore and that’s the only option, it would be something to consider. However, having tried disconnecting hoses from barbed fittings, I’m not sure I’d want to be down in the engine room with the boat taking on water, loosening double-clamped hoses, and prying them off a fitting!
Gasoline Water Pumps
I am only including this category of pump to tell two stories. Gasoline pumps, frequently referred to as Trash Pumps, are those large pumps you see used to drain flooded basements or by fire departments to fill fire engines from lakes or ponds. A 3″ pump can move between 200 and 500 gallons per minute (12,000+ gallons per hour!)
I am not suggesting you carry a gasoline-powered pump on your boat! However, be glad that the Coast Guard, SeaTow, and BoatUS can access them when needed. I have seen a video of the Coast Guard lowering one of these pumps from a helicopter to a boat in distress.
Story 1: When we grounded in Canada.
When we grounded in Canada and cracked our keel, the tow company borrowed the local fire department’s trash pump, “just in case” of a problem, while towing us 75 miles across Georgian Bay. Fortunately, it was not needed, but it made us both worry and feel good that they asked for it before they left.
Story 2: When our rudder post seal failed.
We took on 35 gallons a minute when our rudder post seal failed. Our bilge pumps couldn’t handle that flow, so we called SeaTow. They showed up with one of these pumps that sucked out the water in just a few minutes. (Water was up to the bottom of the engines when they arrived!) Suffice it to say that we upgraded our bilge pump capacity after that event!
What Size & How Many Bilge Pumps Should I Have?
From personal experience, my advice is, “As many and as large as you can afford or fit!” Hopefully, it’s money wasted, but if you need it, the more capacity, the better!
The general suggestions for capacity and number are:
- 16-20 feet: 2500 gph total, using 2 pumps
- 21-26 feet: 3000-3500 gph total, using 2 pumps
- 27-35 feet: 3500-4500 gph total, using 3 pumps
- 36-42 feet: 6000 gph total, using 3 pumps
- 43-49 feet: 8500 gph total, using 3 pumps
- 50-59 feet: 9000 to 10,000 gph total, using 4 pumps
If you have isolated bilge areas (bilges separated by a bulkhead), I recommend a minimum of two pumps per area for redundancy.
Keep Your Bilges Clean,
Test & Inspect Your Pumps Monthly
While we all boat with the best intentions, bilge pumps are one of those things that are out-of-sight-out-of-mind.
Bilges tend to get dirty and damp, which is the perfect environment to make pumps, switches, and electrical connections fail. Unless you check your pumps and switches regularly, you will have a pump failure without knowing it until you need it most!
Dirty bilges, especially those contaminated with oils and fuel, are dangerous and will damage pumps and wiring. Oils and coolant will soften the plastic impellers and degrade wire insulation, causing failures. Debris in the bilge, like dirt, bits of wire ties, the odd nut and bolt, and that cockroach that went swimming and didn’t make it out, can clog the intakes on your bilge pump, reducing its efficiency or even prevent it from removing water.
In the 8 years we have been boating, I know of six boats in our marinas that have sunk or taken on significant water at the slip because of some failure of a through-hull or seal. The bilge pumps either didn’t work, the batteries ran down, the pumps failed, or the leak was more than the pumps could handle.
Bilge Pump Water Level Switches
The switch that activates your bilge pump is as important as the pump itself. Bilge pump water level detectors are probably more prone to failure than the pump itself. There are three types of switches: mechanical float switches, electronic field detector switches, and pressure sensor switches.
All of these switches have distinct advantages and disadvantages.
Mechanical Float Switch
The mechanical float switch is probably the most common bilge pump switch installed in boats. These switches consist of a float that raises as the water level increases. The float either makes an electrical contact or has a mercury switch that completes the connection when the liquid mercury completes the circuit. These switches are cheap and fairly reliable as long as they are maintained.
The benefit of float switches is that they are simple and inexpensive. The drawback is that they are prone to sticking if they get covered in bilge schmoo or if something in the bilge (like that screwdriver you dropped) gets under the float. Another drawback is that wave action can cause short cycling, where the pump will turn on for just a second or two due to waves sloshing water around in the bilge, causing the float to lift.
There are three distinct types of these float-type switches.
Exposed Float Switches
These switches are the most common and least expensive. The float consists of a float that swings on a hinge, which closes a contact when the float rises above a certain level. The major drawback to these types of switches is that they are subject to sticking if debris interferes with the movement of the float. Also, they tend to be large, so they are unsuitable for narrow keel bilges.
Shielded Float Switches
Similar to exposed float switches, these are also mechanical switches that use a float to activate the switch when lifted. The key difference is that the float is enclosed in a protective shield, which keeps most debris from interfering with its function and helps reduce the impact of water movement in the bilge.
They are more reliable and slightly more expensive than standard float switches. You still need to inspect and keep your bilge clean to ensure they do not clog, and as with standard float switches, they tend to be larger and will not fit into small keel bilges.
Ultra Safety Systems Switches
The Ultra Safety Systems bilge switch is considered the “Best, most reliable bilge switch available.”
These switches are truly over-engineered, and I agree they are the most reliable switches on the market. They feature both a bilge pump switch and a high water switch to trigger an alarm if your bilge water exceeds the limits of the bilge pump. The design is solid and prevents most junk in your bilge from affecting the switch. The only drawbacks are that the minimum width for installation is 1.5 inches (so some keel bilges (like mine) won’t accommodate it), and it costs over $200!
Electronic Water Sensing Switches
An electronic bilge switch takes the mechanical sticking issues out of the equation.
These switches use a field sensor to detect the resistance change when water is in the bilge. They are more reliable than mechanical switches as there are no moving parts. However, nothing is perfect, and these switches also have a few drawbacks.
First, electronic switches are more expensive than float switches. A float switch costs between $10 and $50 (except for the Ultimate Switch, which costs $200!). Electronic switches run from $50 to $160.
Electronic Switch
Electronic float switches have a feature/drawback in that they will not activate if oil or fuel is present at the sensor. So, if you have a dirty bilge, the switch may not activate. The plus side is that you won’t contaminate the marine environment!
Since oil floats on water, the switch should trigger once the oil/fuel level is above the sensor. In my experience, the oil can coat the switch and prevent it from tripping even when the water level is high above the switch.
Like float switches, most electronic switches activate at about 1 inch of water. A feature of many of these switches is that there is a delay both in activation and shutoff. Water must be on the sensor for a few seconds before the switch will turn on the pump. This prevents short cycling of the pump due to wave action. When the water is below the off level, the switch will continue to send power to the pump for 15-30 seconds, allowing more water to be pumped out of the bilge.
One of the benefits of electronic switches is that they fit into small areas. Most are about one inch thick. The Water Witch 101 is only 2″ W x 1.4″ H x 1″ D, so it will fit easily into most tight keel bilges!
Another thing to note with electronic field sensor-style switches is that if most of your bilge water is from air conditioning condensation or rainwater, the switch may not trigger. These switches need some level of contaminants, like salt or algae, in the water to detect the water level. One trick is putting a small amount of Dawn dish detergent or one of those toilet deodorant cakes in your bilge. The soap will remain for a long time, and it will help to clean your bilge!
Pressure Sensor Switches
These are the third type of switches. Pressure Sensor Switches work when water in the bilge increases the air pressure in a tube and trips a switch. These switches can detect very small water level changes. Some as little as 1/4 inch! They are, however, prone to clogging, and if oil gets into the sensor tube, it can either keep the switch from tripping or keep it engaged even after all the water is out of the bilge, causing the pump to overheat.
Bilge Pump System Control Switches
The next part of the bilge pump equation is the switch that controls sending power to the pump. It sits between the battery and the pump and/or float switch. These also vary widely in design, from a wire (hopefully with a fuse) connected directly to the battery terminal to soft buttons on screens in sophisticated boat management systems.
Direct Wire To The Battery
This is by far the simplest solution. It connects to the battery and ground. When the pump switch detects water, it turns on. While simple, this arrangement has some definite drawbacks.
First, you have no idea when the pump turns on, except perhaps for the pump noise or water coming out of the through-hull. Second, if the float switch sticks and the pump stays running, you must track down the fuse to turn it off. And your pump motor or impeller might burn out before you find it.
On boats with two or more bilge pumps, the pump lowest in the bilge may be wired this way, while the upper bilge pump is hooked to a three-way switch.
Simple On-Off Switch
An on/off switch lets you turn the pump off if the float switch gets stuck. It is a simple toggle switch typically mounted in the electrical panel or on the dash.
In this configuration, the circuit may also include a light or buzzer that indicates when the pump is running.
Three Way – Manual/Automatic/Off Switch
This is the recommended configuration, as it gives you the most flexibility. The Three-Way switch lets the float switch control the pump and lets you run the pump manually (bypassing the float switch). You can also turn it off (in case the automatic switch is stuck).
These switches typically have a fuse or circuit breaker to protect the pump and an indicator light to tell when the pump is running. (The red light will come on anytime the pump is operating) It’s the best of everything! Standard panel-mount versions are also available in many sizes and styles.
The only thing to be careful of is the switch’s location. If it is in a place easily bumped, you might accidentally set the switch to the off or manual position and not notice.
High Water Alarms
Another common use for float and electronic water level switches is to trigger a high water alarm. A separate float switch is installed above the highest bilge water level expected. This switch is linked to a light, buzzer, or both in the main living area of the boat. If water reaches that level and activates the switch, a signal is heard in the boat warning that the primary bilge pump is either not working or the water flow has exceeded its capacity.
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The Ultra Safety Systems Switches have a two-position sensor. The first will turn on the bilge pump, and the second will trigger an alarm. Several yacht monitoring systems like SirenMarine, Sensar, and ZigBoat have bilge sensors that detect water levels or will alarm if the bilge pump runs too long. You can also find WiFi water sensors on Amazon that will alert you to high water levels in your bilge, even when you are away from your boat!
Backflow Prevention
Backflow preventors are valves placed in the bilge drain hose that will only let water flow in one direction (out of the boat!). They serve two purposes. First, they keep water outside the boat from entering if the through-hull opening submerges. This can happen if the boat is riding low in the water due to excess weight, such as overloading or taking on water. Also, when a sailboat heels over, the through hull fitting can go below the water line.
The second use for backflow is to prevent water pumped into the bilge hose from flushing back into the bilge when the pump stops. Because pumps are usually low in the boat, water between the hose and through-hull will flush back into the bilge after the pump turns off. A long hose running to the through-hull can hold enough water to start the pump again when it drains back, causing an infinite loop.
A small amount of water running back through the pump is advantageous as it can flush out any debris stuck to the pump screen. We mount our backflow preventer 12-18 inches from the pump to allow for this flushing effect.
There are two styles of backflow valves. The flapper valve and the joker valve. Both work equally in a bilge situation where no large foreign objects are being pumped through them.
Make sure to read the directions on the valve. Some valves, especially the flapper style, may need to be mounted in a specific orientation to allow gravity to work on the flapper.
Note: The American Boat and Yacht Council (ABYC) does not recommend installing back-flow preventers, as they can become clogged or fail. Be sure to test your bilge pump regularly.
Anti-Siphon Loop/Device
Another component of back-flow prevention is the Anti-Siphon loop or device. If the through-hull goes below the water line, this will break a siphon condition, preventing the boat from flooding with water through the bilge pump.
The simplest of these is a loop in the bilge pump drain hose that brings a portion of the hose above the level of the through-hull. This can be a hump or a loop.
Another method is to use an anti-siphon valve. This device has a loop with a vent valve installed above the boat’s waterline. The valve will close when pressure from the pump pushes water through the loop and will open to allow air to come in when there is suction in the hose, stopping a siphon effect.
In situations with no installation location above the water line, some models are equipped with an air hose that attaches to the valve and allows you to run it to a location higher in the boat.
Through-Hulls
At last, we reach the point where nasty bilge water exits the boat! Through-hull fittings for bilge pumps ensure safe and efficient water expulsion from a boat’s bilge. Proper installation is vital to prevent leaks and maintain the vessel’s integrity.
Through-hull fittings are available in various materials, such as bronze, stainless steel, or high-strength marine-grade plastic, each offering different levels of durability and resistance to corrosion. Bronze and stainless steel are favored for their long-lasting properties, especially in saltwater environments, while high-strength plastic is valued for its lightweight nature and cost-effectiveness.
Through-Hull Location
Best practices for installation start with selecting a location above the waterline yet easily accessible from inside the hull. This placement helps prevent water from siphoning back into the bilge. Ensure the fitting is properly sealed with a high-quality marine sealant to avoid water intrusion. When drilling the hull, take precautions to avoid splintering or compromising the hull’s strength, potentially using a backing block to distribute stress.
Additionally, while running multiple bilge pumps to a single through-hull fitting to simplify the discharge system might be tempting, this practice is generally discouraged. Multiple pumps feeding into one fitting can create a bottleneck, reducing the efficiency of water expulsion. Also, one pump can force water back down the hose to the second pump, potentially flooding the bilge.
Instead, it is recommended that separate through-hull fittings be installed for each pump. This ensures that each pump operates at its full capacity and enhances overall safety.
If you want to run multiple pumps to a single through-hull, oversize the through-hull to allow the water to pass freely from both pumps simultaneously. Use a 2″ through-hull for two 1″ bilge pump hoses. Also, a back-flow preventer should be used on both bilge hoses to prevent cross-pumping.
Grey Water Tanks and Associated Pumps
Grey water pumps are similar in design to bilge pumps, so I’ll just touch on them briefly.
Gray water is wastewater from a boat’s sinks, showers, washing machines, air conditioning condensation, and other sources, but not toilets. The grey water tank or system collects this water from sources typically below the boat’s waterline and pumps it out of the boat.
Most cruiser-style boats have a small tank under the floorboards near the heads that collects gray water. A float or electronic switch activates a pump when the water in the tank reaches a set level and is discharged overboard.
There are two styles of these tanks: internal and external pumps. Internal pump tanks will have an impeller-style bilge pump and water level sensor built into the tank. External pumps will have a sensor in the tank, but the pump is typically a diaphragm pump located nearby.
These tanks need regular maintenance as the buildup of soap scum and debris can clog the filter screens common in these tanks. Also, if not used frequently, grey water tanks can become stagnant and smells will escape from drains. Unlike household drains, boat drains frequently do not have P-traps to prevent the escape of smells.
To clean your grey water tank, remove the cover and remove and clean the filter screens. Also, wipe down the float switch or sensor with a mild detergent and a rag. I like to flush several hot water cycles through the tank with the cover off to ensure it is clean.
You may need to use a wet vac to suck out accumulated debris, especially sand from dirty feet that can ruin your pump!
I like to put a dissolving toilet deodorant cake (like the type you drop into a toilet tank or hang on the bowl rim) into my gray water tank to keep it smelling pretty(ish). I’ve also heard of boaters who pour an ounce or two of fabric softener down the drain from time to time for the same reason.
Dry Bilge Systems
Last on our list is Dry Bilge Systems. Most bilge pumps will leave between 1/2 inch and 2 inches of water in the bilge. Wet bilges can smell and cause mold and mildew. Unless specifically designed for it, water sitting in a bilge can also seep into stringers and fiberglass, causing damage over the long term.
Several manufacturers have developed “Dry Bilge Systems.” These systems consist of small diaphragm pumps connected to pickups that either stick to the very bottom of the bilge or use sponges to absorb bilge water to be extracted by the pump. These systems can eliminate most of the excess water in the bilge. However, they are NOT for use as primary bilge pumps, as they typically only deal with around 1 gallon per minute.
Depending on the manufacturer, these units have sensors that detect small amounts of bilge water or operate on timers that periodically turn on the pump to remove water.
Building your own D-I-Y dry bilge system is a popular boat project. You can find videos and instructions by searching “DIY Dry Bilge Systems.” I built mine from the instructions on LifeEmpirically.com.
Show Your Bilge Pump Some Love!
Like everything, proper maintenance and regular testing will ensure that this crucial safety system operates reliably. Inspect your bilges and pumps regularly and keep them free of dirt, oils, coolant, and especially fuel!
At least once a year, preferably at the beginning of the season, take a hose and run water into your bilge to test that your bilge system is operating correctly.
If you plan to leave your boat unattended in the water for an extended period, test your bilge system and batteries before leaving, and consider a remote water level alarm.
See our article on building your own Portable & Flexible Bilge/Utility Pump
Does your boat have the proper number and volume of bilge pumps?
What type of water sensing system do you have?
Let us know in the comments below! 
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