How to Select a Bilge Pump

By Tom Burden, Last updated: 6/1/2020

Virtually all boats end up with unwanted water in the bilge, which must be removed. Water gets in from a propeller shaft packing gland, a leaky portlight, leaky seams in a wood boat, melting ice from the icebox and a multitude of other sources.

Leaving large amounts of water in the bilge can have several undesirable effects on your boat, including destabilizing it, lifting spilled fuel and distributing it throughout the bilge (where it can attack, among other things, the insulation on electrical wiring) and promoting the development of osmotic blisters in fiberglass hulls. And did we mention having the boat sink? Getting this water (we call it nuisance water) out of your boat is the primary function of a bilge pump.

In some cases, the leak may be of a catastrophic nature caused by a hole in the hull from a collision or a wave taken on board in a storm. These situations call for very large pumping capacity. For the most part, the largest pumps will only buy you a little time to effect repairs to the leak, limp back to shore or make preparations to abandon ship. For emergency situations like this, it is advisable to have multiple electric pumps and at least one high-capacity manual diaphragm pump.

Therefore, we’d like to reinforce the following words of wisdom regarding bilge pumps so you’re not disappointed in the future:

  • Virtually no boat has a bilge pump system which is large enough to keep up with a leak caused by hull damage. Bilge pumps are designed for small quantities of water and are not damage control pumps like the ones Coast Guard helicopters drop. The American Boat and Yacht Council standards for electric bilge pumps state that they are “intended for control of spray, rainwater and normal accumulation of water due to seepage and spillage.”
  • Never leave a boat with a known leak alone with an automatic bilge pump in the hope that it will keep your boat afloat. We receive an occasional angry letter from a customer who claims that his boat sank because the pump we sold them did not keep their boat afloat. Of course, there are lots of reasons that a pump could fail to remove water from a boat, and we know you’d never trust a pump to keep up with a leak, but some other boaters do. If your boat has a leak, haul it out and get it fixed. Don’t depend on a pump!

Small Powerboats

small powerboat

Planing runabouts and ski boats use one submersible pump located aft. Small boats with large cockpits or low freeboard need bigger pumps.

  • Small internal volume means they can sink more rapidly than bigger boats
  • Limited bilge capacity to hold water away from engine/electrical system
  • Lower freeboard allows them to ship more water over the gunwale
Boat LengthPump Output - Gallons Per Hour
To 18'300 to 500
18'-23'450 to 700
22'-26'700 to 1,100

What type of boat do you have?

A small racing keelboat that sees lots of rough water action will need a better pump system than a large powerboat that is mainly operated on a calm lake. Here are some suggestions:

  • Daysailers and open outboard-powered skiffs generally use a portable piston pump, bucket or hand bailer.
  • Runabouts and ski boats use a single submersible electric pump in the stern or in the lowest point in the bilge. Boats with stern drives may have the pump under the oil pan of the engine.
  • Small cruising and racing sailboats can use one large manual diaphragm bilge pump mounted in the cockpit.
  • Offshore racing sailboats are required by the ISAF Special Regulations to have two permanently installed manual pumps, one operable from the cockpit and one operable from down below.
  • Coastal and offshore boats will generally want automatic electric bilge pumps located in each compartment (bilge area) that can hold water, and a large manual pump for backup

What size boat do you have?

Electric bilge pumps are rated by their capacity, which is measured in gallons per hour, or gallons per minute, under open flow conditions with no restrictions to the discharge. The rule of thumb is to get the largest model that is practical for the boat. Your selection will depend on size constraints and power limitations, but choose a 1,000gph model or larger, if possible. Submersible pumps generally have great open flow capacity, which rapidly decreases depending on how high the water is lifted.

Remember that small boats need good-sized pumps because:



Medium and large sailboats use multiple manual and automatic pumps.

  • Each compartment that can hold water needs its own pump
  • Additional large manual backup pump in accessible location
  • Engine driven pump or second, larger electric pump recommended for emergency backup, especially for coastal/offshore sailing
  • Head height affects flow. Sailboats have a deeper bilge
Boat LengthPump Output - Gallons Per Hour
22'–26'600 to 800
26'–30'800 to 1,000
30'–35'1,000 to 1,600
35'–40'1,600 to 2,200
40'–45'2,000 to 4,000
  • Hull volume is smaller, so a small boat will sink more rapidly than a large boat.
  • Less freeboard means small boats get swamped more easily.
  • The bilge is smaller, so water will flood the engine and electrical system sooner.

Do you have to replace an existing pump?

Match existing hose sizes and wire capacity, unless you want to upgrade the wiring and the thru-hull size too. Buy the largest capacity pump that uses the same hose diameters. For a few dollars more you can increase pump capacity significantly and still keep the same hose diameter. For a new installation, use the largest practical pump size.

Manual or electric power?

High-capacity centrifugal pumps are relatively inexpensive and the easiest pumps to install, but they are rendered ineffective if your boat’s electrical system fails, and this is a likely occurrence if your boat takes on a lot of water. For this reason, we recommend that, in addition to any electric pumps, you have at least one high-capacity manual diaphragm pump.

Manual pumps can move substantial amounts of water (up to 30 gallons per minute or so). With the exception of the higher-end pumps, most are very tiring to use, so be careful in your selection. Each gallon of water weighs over 8lb. and pumping it 15 feet or so is a challenging task for both the pump and the pumper. This is especially true if the pump’s location requires you to be on all fours or in some awkward position while pumping. An appropriate installation location and an effective pump is critical. When selecting a manual pump, it is most important to consider the gallons per stroke and the ergonomics of the pump.

Automatically Activated Pumps

If you want automatic operation, you need to install a bilge pump with a built-in or external float- or electronic-water-sensing bilge pump switch. Automatic pumps are great for removing small amounts of water but do not allow your automatic pump to lull you into a false sense of security. "Things happen." For example, in the face of an unknown leak, such as a thru-hull valve that is failing due to electrolysis, your boat will be fine—as long as the rate of water ingress does not exceed the capacity of the pump. But when it does, a potential sinking awaits.

The moral here is trust but verify. You can do this in several ways. First, you can make frequent physical checks of your boat. Second, you can install a cycle counter to record how often your pump turns on and off, and log this information over time. Another option is to install a light or buzzer that turns on when your pump is operating.

Beyond cycle counters, lights and buzzers you also have a high-tech option, which is to keep tabs on your boat's bilge with a remote monitoring system, such as Siren Marine's Connected Boat System. This system relieves worries about leaving your boat unattended. Using available sensors and an iPhone connection, it not only gives you the ability to connect to a bilge pump and or a high-water alarm, but also to monitor and control other systems, such as your boat's GPS, battery, engine metrics, shore power and more.

Do you have multiple bilge compartments?

Boats that have segmented bilges need modifications to the pump system since a single intake point may not get all the water. For a diaphragm pump, use either a flexible intake hose that can be manually relocated to different parts of the boat, or an intake hose manifold with several hoses connecting at a common point upstream of the pump. Another option would be to use several centrifugal pumps.

Large Powerboats

large powerboat

Larger powerboats use one or more pumps.

  • Each compartment that can hold water needs its own pump
  • Engine driven pump or second, larger electric pump recommended for emergency backup, especially for open water boating
Boat LengthPump Output - Gallons Per Hour
26'–30'800 to 1,200
30'–35'1,000 to 1,600
35'–40'1,600 to 2,200
40'–45'2,000 to 4,000

Types of Bilge Pump Switches

Integral automatic switches: Many pumps are available with a float switch pre-wired to the side of the pump. This makes it simpler to install, especially in tight vertical bilges. Another way to accomplish this is with the Rule Automatic Bilge Pumps, which spin their motors every few minutes and use the current draw from the motor to detect whether there is water present. This has the minor disadvantage of running the motor whether there’s water present or not.

Separate float switches: A popular method of converting a non-automatic bilge pump to automatic operation is to connect the pump to a separate float switch. These switches consist of a hinged float that completes a circuit to energize the pump when the bilge water rises. Formerly, many of these switches relied upon liquid mercury to complete the circuit, but due to environmental concerns, they have been eliminated. Environmentally safe switches have taken their place.

Electronic water-sensing switches use a pair of Mirus™ detector cells that sense the presence of water through the plastic housing using a low-impedance electrical field. Switches like the Johnson Ultima Electronic Switch have no moving parts to wear out and do not sense petroleum products, so you won’t accidentally pump fuel overboard if it is spilled in the bilge.

Centrifugal Vs. Diaphragm Pumps

Centrifugal pumps are submersible and non-self-priming, so they must be sitting in the water in order to pump it, and can usually remove all but the last inch of water. They work best when the bilge has a small sump where water collects. Centrifugal pumps use whirling vanes to draw fluid into the center of the pump and then push it outward from the center through an outlet port. They have a built-in strainer in their base that can be removed quickly for cleaning, which is important because the small impeller can get clogged with debris.

Diaphragm pumps are self-priming, which means they can lift water up an intake hose and expel it outside the hull. They use a membrane to increase and decrease the volume of a pumping chamber, drawing fluid in and pushing fluid out through a set of one-way check valves. Diaphragm pumps require an external strainer at the end of the intake hose since a small amount of hair or bilge debris can cause the valves to clog.

How a Bilge Pump Can Fail

Flow restrictions: Diaphragm pumps try to move the same volume continuously. If something restricts the flow, they work harder and harder until they stop pumping, at which point something usually fails. Centrifugal pumps respond to increased head pressure by moving less and less fluid until they stop pumping. The motor will not burn up, but the pump will stop moving any fluid.

Siphoning: The effect of siphoning occurs when a hose is led to a body of water above the starting point. If this water ever starts to flow backward it produces suction that draws water into the boat. This is one reason why bilge pump discharges frequently exit high on the hull well above the waterline. A vented loop can also be used to prevent back-siphoning.

Plugged Intakes: Bilge pumps with a remote water intake should have a strainer or strum box at the intake end of the hose, to prevent debris from clogging the pump or hose. Bilges are full of junk more often than not, so bilge pumps must be easy to clear and clean. The valves in some diaphragm bilge pumps are designed to pass objects as big as a marble, but hair and fiberglass strands are very difficult to defend against. Centrifugal pumps such as many we sell by Rule, Attwood and others normally snap into a base that also serves as an intake strainer. Keep it clean!

Faulty wiring or a dead battery: Corrosive saltwater and contaminants from fuel, cleaning chemicals and oil from engines can wreak havoc on wiring. So along with keeping your bilge clean, make sure to use tinned marine-grade wire and seal your connections with waterproof heat shrink butt connectors and terminals. Keep your battery charged and monitor its status frequently.

Bilge Pump Installation Tips

  • Keep discharge hoses as short as possible. Long hoses have more resistance, which reduces pump performance.
  • Use smooth bore hose. It is common to install corrugated hose for price reasons, but the ribbed bore reduces flow capacity and pump performance.
  • Use a vented loop if the thru-hull can be submerged, especially on sailboats where heel might immerse the fitting that is well above the waterline when the vessel floats on an even keel.
  • Follow the manufacturer’s recommendations for wire sizes to prevent voltage drop, especially on pumps that draw 10, 15 amps or more. Use the wire charts in the Electrical section of this website and size wiring for a three percent voltage drop.
  • Use waterproof butt connectors for any wiring connections in the bilge area. These wires are notorious for getting water inside the insulation and turning into green mush.
diagram of a dual bilge pump installation

Dual bilge pump installation

3,500gph (or larger) emergency pump located above sump, and connected to manual switch. 400gph (or smaller) pump with integral float switch, connected directly to battery.

Bilge Pump System Components

The complete installation will consist of several components:

  • The pump itself.
  • A connection to the boat’s battery.
  • A switch to be turned on and off. Most pumps will utilize a manual on/off switch, a float switch, or both. A float switch automatically activates the pump once the water level rises above a certain point and turns the pump off when the water level falls.
  • An intake hose, if you are installing a diaphragm pump, leading from the lowest possible spot in the bilge to the pump. It can be a corrugated hose, especially when the pump is mounted high above the bilge.
  • A discharge hose, which should be smooth leading from the pump to the thru-hull for overboard discharge of the extracted water.
  • Thru-hull fitting above the waterline

What size wire do you need?

The sizing of wire depends on amp draw and pump capacity. Use the following chart as a guide. It assumes a wire run of about 10 feet. Longer runs need larger wire sizes.

Pump CapacityWire Size
< 800gph16 ga
800–2000gph14 ga
2000–3500gph12 ga
4000gph10 ga