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Everything You Need To Know About Your Boat's Bilge Pump

While having this basic safety equipment installed on your boat isn't required by law, it is essential.

Bilge pumps

Photo: Mark Corke

When most boat owners work up the nerve to reach down into the questionable waters of their bilge, it's usually to determine why their pumps failed, rather than how to keep them from doing so in the first place. After hull integrity, bilge pumps are often the first and only line of defense against sinking, yet many boaters develop a frighteningly cavalier attitude toward their maintenance and operation.

New boats often come with too few or poorly installed pumps, while owners of older boats need to make sure previous owners didn't skimp on capacity when forced to replace one. Knowing how different bilge pumps work, how many you should have, and how to spot installation problems are good places to begin understanding these vital pieces of safety equipment.

Know Your Type

Bilge pumps perform two functions. The first and primary function is ridding the boat of normal accumulations of nuisance water (such as rain or packing gland drips). The second is to assist emergency pumps during a flooding situation (more on these pumps later).

The most common types of electrical bilge pumps are centrifugal and diaphragm pumps.

Centrifugal pumps move water by kinetic energy. Water enters the pump, picks up speed as the impeller rotates, and is then forced out by its own momentum. These pump a lot of water, are relatively inexpensive, and are designed to operate while completely submerged. Most centrifugal pumps have large internal tolerances and can pass small amounts of debris. This, however, makes them highly sensitive to vertical or static head — in other words, the higher they have to push water vertically, the less effective they become.

Diaphragm pumps are self-priming and work on a principle called "positive displacement," meaning they act like little vacuums to suck out bilge water. The action of the diaphragm pulls water into the pump through an intake valve and then pushes it out through an output valve. These pumps can pass small amounts of debris, but their check valves are prone to clogging and failure if too much is present.

Pros for diaphragm pumps include the ability to mount them up and away from the corrosive environment of the bilge (which increases ease and likelihood of maintenance), more robust construction, and a longer life cycle than most centrifugal pumps. They're also less sensitive to the effects of static head. Cons are a smaller rating capacity (as compared to centrifugal pumps), more moving parts, and higher cost. They're also physically larger, meaning they take up more space and require more power to operate. They're also not waterproof and must be mounted in a dry location.

A third type of electrical pump is the flexible impeller pump. Combining the priming features of a diaphragm pump with the capacity of a centrifugal pump, these utilize a flexible impeller to draw water in, prime the pump, and keep it moving toward the discharge outlet. Engine raw-water pumps are a good example of this type.

The benefits are that they can be mounted at any angle and are efficient at low or high speeds, assets that make them a popular choice for pressurized freshwater systems. As they're not waterproof, however, they must be mounted in a dry location, well above bilge water levels. They're also less tolerant to passing debris and can burn up in a couple of minutes if run dry — characteristics that limit their popularity as bilge pumps.

If you don't want to count entirely on your electrical system to power your pumps — and you probably shouldn't — you'll need manual pumps, too. The two most common manual pumps are diaphragm and piston types.

Manual diaphragm pumps work on the same principle as the electrical versions; they can be fixed or portable, are self-priming (that is, they develop suction and prime themselves when dry), and can be single or double action, using single or multiple diaphragms. Some larger ones are rated at 50 gallons per hour (gph). Piston pumps work just as their name implies: You pull on a handle that draws a piston up a tube, sucking water along with it and flushing it out of a hose. These pumps are portable and can move about a gallon of fluid with every four strokes.

Pump Up The Volume

Now that you know about the different types of bilge pumps and how they work, you'll need to determine how many your boat should have. The first thing to understand is that just because the writing on the bilge pump box says it can pump 500 gph, that doesn't mean it will. Several factors affect pumping capacity, and while a manufacturer might get a pump up to 500 gph in the ideal horizontal conditions of a test lab, the real world is another matter. (See illustration below.)

Bilge pump math illustration

Several factors will lower the listed gph on your pump.

On most boats, water has to be pumped up before it can be pumped out — and that's a critical factor. The distance of vertical travel, called "static head" by the guys in lab coats, can decrease a pump's rated performance by as much as 75%. Just 2 feet of static head will cut the output of a small centrifugal pump by half, and 15 or 20 feet might neutralize the pump entirely, depending on its size. That's why checking the manufacturer's flow rates for various lifts and installations is critical in pump selection. Other factors degrading performance include bends and restrictions in the discharge line, as well as hose size and interior surface. (Small hoses with ribbed insides are the worst!)

So how much pumping capacity should your boat have? That's a good question, but one with no clear or easy answer, mainly because boats are so different. Any compartment that's essentially watertight — where water can't escape into another area — should have its own pump or two. The American Boat and Yacht Council (ABYC) has not set requirements concerning bilge pump capacity, though the American Bureau of Shipping recommends one 24 gallons per minute pump (that's about 1,440 gph) and one 12 gpm pump for boats under 65 feet. The Lloyd's standards relate to vessel length and volume, recommending a 50 gpm pump for a boat about 50 feet.

What Size Bilge Pump Do I Need On My Boat?

To me, it's a simple case of bigger is better — within reason, of course. Based on size alone, I'd recommend a minimum 5,500 gph electrical pumping capacity for a 40-foot vessel — divided among a 1,500 gph primary pump and two 2,000 gph backup pumps.

This is a minimum, so feel free to bump it up, especially where extenuating circumstances apply, such as increased vertical lift due to the deeper bilge of a sailboat.

Ready, Willing, And Able

Once you've determined the types of pumps you want and how many, the next step is making sure they're installed and maintained properly. Maintenance begins with the bilge itself; no pump can overcome a bilge choked with trash, so a good old-fashioned cleaning is the first order of business. Periodic bilge cleaning is a fact of life with older vessels, but even the bilge of a new boat can be littered with wood shavings, bits of fiberglass, globs of adhesives, and other construction trash that can plug up a pump. Oil in the bilge is just as bad; it combines with dirt to form a gooey sludge that can clog pumps and prevent float switches from operating properly.

Bilge pump debris

Bilge pump debris from new boat construction

Keeping your bilge clean is essential to ensuring your bilge pump(s) work properly, whether your boat is older (top) or new construction (bottom). (Photos: Frank Lanier)

Make sure all the pumps, float switches, and strainers are easily accessible — something essential for routine maintenance and emergency repairs. Remove, disassemble, and inspect pumps at least twice a year for broken or worn parts. Not only is this good preventive maintenance, but it also gets you familiar with disassembling and reassembling before the feces hit the rotary oscillator. Pay particular attention to rubber parts, such as diaphragms and internal valves, which can crack and deteriorate due to age, even if the pump is not often used.

Check that all pumps are firmly mounted, paying particular attention to the mounts themselves, which may develop hairline cracks. List each pump by type, location, and size for future reference — don't forget shower and galley sumps — and make sure you have sufficient spare parts onboard for each. Verify that each of the intake strainers is securely fastened in place, and while you're at it, clear them of any gunk that may have accumulated since the last check.

Float switches must be securely mounted and installed clear of wires, hoses, and other obstructions that can impede operation of the floating arm or flapper switch. Orient the switch fore and aft, with the flapper pointed toward the stern. This is especially important on powerboats: During jackrabbit takeoffs, surging bilge water can damage the flapper mechanism. Installing them close to a bulkhead or frame also helps protect the switch from a torrent of water. Enclosed switches eliminate this worry, but they're difficult to inspect and test. I like having the float in plain view, where I can keep an eye on it.

Unmounted bilge pump

Bilge pumps and switches should be properly secured.

Advances in switch technology range from microchips and magnetic relays to air-pressure sensors and even water conductivity sensors, while other switches have sensors that let them distinguish between water and other fluids, reducing the chance of illegal discharges. Regardless of the type of automatic switch you choose, make sure each pump has a manual on switch as well, which allows you to power up the pump should the automatic switch fail.

Inspect all bilge pump hoses (ensuring each is clamped with marine-grade stainless steel clamps), as well as the discharge thru-hull fitting. Composite thru-hull fittings can develop stress cracks from supporting the weight of the hose (securing the discharge hose to keep it from bouncing around will go a long way toward preventing this problem).

The discharge thru-hull itself should be well above the waterline to prevent water from siphoning back into the bilge. Boatbuilders often install them low to eliminate hull-staining backsplash, but that's a dangerous concession to tidiness; a change in waterline (by adding many pounds of extra gear, for example) could easily put the thru-hull at or below water level.

Vented loops and riser loops add some margin of safety, but be sure they reach at least 18 inches above the static waterline. If your bilge pump discharge hose uses a vented loop, be sure to clean and inspect it for proper operation as part of your routine maintenance.

Another thing you may see in your bilge pump discharge hose is a one-way check valve, which owners often install to prevent water in the discharge hose from flowing back into the bilge. Check valves are not recommended and should be removed as they can fail or become clogged.

Most bilge pump failures can be traced to corroded electrical connectors, either at the float switch or at the pump itself, so inspect all wiring and connections to make sure they're properly sealed and installed well above the normal accumulation of bilge water. If you find connections made with electrical tape and household twist-on connectors, replace them with proper marine grade heat-shrink connectors.

Bilge Pump Not Working? Here's How To Troubleshoot the Problem

ABYC standards recommend providing circuit protection for each pump. You can do this by powering them through the main breaker or fuse panel, but then you'll have the potential problem of someone inadvertently killing power to the pumps by throwing the main breaker. To prevent this, some advocate wiring them through a dedicated fuse to the "all" or hot terminal of the battery switch (the one that's always energized), or even directly to the battery itself.

The switch is the better option, as it's generally poor practice to wire equipment directly to the battery. The battery switch option also ensures the pump can draw power from both batteries, rather than limiting it to one.

I took the dedicated-line-and-fuse approach one step further on my last boat. For all the pumps onboard, I installed a small dedicated breaker panel, which I then wired directly to the battery switch, bypassing the main circuit breaker. I also fashioned a small Plexiglas cover and stenciled "do not turn off" on it. This keeps all the pump fuses and breakers together, easily accessible, and eliminates the potential for someone to accidentally cut off the bilge pumps by shutting down the main breaker panel.

Emergency Bilge Pumps

As mentioned earlier, backup or emergency pumps differ from regular bilge pumps in that they provide enough pumping capacity to dewater (or at least maintain) bilge water levels in the event of a hull penetration, failed thru-hull, or similar breech.

There are a number of choices when it comes to backup pumps, from engine- driven units to manual pumps. However, electric powered centrifugal pumps are probably the most common. They pump a lot of water, are relatively inexpensive, have large internal tolerances, and can pass small amounts of debris (a plus for emergency pumps).

Flast Flow emergency bilge pump installation

An electric emergency bilge pump such as this Fast Flow ( can dewater a bilge in case of a hull breech. (Photo: Frank Lanier)

In addition to electrical pumps, you may also want to consider having a large-capacity manual pump on board. Though useful as backups, keep in mind they're powered by elbow grease, and even the fittest crewmember will have a hard time keeping up with them after a while — assuming you can spare a crewman to man the pump in an emergency situation.

Engine-driven emergency pumps allow you to harness the power of the engine in the event of hull breach. They can be powered off the crankshaft pulley (using a manual or electric clutch assembly) or the engine drive train. Another option is installing a suction takeoff from the engine's raw water pump. Just remember that running the engine pump dry will damage the impeller, and that the intake connection should be in front of the sea strainer (so you don't suck bilge debris into the engine). This is a proven and accepted pumping method, but know what you're doing before you do it, otherwise you can seriously damage your engine.

Backup pumps should be securely mounted and configured to automatically turn on when bilge water level reaches a predetermined height above the cut-on point for the primary pump (typically 3 to 4 inches, but low enough to prevent water from overflowing the bilge and damaging furnishings or equipment).

This lets the smaller primary pump take care of normal seepage (with less battery drain) and leaves the larger pump to kick in only when needed. It also keeps the backup pump from resting in the normal accumulation of bilge water, where it can become clogged with sludge and debris or seized from disuse.

In addition to primary and backup bilge pumps, it's always good to include a visual/audible high-water bilge alarm as part of your emergency dewatering strategy. Alarms should be loud enough to be heard over engine noise while underway and ideally by passers-by or marina personnel when docked. Installing a visual "bilge pump on" indicator at the helm for each electric bilge pump is also a good idea, one that can provide even earlier indications that something is amiss. A bilge pump on/off counter for primary bilge pumps is also desirable to indicate how often bilge pumps are cycling (making a leak more noticeable).

Final Thoughts

Many make light of bilge pumps by quoting the old nautical adage that the best bilge pump is a scared sailor with a bucket. That's funny and perhaps true to a point, but scared folks get tired, too, and often long before whatever's letting all that water in is fixed. So to keep passengers from having to choose between grabbing their life jackets or joining the bucket brigade, be serious about your bilge pumps. Someday, they may help save your boat, or even your life.

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Frank Lanier

Contributing Editor, BoatUS Magazine

Capt. Frank Lanier is a SAMS Accredited Marine Surveyor with more than 40 years of experience in the marine and diving industries. He’s also an author, public speaker, and multiple award-winning journalist whose articles on boat maintenance, repair, and seamanship appear regularly in numerous marine publications worldwide. He can be reached via his YouTube channel “Everything Boats with Capt. Frank Lanier” and website