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Bilge Pump Basics

They're your last line of defense against sinking, so give them some respect — and a little love from time to time.

Primary and secondary bilge pump strainer

Due to the lack of maintenance they receive from the average boat owner, I often refer to bilge pumps as the Rodney Dangerfield of boat equipment, meaning "they just don't get no respect." It's a funny but also troubling statement, particularly because bilge pumps are often a boat owner's first and only line of defense against sinking. To prevent you from being that proverbial scared sailor with a bucket, here's a look at bilge pump basics — from what they do, to selection, installation, and maintenance.

What They Are (And Aren't)

The primary job of most bilge pumps on most boats is to clear nuisance water from the bilges (packing gland drips, spray from an errant wave, etc.). The one that came with your boat may be up to that task, but a bilge pump should also be able to provide crucial extra time if your boat is taking on water, allowing you to find and deal with the source of a leak or, in extremis, to don life jackets and hopefully keep your boat afloat long enough for help to arrive. Most people upgrade to a larger bilge pump or add a backup bilge pump to give them time to deal with an emergency. Even that pump should not be confused with an emergency pump, which provides much greater dewatering capacity. If you plan on fishing the Canyons or sailing offshore, then you need a true "crash" pump, one that can move hundreds of gallons per minute.

This article will focus on the middle of these three options — the bilge pump meant to remove nuisance water but also to give you time to deal with an emergency. See the article "Bilge Pump Capacity: Do The Math" to figure out how much pumping capacity you need.

What Type Of Bilge Pump Do You Have?

The most common types of bilge pumps (and the primary focus of this article) are centrifugal and diaphragm electrical pumps. Centrifugal pumps move water by kinetic energy using a rotating, solid impeller, similar in design to a turbine. Water enters the pump, picks up speed as the impeller rotates, and is then forced out by its own momentum. Centrifugal pumps are submersible but not self-priming, meaning they must be sitting in water in order to pump it.

Centrifugal pumps are relatively inexpensive and can move a lot of water. Other advantages include low maintenance, excellent reliability, and the ability to pass small amounts of debris without clogging. They can also run dry for extended periods without damage, although this does wear the bearing and will cause it to fail eventually. Disadvantages include their inability to self-prime and their loss of effectiveness the farther they have to push water vertically.

A diaphragm pump acts like a little wet-vac to suck out bilge water. Water is pulled in through an intake valve, then pushed out through an output valve. Diaphragm pumps are self-priming (meaning they develop suction and prime themselves when dry), can be run dry without damage, and are better able to push water uphill than centrifugal pumps. As to downsides, they can't move as much water as a comparable centrifugal pump and most can't tolerate even small bits of trash or debris, which can lodge in the pump's valves, causing leaks or failure.

Nothing beats the simplicity and pumping power of a centrifugal pump. However, diaphragm pumps can be a better choice where water has to be pumped more than a few feet uphill, and they can be installed in a drier, more convenient place than the bottom of the bilge. Adding a filter before the pump to remove debris reduces the risk of failure.

Do You Have Enough Capacity?

Now that you know how different bilge pumps work, the next step is determining how many and what size your boat should have. The first thing to understand when shopping for a bilge pump is that just because a pump is rated to pump 1,000 gallons per hour (gph) that doesn't mean it will. See the "Bilge Pump Capacity: Do The Math" to figure out how much a bilge pump is likely to actually move when installed on your boat compared to its rated capacity.

So how much pumping capacity should your boat have? It's a good question, but one with no clear or easy answer, mainly because boats are so different. Any compartment that's essentially watertight (i.e. where water can't drain into another area) should have its own pump or two. While the American Boat and Yacht Council (ABYC) hasn't set requirements concerning bilge pump capacity, the American Bureau of Shipping recommends one 24-gallon-per-minute (gpm) pump — or about 1,440 gph — and one 12-gpm (720 gph) pump for boats under 65 feet. To me, it's a simple case of bigger is better — within reason, of course. Based on size alone, I'd recommend a minimum of 5,500 gph pumping capacity for a 40-foot vessel, divided among a 1,500-gph primary pump and two 2,000-gph backup pumps. I'd also throw a high-volume manual pump in the mix for good measure, since all that pumping capacity will be useless if something happens to your batteries.

Based on this, you'll probably find you don't have enough pump capacity. If you decide to install more, make sure to follow tips below. And if you are happy with what you have, take a look to see if your current bilge pump needs a bit more respect.

Are Your Bilge Pumps Installed To Minimize Problems?

The first step on the path to bilge pump nirvana is making sure your boat's bilge is clean and free of trash and debris. Routine bilge cleaning is a fact of life for older boats, but even that new boat you're purchasing can have a bilge littered with pump-clogging bits of construction material — wood shavings, bits of fiberglass, and gobs of epoxy. Oily bilge residue should also be cleaned up and disposed of properly. In addition to the ecological concerns of accidentally pumping it overboard, oil combines with dirt to form a gooey sludge that can clog pumps and prevent float switches from operating properly. In addition, some newer bilge pumps are designed not to pump automatically if they sense oil in the water, so keep those bilges clean!

Bilge choked with trash and debris

No pump can work properly in a bilge choked with trash and debris.

Sludge clogged bilge pump

Oil combines with dirt to form a gooey sludge that can clog pumps and prevent float switches from operating.

Use smooth-walled marine-grade hose for pump discharge runs and secure them at each end with marine-grade stainless steel hose clamps. Hoses should be routed as directly as possible to their discharge thru-hull and should also be properly supported (approximately every 18 inches) to prevent chafe and excessive movement. Speaking of discharge thru-hulls, they should be situated well above the waterline to prevent water from siphoning back into the bilge. Siphon breaks and riser loops are also recommended and should reach at least 18 inches above static waterline where possible.

If your bilge pump uses a flapper style automatic float switch, it must be securely mounted and installed so that the floating-arm is clear of wires, hoses and other obstructions that could impede its operation. 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. Regardless of the type you choose, make sure each pump has a manual switch as well; none of the automatic systems are failsafe.

Make sure all bilge pumps not only have intake strainers or strum boxes installed, but that they can be easily reached and cleared of debris. As a marine surveyor I often see centrifugal-style pumps mounted beneath engines and completely inaccessible, even to simply clean the strainer — if your boat has similarly inaccessible pumps, relocate them for better access.

Common bilge pump problems

Common bilge pump problems include physical damage, loose hoses and failure to securely mount the pump to ensure proper orientation.

ABYC standards require circuit protection for each bilge pump. You can do this by powering them through the primary DC circuit breaker panel, but then you'll have the potential problem of someone inadvertently killing power to the pumps by turning off the primary battery switch. To prevent this, you can also wire each 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 battery switch option ensures the pump can draw power from both batteries, rather than limiting them to one. An even better option is installing a small, dedicated bilge pump breaker panel (which has the added benefit of keeping all bilge pump fuses and breakers together and easily accessible). This panel would then be wired directly to the battery switch, bypassing the primary DC circuit breaker panel.

When wiring your bilge pump, ensure all electrical connections are located well above normal bilge water levels (to reduce corrosion issues) and properly terminated with marine-grade connectors — leave those wire nuts and electrical tape joints at home! Finally, while it's fine to go with oversized wire, don't upsize the fuse. The BoatUS Marine Insurance claim files include many fire claims caused by centrifugal bilge pumps that overheated when something got caught in the rotor assembly and the fuse didn't blow because the installer thought a slightly bigger fuse would be better. When it comes to fuse size, follow the manufacturer's recommendations exactly.

What Maintenance Should I Be Doing?

Problems with centrifugal pumps typically involve clogging, defective automatic float switches (if installed), or corroded electrical connections, a common problem with any electrical gear installed in corrosive environment of a vessel's bilge. Maintenance is generally limited to clearing the strainer (centrifugal pumps have one built into the base) and waterproofing all connectors. When it comes to repair, with the exception of the larger, rebuildable units, most centrifugal pumps are so inexpensive that it usually costs less to replace a damaged pump than repair it.

Maintenance and repair of diaphragm pumps typically involve opening up the pump body, clearing the pump chamber of debris, and checking the diaphragm and valves for damage or deterioration. Other than clogging, most problems will be caused by torn or damaged check valves. The diaphragms can also fail though they will typically outlast several valve changes. Pump disassembly for maintenance is normally straightforward; however, some are more complex than others (multi-chambered units, for example), so be sure to read all instructions carefully to avoid common mistakes, such as improper orientation of check valves during installation.

So give that bilge pump a little respect, and it will keep your bilge dry and maybe even keep your boat afloat long enough for you to figure out where that water's coming from!

Keep Your Bilge Pump Pumping

  • Test and verify operation of all bilge pump systems at regular intervals (monthly at a minimum). Testing should verify the actual pumping of water overboard, rather than (in the case of electric pumps) simply switching the pump on and listening for motor operation.
  • Disassemble and inspect pumps periodically for worn or damaged components. Pay particular attention to neoprene or other soft parts such as diaphragms, check valves, etc. This is not only good preventive maintenance, but also lets you learn the ins and outs of disassembling and reassembling before an emergency occurs.
  • List bilge pump by type, location and size for future reference and make sure there are spare parts or complete rebuild kits onboard for each. If you really want to go the extra mile, pack a complete spare pump assembly. Being able to swap out a defective pump lets you quickly bring the system back up, while giving you the option of repairing the damaged pump later at your convenience.

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Frank Lanier

Contributing Editor, BoatUS Magazine

Capt. Frank Lanier is a SAMS-accredited marine surveyor with over 40 years of experience in the marine and diving industries. He’s an author, public speaker, and multiple award-winning journalist whose articles on boat maintenance, repair, and seamanship appear regularly in numerous marine publications worldwide. Contact him via his YouTube channel “Everything Boats with Capt. Frank Lanier” or at