Bilge Pumps: Selecting One With the Right Stuff

Whether we like it or not, boats leak. Yep, that’s right; even the finest, hand-built megayachts get water in their bilges for one reason or another. Sometimes that pesky water comes in from a propeller shaft log or a leaky raw water pump, while other times an icebox or fish locker drain is the culprit. And however water finds its way into your boat, it will eventually filter into your bilge, where it will require pumping out. That’s why a bilge pump is an essential piece of boating gear.

A Rule submersible bilge pump. These submersible pumps are designed to be mounted where the action happens: right down in the bilge.
A Rule submersible bilge pump. These are designed to be mounted right where the action happens: right down in the bilge.

A bilge pump’s primary function is to deal with the types of nuisance water we mention above—it shouldn’t be relied on as safety gear, although many boaters consider it just that. That’s because even the largest capacity bilge pumps won’t save your boat from sinking. Sure, they can certainly buy you some time—enough to haul out, perhaps—but their primary duty is to keep your bilge from turning into a pool of foul-smelling (and equipment-damaging) goop.

High-Capacity Submersible Centrifugal Pumps

The most common type of bilge pumps used on boats today are called “submersibles.” And, as the name suggests, these pumps are designed to live directly in the bilge and operate underwater (or under whatever funkiness lives in your bilge). The advantages of these pumps are that they have fairly high pumping capacities, and are also generally reliable, inexpensive, small, and easy to install.

These pumps do come with downsides. The biggest is that they will easily become clogged with debris: fish scales, bits of trash, sediment. And because of their location deep in the bilge, it can be extremely difficult to waterproof (and keep waterproof) the wiring that supplies the electrical current needed to run them.

Like all pumps, these are rated by how many gallons per hour (gph) they can pump. A good general rule of thumb is 300 to 500 gph for powerboats up to 18 feet, 500 to 700gph for powerboats up to 23 feet, and 700 to 1600 gph for powerboats up to 26 feet. If yours is larger than this, consult your marine shop pros for advice. Some sailboats and hollow-keel powerboats, due to their large bilges, have even greater gph requirements, since they are capable of holding more water.

Keep in mind that these gph ratings are “ideal world.”  The rating assumes two things: 1. There’s no lift involved (bilge pumps often have to push water up a hose several feet, which reduces the gph capacity significantly). 2. The pump is powered by a fully charged battery (around 12.6 volts DC).

Diaphragm Pumps

While somewhat rare today, some boats are equipped with self-priming diaphragm pumps that are mounted somewhere other than the bilge. These have a pickup hose that runs down to the bilge and a discharge that runs overboard, generally via a thru-hull of some sort. One key advantage to these pumps is that most of the wiring and plumbing is completely out of the bilge, avoiding some of the problems that befall submersible pumps.

An ITT Jabsco diaphragm bilge pump. These pumps are mounted up and out of the way of nasty bilge conditions, but generally have lower capacities than submersible centrifugal bilge pumps.
An ITT Jabsco diaphragm bilge pump. These pumps are mounted up and out of the way of nasty bilges, but they generally have lower capacities than submersible centrifugal bilge pumps.

Downsides to diaphragm pumps include a relatively high cost; the ease with which they can clog; and generally low pumping capacities when compared dollar-for-dollar to submersible pumps. Additionally, they have lots more moving parts, so they bear watching closely for failures. If you do encounter a problem, repair kits are generally readily available.

Manual Pumps

Having a backup is never a bad idea. In fact, we recommend it. The simplest manual pump is a portable, hand-actuated bilge pump such as Beckson’s Thirsty-Mate pump. There’s a pick-up at one end that gets submersed in the water and a discharge end with a hose for pumping overboard. They’re great for smaller boats where it may not be feasible to install a dedicated electric pump.

You can also find hand-actuated diaphragm bilge pumps that can be permanently installed and plumbed with a pickup and overboard discharge. They’re great as a backup in case your electric pump fails (and the electricity often fails on a sinking boat), or for smaller, deep-bilge boats where a portable isn’t feasible. Some of these pumps can move up to 1,000 gallons of water per hour, though that’s assuming you and your crew have the endurance to operate them for that long.

This Edson manual bilge pump can pump a heck of a lot of water, as long as you have the endurance. Manual bilge pumps such as this one make great back ups.
This Edson manual bilge pump can pump a heck of a lot of water, for as long as you have the endurance to move the handle. Manual bilge pumps such as this one make great backups.

Automation Nation

Most electric bilge pumps (submersible or otherwise) are installed with some sort of switch that automatically turns the pump on when water is present, and turns it off once that water is completely evacuated. The most common bilge pump switch setup (with either a submersible or diaphragm pump) is a float switch. Some submersible bilge pumps even come with their own internal switches.

Float switches work by rising or falling with the level of water in the bilge. When the switch floats upward, an electrical circuit is completed, and the pump turns on. As it lowers, that circuit is broken and the pump turns off. While float switches are generally reliable, they do have some problems.

A float switch’s location in a watery bilge makes it somewhat susceptible to electrical connection issues where power leads are attached, and water can sometimes find its way inside a switch and ruin it. The switch mechanisms inside the floats can go bad, and debris can also jam a float switch and render it inoperable.

A common bilge pump float switch. These switches can be very reliable, but require frequent monitoring.
A common bilge pump float switch. These switches can be very reliable, but they require frequent monitoring.

To counter some of the inherent problems, manufacturers started building pumps with integral microprocessor switches that can sense the presence of water, either by “feeling” pump resistance against water, or by creating a low-impedance electrical field to sense the presence of water.

The good part about having any type of fully automatic bilge pump setup is that you don’t have to worry about nuisance water gathering and potentially damaging your boat. It can even help save your boat if it starts to take on water when you’re not onboard. The bad part is that automation can make you complacent and unaware of slowly developing leaks that should otherwise be investigated. You can circumvent this problem by installing a cycle counter, buzzer, or indicator light on your bilge pump circuit, so you know how often it is running, even when you are away.

Now that you have a PhD in bilge pumps and bilge pump systems, you’re likely curious about installing them—the right way. Check in here next week when we’ll discuss wiring, installing, and plumbing a bilge pump like the pros.

Written by: Gary Reich

Gary Reich is a Chesapeake Bay-based freelance writer and photojournalist with over 25 years of experience in the marine industry. He is the former editor of PropTalk Magazine and was the managing editor of the Waterway Guide. His writing and photography have been published in PassageMaker Magazine, Soundings, Fly Fishing in Salt Waters, Yachting Magazine, and Lakeland Boating, among others.


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