Combining the early warning of a high-water bilge alarm with the additional pumping capacity of a backup bilge pump can add valuable minutes when faced with a flooding situation. You may gain extra time to find a leak (while still above incoming water), don life jackets, send out a distress call, or hopefully give marina personnel enough warning to keep your boat from sinking while at the dock.
Bilge alarms can be installed as a standalone system, but combining one with that new automatic backup bilge pump installation you’ve been putting off is proactive and easy – particularly as both can use the same automatic float switch.
Before planning your alarm and backup pump installation, now is the perfect time to check your existing bilge pump system for issues. Take a moment to verify that it’s installed correctly, operates properly, and that it’s of sufficient capacity for the job at hand. Keep in mind that just because the boatbuilder installed a certain sized pump, that doesn’t mean it’s the correct size; a larger pump may well be in order, and a smart upgrade.
Once you’re sure your primary pump is of the right size, you’re ready to start your backup installation by sketching out a simple diagram. This not only makes it easier to visualize and plan, but also aids in future troubleshooting should the need arise.
Show all wire runs, electrical connections, circuit protection, bilge alarm indicator(s), the automatic float switch, and backup bilge pump. Include all plumbing required for the pump (discharge hose run and thru-hull, for example) and where each component will be physically located.
When choosing a location for the bilge pump and automatic float switch, ensure it is both functional and that all components are easily accessible for future maintenance and repairs.
Bilge alarms can be configured to operate both audible and visual alarms (a helm or mast-mounted strobe light, for example). Whatever alarm you choose, the goal is to get someone’s attention, whether at the dock or while roaring along at full speed. Commercially available bilge alarm kits consisting of a float switch and a small control panel with buzzer can be purchased at most marine outlets for around $70. The problem is that many of these off-the-shelf units aren’t very loud or weatherproof, two factors that mean they’re usually installed belowdecks where they are difficult to hear from the helm – particularly while underway.
To address these concerns, I installed a custom alarm system on my last boat. Don’t let the word “custom” scare you, though. A simple flapper-style bilge pump float switch, a used 12-volt DC horn (like you’d find on a small runabout), marine-grade wiring, a few other miscellaneous items (e.g., connectors, heat shrink), and an hour or so of work was all it took (Photo A).
There are several choices when selecting a backup bilge pump, however, DC-powered centrifugal pumps are the most common and the focus of this article. They pump a lot of water, are relatively inexpensive, and are designed to operate while completely submerged. They also have large internal tolerances and can pass small amounts of debris (a plus for backup pumps). However, this benefit also makes them highly sensitive to vertical or static head. In other words, the higher they have to push water vertically, the less efficient they become.
As to size, more pumping capacity for a backup pump is better – within reason. If the installation location you’ve chosen can accommodate a 3,000 GPH pump, then go for it. Just keep in mind that while it will assist the primary pump in removing water, a backup bilge pump is not an emergency bilge pump.
A backup pump is meant to provide you with an additional margin of safety in the event of moderate water ingress. An emergency pump, on the other hand, is designed to remove large amounts of water and (in combination with damage-control measures) hopefully prevent the loss of your vessel in the event of a major hull breach.
Don’t let the word ‘custom’ scare you. A simple flapper-style float switch, a used 12-volt DC horn, marine-grade wiring, a few other items, and an hour or so of work was all it took.
Frank Lanier
1. Start by installing the backup pump and automatic float switch. Both should be securely mounted and located approximately 2 to 3 inches above the cut-on level for the primary bilge pump. This allows the smaller primary pump to take care of nuisance bilge water accumulation (with less battery drain), while leaving the larger pump to kick in only when needed. The incorrect installation (Photo B) allows the backup pump to rest in the normal accumulation of bilge water, where it can become clogged or seized from lack of use.
2. Select a suitable location for installing the backup pump discharge thru-hull. While it’s possible you’ll have an unused thru-hull available, chances are you’ll have to install one. Never “T” the backup pump discharge into your primary pump discharge (or any other utilized thru-hull for that matter). Doing so reduces maximum discharge capacity, such as the case where a single 1-inch thru-hull is used for two 1-inch discharge bilge pumps. Such an arrangement also introduces the chance that the bilge pump discharge from one pump can backflow into the vessel via the other pump discharge.
The best location for the overboard discharge thru-hull will depend on your installation, however, it should be situated well above the waterline to prevent water from siphoning back into the bilge. Installing a riser loop or siphon break (vented loop) is also recommended for this reason and should reach at least 18 inches above static waterline where possible (Photo C). Use smooth interior marine-grade hose for pump discharge runs and double clamp (where possible) on each end with marine-grade stainless-steel hose clamps.
3. Mount the horn or alarm at the helm, cockpit, or other area where it can easily be heard (Photo D). Installing a cutoff switch for the horn is also a good option – it will be loud, and being able to shut it off in an emergency is a worthy upgrade.
4. Measure the entire length of the wire run (round trip distance from the battery to the pump, float switch, and horn) then select the wire size accordingly. Marinco offers a free wire calculator on its website (marinco.com/resources/wire-data).
5. Starting at the battery (don’t connect it yet), run a two-conductor wire of appropriate size to the float switch, back-up pump, horn, and back to the negative terminal of the battery. Keep the wire run above the normal accumulation of bilge water (to reduce corrosion issues), and provide support at least every 18 inches.
6. Install wire connections at the float, bilge pump, horn, and battery using marine grade connectors – preferably the type featuring heat shrink (Photo E). A coating of liquid electrical tape is also useful to help prevent corrosion (Photo F).
7. Install an appropriately sized fuse in the positive or “hot” wire at the battery connection to protect the circuit. While power can also be supplied from your vessel’s DC breaker panel, connecting the alarm and backup pump directly to the battery ensures continuous power to the system – even when the battery switches are placed in the “off” position.
8. Attach the positive and negative connections at the battery securely using marine-grade connectors.
9. Lift the float switch to test both the back-up pump and bilge alarm.
Published: February 2026
Contributing Editor, BoatUS Magazine
Frank Lanier is a marine surveyor with over 30 years of experience in the marine and diving industry. He holds a 100GT master's license, and has captained and maintained many different types of vessels.