Installing a High Water Bilge Alarm

By Frank Lanier
Chesapeake Bay Magazine (2006)
Offshore Magazine (2005)
Lattitudes & Attitudes (2006)


If keeping water outside the boat is the first rule of common-sense boating, the second must be having a high water bilge alarm installed for those times rule number one are broken.  When it comes to dealing with leaks quick detection is the key, meaning the early warning provided by a bilge alarm can add valuable minutes to handle flooding situations – extra time to find the leak (while it’s still above incoming water), don lifejackets, bring additional pumps online, send out that last-ditch distress signal, or to hopefully give marina personnel enough warning to keep your boat from sinking while at the dock.

Installation of a bilge alarm is a straight-forward affair that’s easily within the ability of most boaters.  They can be installed as a standalone system (i.e. an alarm and dedicated float switch), as an add-on to an existing primary or backup bilge pump and automatic float switch, or as part of that new backup bilge pump installation you’ve been putting off.  My preference is to have a dedicated float switch separate from the bilge pump, that way the bilge alarm will sound even if the automatic float switch for the pump fails.  Bilge alarms can also be configured to operate both audible and visual alarms (sirens, strobes, etc) the goal being to get attention, whether tied at the dock or roaring along at full speed.

The goal of my bilge alarm project was installation of an alarm in the cockpit, one loud enough to be heard above engine noise or from the dock when moored.  The first thing I did was to sketch out a simple wiring diagram showing all electrical connections for the horn and switch, as well as where each component would be physically located (see illustration 1).  Drawing out the installation and filing it for future reference not only makes it easier to visualize the installation now, but also aids in troubleshooting later on should the need arise.

Bilge alarm kits consisting of a float switch and alarm or a small panel with buzzer and bilge pump control switch can be purchased at most marine outlets for around $70.  The latter are typically not weatherproof and are often installed inside, where they’ll likely not be heard from the helm (particularly with the engine running) or by folks on the dock.

To overcome these issues, I felt the simplest way to go was to install a customized alarm system meeting my particular needs.  Don’t let the word “customized” scare you though – my install consisted simply of a typical bilge pump float switch and a compact, 12 Volt marine horn (like you’d find on a small runabout).  The float switch, horn, wiring, and miscellaneous items to compete the install (connectors, heat shrink, etc) were readily available and cost around $50 (I picked up the horn from a local marina’s bargain bin for $10.00).

Once I had my plan in place, the installation itself was relatively easy.  I mounted the automatic float switch approximately 3 inches above the cut-on point for the primary bilge pump float switch so that the alarm turns on only if the pump is overwhelmed or becomes inoperable.  The height of the alarm float switch above the pump float switch will vary, based on how quickly you want your alarm to turn on.  As for the horn, I decided to mount it in a small compartment just beneath the helm seat where I’d be sure to hear it (and likely have a heart attack) if it ever goes off.

Following the directions that came with the horn, which provided the gauge (size) of wire needed based on total length of the wire run, I measured the round trip distance from battery to the float switch, float switch to horn, and from horn back to the battery.  Total round trip length of the wire run was 20 feet, which as per the horn manufacturer required the use of 14 AWG (American Wire Gauge) marine grade wire.  Another good option to find out what gauge wire to use for your particular installation is the free wire calculator found on Ancor Marine’s website ( under the “Technical Info” tab.

Once the float switch and horn were in place, next up was the wiring.  Starting at the positive terminal of the battery, I ran a wire to the pump, then up to the horn, and finally back to the negative terminal of the battery, keeping the wire run above the normal accumulation of bilge water (to reduce corrosion issues) and support at least every 18 inches with wire ties.  I then made the connections at the float, horn, and finally at the battery using marine grade connectors (the type with heat shrink pre-installed) holding off on completing connections to the battery until all work was finished.  You may notice that the wire leads for most float switches are pretty long – the reason manufacturers do this is so you can mount the connectors up and away from the corrosive environment of the bilge.  After activating the heatshrink of the connectors with a heat gun, I then applied a liberal coating of liquid electrical tape at each connector in efforts to make them as waterproof (and hopefully corrosion-proof) as possible.

Next up (as called for in both the horn and float switch instructions) was installation of a fuse in the positive or “hot” wire to protect the circuit.  As I wanted to connect the alarm directly to a battery (so it would have power even with the battery switches in the “OFF” position) I opted to install an inline fuse holder near the battery in the wire I’d ran from the positive terminal to the float.

The last step was installing the positive and negative connections at the battery.  Once completed, I gingerly raised the alarm’s float switch and was rewarded with a deafening howl that had boaters in a 20 slip radius popping up through their hatches like seagoing prairie dogs.  Ah, the nerve-wracking sound of success.



In addition to a high water bilge alarm, there are few other simple installations boaters should consider in their goal to stay afloat.

The first is installation of a visual “bilge pump on” light at each helm position.  My boat has one as part of the three position control switch panel for the pump, but as the panel isn’t weatherproof, it’s mounted below in the main cabin and not visible from the helm.  As my bilge pump rarely turns on, a light at the helm would give me even more of a heads up that something isn’t quiet right in the bilge water department.

A bilge pump on/off counter is also desirable to indicate how often bilge pumps are cycling (making a leak more noticeable).  Finally, installation of back-up bilge pumps are always desirable, however keep in mind that back up pumps should ideally be mounted and configured to turn on when bilge water level reaches around 4 to 6 inches above the cut on point for the primary pump.  This prevents the back up pump from resting in the normal accumulation of bilge water, where it can become clogged with sludge and debris or seized from disuse.