A bilge high water alarm is a simple device that can alert you to water rising faster in your bilge than your bilge pump can pump it out.
Most boats large enough to have an inboard engine also have through-hulls and other means by which water can enter the boat. In event of a failure in any of these systems, or a problem in the integrity of the hull, water can overwhelm an automatic bilge pump and, in extreme cases, sink the boat. It can be difficult to find and correct a leak once the water becomes deep enough to obscure visibility in all areas.
To guard against this emergency, consider installing a bilge pump counter, bilge pump alarm, and/or high water alarm. These three systems work in different ways and offer different benefits, and you may want to use more than one. This article describes the use of a bilge high water alarm.
Start with an Automatic Bilge Pump
Every boat gains from an automatic bilge pump that comes on when an internal or external float switch or sensor indicates water has risen to a certain level in the bilge. On many boats the bilge pump is wired into the electrical control panel, tempting the owner to shut it off when leaving the boat or at other times - defeating the whole purpose of being an automatic pump. Or even if the switch is left on, the power to it may be cut if you shut the main battery switch off when leaving the boat, as generally should be done to prevent losing power to a short or other systems left running.
A simple solution is to wire the automatic bilge pump directly to one of the boat's batteries with an inline fuse. No matter what is done with the panel or battery switch, the pump will run as long as the battery has power. The only downside is that the pump may get stuck on and drain the battery completely (and/or overheat the pump). If you have multiple batteries the risk is minimal if you shut off the battery switch so that they are not connected in parallel to the pump. The risk is preferred to potential damage from a leak when you're away from the boat.
Why Use a Bilge High Water Alarm?
A bilge high water alarm is a simple device that sounds when the bilge water level rises above a level that is typically controlled by the automatic bilge pump. This is usually an emergency situation, and you need to act fast to locate and fix the leak before the boat - and perhaps your life - is threatened.
Without such an alarm, by the time water is over the floorboards in most boats, the water level is already so high that it can be difficult to locate its source. As well, because boat batteries are usually located fairly low in the boat, there is also a risk of water flooding the batteries and shorting them out, thus stopping the bilge pump and increasing the rate of flooding.
The basic systems include the alarm itself and a sensor that is mounted in the bilge at a level water should not normally reach when the bilge pump is functioning. Water reaches this level only if the pump has failed or is running but is unable to keep up with the ingress of water due to a major leak.
Install the system as instructed by the manufacturer. While it is possible to build your own system with a sensor designed for other applications, such as a home basement water sensor and a loud 12-volt alarm, it is generally more efficient and safer to purchase a more rugged system designed for marine use. Remember to locate the alarm itself in a place where it can be heard from both inside and outside the cabin.