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Chuck Husick: Techno-Talk, November 2005 from BoatUS Magazine - Updated November 2008

Automatic Bilge Pump Switches

This column usually concerns itself with relatively complex marine electronic devices like radar, radios, chart plotters and autopilots rather than some of the more prosaic products usually found on boats. But, one apparently simple device, the “automatic” bilge pump switch, is anything but basic.

Automatic bilge pump switches have a seemingly routine job to do. They sense the rising level of water in the bilge, turn the pump on, then turn the pump off when the water level has been reduced to an acceptable level. It’s important for the switch to work properly every time. If it fails to turn on, the rising level of water in the bilge could damage equipment or even flood and sink the boat. If the switch activates the pump and then fails to turn it off, continued operation of the pump motor can drain the boat’s battery, eliminating the protection assumed to be provided by the automatic bilge pump system.

Bilge pump switch problems often occur when the liquid in the bilge is not just water. An accumulation of debris in the bilge can block the action of some types of switches. An extreme example is provided by draining the galley sinks and shower drains on some boats into a bilge sump. Even if food scraps are meticulously excluded from the water, the inevitable accumulation of waxy, soap-like material that results from the use of dish-washing detergents will interfere with almost any conventional water level sensing switch.

The most common type of bilge pump switch uses a pivoted float to sense water level. It’s obvious that this type of switch can fail if the float’s movement is impeded by debris in the bilge. However, an additional and insidious failure can occur when a float switch is used in a boat kept in salt water. In virtually every design, the wires connected to the movable float are immersed in salt water. Over time the insulation on these very flexible wires degrades, allowing a small current to flow through the salty bilge water. The electrolytic reaction that ensues eventually dissolves the copper wires inside the insulation, leaving behind what amounts to two rubber bands and a totally useless bilge pump switch.

Alternatively, some automatic bilge pump switches operate by sensing the increase in air pressure in a small plastic dome caused by the rising water level in the bilge. The reliability of this type of switch benefits from the fact that electrical contacts and wires are remote from the bilge water. However, an accumulation of debris in the small diameter air pressure sensing tube can cause a failure. Other switches sense the electrical conductivity of the water in the bilge. Although generally reliable, oil or grease in the bilge water can interfere with the operation of this type of switch, delaying or preventing it from activating the pump or failing to turn the pump off when the bilge has been sufficiently dried.

Fortunately, electronic technology now provides “no moving part liquid level sensors” that can reasonably be expected to be immune to accumulations of debris, grease, oil or virtually any other contaminant that might be found in a bilge or in a galley sink sump. The Ultima switch made by Johnson Pump (and sold by West Marine under their part number 7865637) uses field effect detector cells to sense the presence of water in the bilge. The switch activates the bilge pump when the rising water level reaches a depth of two inches. The pump will continue to run until the water level drops to 3/4”. The switch consumes power from the battery, however the current drain, 16 microamperes is far too low to be of concern. An 8 second delay minimizes pump cycling caused by sloshing of water in the bilge.

An alternate approach based on sensing the electrical capacitance of water is used in the Oil Smart switch made by See Water Inc. ( The manufacturer claims that the switch will differentiate between water and oil, preventing the discharge of oil from the bilge.

Regardless of the type of automatic bilge pump switch installed in your boat it is always a good idea to install a backup switch a few inches above the main switch. Wire this switch to the bilge pump and to alarm buzzers above and below deck and be sure to check the operation of the automatic bilge pump switch every time you board your boat.

By Chuck Husick

Chuck Husick is a pilot, engineer, sailor and former president of Chris Craft Boats.

© Copyright BoatUS Magazine 2005

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