Electric Bilge Pumps - The Basics
Any boat that is not self draining and that spends time afloat and unattended will need a bilge pump to keep rainwater out. Most other boats will still have one, partly as a safety measure, and partly in case any water gets below the self draining deck. A self draining boat is normally a boat where the deck level sits above the waterline. This means that rainwater can run out of a hole in the transom. If the deck sits below the waterline, water cannot run out, as a hole in the transom would have the opposite effect and sink the boat!
Electric bilge pumps can either be manually activated by a switch, or automatically activated by a device called a float switch (or both). A float switch is activated by rising water, that in turn switches on the bilge pump.
Many boats have their bilge pumps set up to be either manually activated, or automatically activated, so the operator can choose how to use the pump. Needless to say, you must remember to leave the pump in ‘auto’ mode when you leave the boat unattended. This type of setup must also be wired to bypass a battery cut-off switch (if fitted), as you still want the pump to work even if the isolator switch is in the ‘off’ position.
Bilge pumps with built in switches
Some bilge pumps combine the pump and float switch into one neat package. These are great when space is at a premium, and they normally have the option of a manual override built in to them too. Some take this even further and replace the float switch with a different technology such as a field sensing switch that senses water with no moving parts. Others measure the resistance of the motor, and determine if water is present that way - this type will run for a second or two every few minutes to check for the presence of water, so may be best avoided near beds or bunks as some find them annoying when trying to sleep!
What size bilge pump?
Bilge pumps are available in a wide range of sizes to suit different sizes of vessel. They are often measured in gallons per hour or litre per minute of flow. Choosing a size depends on several factors such as the type of boat, surface area and climate, but something around 500GPH is ideal for most smaller, trailerable size vessels. As the vessels get larger, the intended us moves away from clearing general rainwater (although still useful for this) and more towards a safety device to help clear water if the boat is holed below the waterline. These pumps can be much larger, often over 2000GPH, and sometimes several are fitted, in different compartments of a boat.