How does the bilge pump work

The function of the bilge pump is to remove water that collects in the bilge, which is the bottom of the inside of the hull. Most of the time, water that collects in the bilge is incidental:

It could be rain water or water from spray collecting in the boat;

Wash-down water that collects in the bilge while the boat is being cleaned; 

Or water from built-in coolers that drain to the bilge.

Usually the voltage will be 12V or 24V dc type 

Where are Bilge Pumps Located on a Boat?

At least one pump or bilge pump pickup should be installed at the lowest point in the bilge. Larger boats should have one in each enclosed area that can retain water. Discharge outlets need to be a minimum of eight inches above the waterline.

All bilge pumps on the boat should be accessible, so you can inspect the pump and its float, and clear debris from around the pump pickup. If the pump is located in a spot that’s inaccessible or hard to reach, such as below the engine on a sterndrive boat, consider relocating it if possible.

Selecting a Bilge Pump for Your Boat

A bilge pump is rated by its ability to move water, expressed in gallon per hour, or GPH. The rating is relative, however, and a good rule of thumb is that the pump will move about 60 percent of its rating. The distance and height it must move the water, and the type of discharge hose, all affect the real-world pumping rate.

If the discharge hose is a corrugated type, replacing it with a smooth-wall hose can increase the flow rate by up to 30 percent. It seems logical that a small boat can make due with a pump with a lower flow rating, but the opposite is true; in an instance of a hole in the hull or another major incident, that water is going to impact a small boat much faster than it will a larger boat.

Experienced boat owners use a two-pump strategy, especially if they head far from shore. A smaller (400-500 GPH) automatic bilge pump is mounted low in the bilge to pump out rain other incidental water, and a high-capacity pump (3,500 GPH) mounted higher to handle a more-serious situation.