A
bilge pumping system is used to remove water from a
ship's inner
hull. This is important, as water in the hull is a leading cause of
sinking (hmm...) Bilge pumps have a capacity ranging from 2500
GPH (
gallons per hour) for a small
sailboat, to 10,000 GPH or more for a larger 50-60ft boat.
Bilge pumps may be manually activated, or set to trigger at a certain water level. It's generally a good idea to have a bilge pump in each hull compartment that has no free flow of water to another compartment (i.e., may fill up with water and weigh the boat down).
Information from David H. Pascoe's writing on the subject at http://marinesurvey.com/yacht/bilge_pumps.htm