Rather than using an internal or external float switch this centrifugal bilge pump measures water pressure against the impellers to determine whether it needs to activate. Due to the rotation of the impeller the velocity of the liquid increases.

Jabsco Rule Mate Bilge Pump
How does an automatic bilge pump work. Its automatic feature is made possible by the reed sensor system that is controlled electronically. How does a bilge pump work. The liquid after entering flows radially out and enters the vanes of the impeller. An excellent choice for smaller watercraft the rule 25s marine 500 automatic bilge pump is easy to install and maintain. This then enables the pump to work easily and operate automatically. The float switch on this equipment is inbuilt which is a great convenience to the buyer.
As a result of the impeller rotation there is an increase in the velocity of the liquid. After entering the fluid radially enters the impeller vanes. Working of bilge pump the liquid enters the pump through the center of the impeller also known as the eye of the impeller. How does a bilge pump work the liquid gets into the pump using the impeller center which is in other terms referred to as the eye of the impeller. The pump also has a very easy maintenance feature. Bilge pumps are placed in the hold or lower levels of a ship to remove accumulated water from the lowest.
Some bilge pumps require priming to encourage the draw of water through the pump and out the discharge.