Bailing is the process of removing water from a vessel.
A hand bailer is a device used for manually removing water which has entered a vessel.
In the simplest case, it is merely a smaller container which can be filled and then emptied.
This kind of device has been in use since early times.
Self-bailing boats are shaped so that they will drain completely if filled with water; powered by the venturi effect and the motion of the boat, they are distinct from the powered bilge pumps used on non-self-bailing boats.