Most small boats have these. Even boats with
electric bilge pumps often have hand operated Bilge Pumps for emergencies, and
lifeboats have them too.
A flapper valve (rubber disk) at the base of the
pump keeps water from falling out when the handle is depressed.
At the same
time, a sliding disk valve at the bottom of the traveling piston, is forced open
by water pressure allowing water to enter the upper part of the cylinder.
On the upstroke the disk valve on the piston is forced closed (down) and water
is forced up and out the top. At the same time the flapper valve opens and the
decreased pressure in the lower part of the cylinder sucks water in from the bilge.
(This is a still representation and possibly also a low-resolution
image of a pump which is fully animated in the CD-ROM version of this glossary.)