Buoyancy
Hydraulics: Lesson 5
Buoyant Force
Archimedes’ Principle
• For an object partially or
completely submerged in a
fluid, there is a net upward
force (buoyant force)
equal to the weight of the
displaced fluid
A hydrometer is
an instrument
for measuring
the specific
gravity of
liquids
Stability of Immersed and Floating Bodies
When a body is completely immersed in a liquid, its stability depends on
the relative positions of the center of gravity of the body and the centroid
of the displaced volume of fluid, which is called the center of buoyancy.
Floating Bodies
The point of intersection
of the lines of action of the
buoyant force before and
after heel is called the
metacenter M, and the
distance GM is called the
metacentric height. If GM
is positive—that is, if M is
above G—the ship is
stable; however, if GM is
negative, the ship is
unstable.
Floating Bodies
Metacentric height GM
+GM → stable
- GM → unstable
Righting Moment RM
Note:
For large angles of heel,
direct methods of
calculation based on
these same principles
would have to be
employed to evaluate the
righting or overturning
moment.