Page 42 - Introduction to Naval Architecture

4 Flotation and stability

A rigid body floating freely on the surface of a fluid has six degrees of
freedom, three of translation and three of rotation. For disturbances
from a state of equilibrium, the naval architect refers to the movements
in the six degrees as follows:

Fore and aft translation is termed surge.
Transverse translation is termed sway.
Vertical translation is termed heave.
Rotation about a fore and aft axis is termed heel or rott.
Rotation about a transverse axis is termed trim or pitch.
Rotation about a vertical axis is termed yam

The terms heel and trim are used in static or quasi-static conditions.
Roll and pitch are used in the dynamic situation which is dealt with
under seakeeping.
Having suffered a disturbance the body is said to be in stable, neutral
or unstable equilibrium if, when the disturbance is removed, it returns
to its original position, stays where it is or continues to depart further
from its original attitude, respectively. Correspondingly it is said to
possess positive, neutral or negative stability. A body floating freely on
the surface of a fluid must be stable in heave since the buoyany force
must equal weight as explained later. It will also have neutral stability
for surge, sway and yaw. The naval architect is concerned primarily with
bodies floating in water but the results are applicable to any fluid.

EQUILIBRIUM OF A BODY FLOATING IN STILL WATER

A body floating freely in still water experiences a downward force acting
on it due to gravity. If the body has a mass m, this force will be mg and
is known as die weight Since the body is in equilibrium there must be
a force of the same magnitude and in the same line of action as the
weight but opposing it. Otherwise the body would move. This opposing
force is generated by the hydrostatic pressures which act on the body,
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