Ship Stability - Curves of Statical Stability

Load Line requirements for minimum stability conditions

Ship Stability - Angle of Loll

Consider the following vessel in unstable equilibrium condition.

DEFINING THE SHIP GEOMETRY

As with any engineering product, precision is necessary in defining the geometry of a ship. Again, in common with most disciplines, an internationally recognised terminology has grown up over the years to aid this definition.

BUOYANCY AND DISPLACEMENT

It was Archimedes who first realised, in his “eureka” moment, that a body that is immersed completely in water will displace a volume of water equal to the volume of the body and that the apparent weight of the body, when immersed, will be reduced by the weight of water displaced.

Ship Stability - Movement of the Centre of Gravity

Centre of gravity

Ship Stability - Buoyancy

The Laws Of Buoyancy

FINDING A SHIP’S DRAUGHTS

Knowing the weight and cg position for any given ship condition, the draughts can be found by:

ANGLE OF LOLL A ship which is slightly unstable in the upright condition may become stable as it heels over. The GZ curve for such a case is shown:

THE GZ CURVE OR CURVE OF STATICAL STABILITY A typical plot of GZ against angle of heel is shown here.

Defining the Transverse Section

Picture shows typical cross-sections of a ship near amidships and near the bow. It will be noted that:

THE METACENTRIC DIAGRAM

When a ship is heeled through a small angle, the centre of buoyancy moves to a new position B1 and the buoyancy force acts through a point M on the centreline. M is known as the transverse metacentre.

STABILITY STANDARDS

Before one can define the standards for desirable stability, it is necessary to consider the normal operations of the ship and what accidents might befall it.

Ship Stability - Displacement

Mass, Weight, Force and Gravity

TRANSVERSE STABILITY, LARGE ANGLES

When the angle of heel becomes larger a number of the simplifying assumptions made above no longer apply:

TRANSVERSE STABILITY, SMALL ANGLES

So far, consideration has been given only to a ship when in equilibrium or moving slowly from one position of equilibrium to another. Now consider what happens when a ship is subject to a small heeling moment.

Defining the Length

There are three lengths commonly referred to:

HYDROSTATIC CURVES

For a given loading condition the draughts at which a ship will float are determined by:

Coefficients of Fineness

A table of offsets, although accurately defining the hull shape, does not provide an immediate feel of the main characteristics of that shape. There are some “coefficients” which can be obtained for the underwater hull which provide clues as to its general nature and its likely behaviour.